旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � uXu � ATi � NiA � Cha0s � Abuse � cDc � CuD � EFF � Phrack � Telec0m Dige � � st � W0rldview � 2600 � Bad News � B00tleg � C0mputer Anarchist Undergr0u � � nd � Chalisti � Ch -=][][][][][][][][][][][][][][]][=- ip � C0re � Crimin � � al intent � Digita -=] A R R E S T E D [=- lFree Press � Digi � � tal Murder � Death -=] D E V E L 0 P M E N T [=- and Anarchy � FBi � � � Freed0m � Fuct � -=] +31.(0)77.547477 [=- � Future Culture � � � Hack-Tic � Hackers -=] [=- Unlimited � inf0r � � matik � KCAH � L0D -=] F0r All Y0ur H/P/A/V Files. [=- � L0L � Magik � M � � Ci � Panther M0der -=] Sys0p: 0mega [=- nz � NARC � NFX � � � NSA � PHA � Phanta -=] C0Sys0p: Syd Mead [=- sy � Phate � PHUN � � � PPP � SHA � UPi -=] [=- �TridenT � Vide0 V � � indicat0r � Dr. Pf -=][][][][][][][][][][][][][][]][=- alken � MPi � NuKE � � � Crypt Newsletter � B0W � ULTRA � Mega-industries � Phalc0n/Skism � Dem0 � � ralized Y0uth � AR *** N0T! F0R GENERAL DiSTRiBUTi0N *** CV � 40Hex � ins � � ane Reality � Silic0n V0rtex � BlaH � iiRG � CERT � Ne0n Knights � T0xiC � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 뇌頰頰頰頰Release�Inf0:型�THe�ARReSTED�DeVEL0PMENT�X-FiLES�#004型頰頰頰頰頰邂 쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 뇌頰頰頰Common Channel Signalling by S. Welch (Edited/OCRed by Omega)頰頰頰邂 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� This TadXF File is split into two pieces. TADXF #004 which is just the ASCII text and TADXF #005 which are the digitized figures and tables in PCX format. Release Date: 26th of November 1994. The text was taken from a book from Samuel Welch called 'Signalling in Telecommunications Networks' The ISBN number is : 0 906048 46 X. The pages OCRed are page 259-321. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= Table of contents Common channel signalling =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.1 Introduction 11.2 Basic common channel signalling 11.3 Association between c.c.s. and speech (or equivalent networks) 11.4 Network centralised service signalling 11.5 CCITT No. 6 signalling system 11.5.1 Basic concepts 11.5.2 Signal codings 11.5.3 Error control 11.5.4 Analysis of the system 6 error control method 11.5.5 Synchronisation 11.5.6 System 6 analogue and digital versions 11.6 Common channel signalling loading 11.7 Signalling link security and load sharing 11.7.1 Security 11.7.2 Load sharing 11.8 Changeover, retrieval and changeback 11.9 Continuity check of the speech path 11.10 Signal Priority 11.11 CCITT No. 7 optimised digital common channel signalling system 11.11.1 Basic concepts 11.11.2 Signalling bit rate 11.11.3 Error control 11.11.4 Message Structure References =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.1 Introduction =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= Conventional signalling on speech path systems (d.c., v.f., outband, m.f.) have a number of limitations: (a) Relatively slow signalling. This is usually tolerable for telephony pur- poses in most networks in view of the acceptable postdialling delays, but the application of computer-like techniques (stored program control) to the control of switching introduces new factors. (b) Limited information capacity. (c) Limited capability to convey signalling information which is not call re- lated and the inability of some systems to signal during the speech period. (d) The signalling systems tend to be designed for specific application con- ditions, which results in a relatively large number of different systems in the one network with consequential economic and administrative problems. (e) Expensive due to the per-speech circuit provision of most of the systems. Conventional line-signalling systems incorporating decadic address signal- ling are slow and inflexible, but meet the needs of most directacting non- common control switching systems. Wired-logic common control switching systems demand more flexibility, more facilities and the application of interregister signalling systems as discussed, which systems are common provision, line- signalling systems being necessary to deal with the supervisory signals. Time- sharing of conventional analogue line-signalling systems is not usually practicable and they are invariably per-speech circuit provided, and expensive in total network signalling cost. Stored program (processor) control (s.p.c.) of switching and networks has prompted a reappraisal of the signalling technique. Processor control makes possible the concentration of signalling logic for a large number of infor- mation circuits, e.g. speech, with consequent cost-reduction. Unless care is taken, however, an exchange processor may be given an excessive load in merely scanning speech circuits to detect the signalling condition, with a consequent loss of capacity for other functions, such as switching control, to be per- formed by the processor. It would also be necessary for the processor, or other suitable interface, to translate the various analogue conditions of speechpath signalling to digital, and vice versa. Thus with s.p.c., it is in- efficient for the processor which works in the digital mode, to deal with sig- nalling on the speech path. A much more efficient way of transferring infor- mation between s.p.c. exchanges is to provide a bidirectional high-speed data link between the two processors over which they transfer signals in digital form by means of coded-bit fields. A group of circuits (many hundreds) thus shares a common channel signalling (c.c.s.) link in the time-shared mode. In preferred c.c.s., all the signals between two exchanges are passed over a sig- nalling link which is separate from speech, c.c.s. thus replacing speech-path signalling systems such as d.c., v.f., outband and m.f. While c.c.s. could be applied to non-s.p.c. exchanges, the need to provide means of directing the information contained in the data link signals to or from the registers, and, in the case of tandem connections, to transfer such information across the exchange, would tend to make c.c.s. uneconomic; c.c.s. is thus mainly suited for s.p.c. exchanges. Many administrations are at present programming c.c.s. application, but owing to the well known inertia of networks to change, it will require a number of years for c.c.s. to make significant application impact in a net- work, and conventional signalling systems will exist, and continue to be applied, for a long time to come. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.2 Basic common channel signalling =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= General With c.c.s., independent signalling is performed in the respective sig- nalling directions on the respective signalling channels, the two signal- ling channels comprising the c.c.s. link (Fig. 11.1). Modems are used to transmit and receive serial binary data over analogue transmission channels, the multiplex performing this function on digital transmission channels. C.C.S. has the following merits: (i) signalling is completely separate from switching and speech transmission and thus may evolve without the constraints normally associated with such factors ------------------------[See fig11-1.pcx in tadxf005]------------------------ Fig. 11.1 Basic schematic common channel signalling ----------------------------------------------------------------------------- (ii) significantly faster signalling (iii) potential for a large number of signals (iv) freedom to handle signals during the speech period (v) flexibility to change or add signals (vi) potential for network centralised-service signalling (e.g. network management, network maintenance, centralised call accounting, etc.) (vii) is economic for large speech circuit groups (viii) can be economic for the smaller speech circuit groups owing to the quasiassociated and dissociated signalling capabilities (ix) unlike v.f. signalling, signalling line splits are not necessary; this eliminates the problem arising on v.f. signalling systems where a fast nonrepeated verbal answer response may be clipped, or lost, owing to the speech path being split on electrical-answer signal transmission. It was the recognition by the CCITT of the signficance of this problem in the international service which produced the initial thoughts for a c.c.s. system of the type under discussion. (x) allows possibility for signalling rationalisation in networks. C.C.S., however, gives rise to requirements which do not arise with signalling on speech-path systems such as: (a) high order error rate performance (b) signalling link security backup (c) assurance of speech-path continuity as, unlike speech-path sig- nalling, c.c.s. does not establish speech path integrity. A c.c.s. system serving many speech circuits must have a much greater signal dependability than signalling on speech-path systems, as random errors on the signalling link would disturb an appreciable number of signals, and therefore speech circuits. For this reason, provision must be made in c.c.s. to control errors, and as a generalisa- tion, an undetected error rate of the order of 1 in 10^8 to 1 in 10^10 is desired. Automatic diverting of signalling traffic to an alternative signalling facility occurs on excessive error rate, or on complete failure of a c.c.s. link. Time-shared signalling requires each signal to have identification of the speech circuit to which it belongs, the identification being on a time basis when a speech circuit has exclusive use of a signalling facility, as in built-in p.c.m. signalling (Section 6). This time-assigned signalling cannot apply when, as in c.c.s., a speech circuit does not have exclusive use of a signalling facility in the time-shared signalling, and a signal is labelled to give the speech circuit identity. With circuit labelling, the coding of a bit-field in the signal itself gives the address of the speech circuit, the number of bits depending on the number of speech circuits to be identified. This depends on the maximum number of speech circuits per c.c s. link-loading adopted, which is influenced by the capacity of the c.c.s. link to transfer signals. This, in tum, depends on the signalling bit rate and the size of signal unit. With c.c.s., the speech-path connection may be set up in parallel with the signalling connection set-up, or retrospectively. The former is adopted for simplicity and assurance of speech-path avail- ability. Speech-path availability could be assured with the retrospec- tive set-up by marking, but not switching, relevant speech circuits in the connection build up, but there appears to be little point in this. General features of c.c.s. -------------------------- The following general features apply: (a) Information may be transferred as signal messages of varying bit length, or by defined signal units. Signal units will be assumed for the present. (b) Each signalling channel operates in the synchronous mode with its continuous bit stream divided into contiguous signal units which all contain the same number of bits for the same application or service. The two signalling channels of a link need not necessarily be syn- chronised to each other, and, if not, drift may arise between the signal units in the respective directions, which, in certain implementations of c.c s. (e.g. CCITT 6 system) may require compensation arrangements. (c) Synchronisation (idle) units are transmitted when message signals are not being transmitted, to maintain signal unit synchronism on the signalling channel. (d) The signal unit is divided into a number of constituent bit fields each having its own function in the system, typically, heading, signal information, circuit label, parity check, etc. (e) As the c.c.s. link is time-shared, each message signal requires identification of the speech circuit (and thus the call) to which it be- longs. This is by means of a circuit label bit-field of size depending on the number of speech circuits to be identified. (f) Unlike time-assigned, channel-associated, time-shared p.c.m. signalling (Sections 6.2 and 6.3), a speech circuit does not have the exclusive use of a signalling facility in time-shared circuit-label ad- dressed c.c.s. and queueing delays arise; message signals being placcd in a queueing store and offered in turn for transfer over the signalling channel. Thus the heavier the signal loading the greater the queue, and the queueing delay, and the slower the speed of signal transfer. (g) As c.c s. signals do not prove the continuity of the speech path selected, other arrangements (e.g. per-call continuity check, routine testing of idle paths) must be made. (h) Errors are liable to occur in the signal-transfer process and some form of error control is required as the uncorrected error-rate of transmission plant is usually unacceptable for c.c s. Error detection is based on redundant coding, the parity check bits being, typically, part of each signal unit. Error correction can be, and usually is in telephony c.c.s., by retransmission. Despite the incorporation of error control, undetected errors may arise. Even with a high degree of error correction a signalling link could be unusable for varying periods, which requires signalling security backup. (i) For signalling security, signals may be directed, by automatic procedures, from a regular signalling link to an alternative signalling facility when an excessive error rate, or complete failure, of the regular link is detected. (j) On multilink connections, signalling information is transmitted on a link-by-link basis, the signals being transferred from one link to the next only after processing. This is an inherent feature of c.c.s. (k) since the circuit label may require a substantial proportion of the bits available in a signal unit, comparatively few bits may remain for coding the information a message has to convey. It follows that when the information is even moderately extensive, and particularly when it has a data content which can vary from one message to another, it cannot be transmitted efficiently by a succession of single signal unit messages. Instead, the initial circuit label carrying signal unit may be augmented by one or more subsequent signal units in which all the available bits can be used for carrying the information, thus producing an efficient multiunit message. Thus lone signal units (LSU) and multi- unit messages (MUM) may apply with c.c.s., and in the latter case the initial signal unit (ISU) carries the circuit label bit-field for the whole MUM. Each SU in a MUM carries its own check bit-field. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.3 Association between c.c.s. and speech (or equivalent) networks =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= The signal messages relating to a given group of speech circuits between two switching centres using a c.c.s. system can be transferred in the following ways: (a) Associated mode of operation: The signal messages are trans- ferred between the two switching centres over a c.c.s. Iink which terminates at the same switching centres as the group of speech circuits to which the signalling link is assigned (Fig. 11.2a). The term 'associated signalling' applied to c.c.s. should not be confused with speech path, or channel, associated signalling used to describe sig- nalling systems in which the signals are passed over the speech paths comprising the connections to which they relate, or over paths which are permanently and individually related to the speech paths. (b) Nonassociated mode of operation: The signal messages are transferred between the two switching centres over two or more c.c.s. links in tandem, and thus on a different routing from the relevant speech-circuit group, the signal messages being processed and forwarded through one or more intermediate signal-transfer points. It follows from this definition that there may be a range of nonassociated modes of operation which vary in the degree of rigidity imposed on the choice of the path used by the signal messages. The extremes of this range can be described as the fully dissociated mode and the quasiassociated mode. ------------------------[See fig11-2.pcx in tadxf005]------------------------ Fig. 11.2 Examples of associated and quasiassociated signalling Associated signalling between A and B b Quasiassociated signalling between A and B c Quasiassociated signalling between A and B ----------------------------------------------------------------------------- (c) Fully dissociated mode of operation: The signal messages are transferred between the two switching centres via any available path in the switching network according to the routing principles and rules of that network. The great flexibility in the routing of signal messages demands a more comprehensive message-addressing scheme than is needed for associated signalling. This is the ultimate of the principle of separate paths for signalling, and a completely separate c.c.s. network has been suggested. (d) Quasiassociated mode of operation: The signal messages are transferred between the two switching centres over two or more c.c.s. links in tandem, but only over predetermined paths and through predetermined signal transfer points. The predetermined routing permits the same method of addressing (circuit labelling) the signal messages as in the associated mode of operation. The constituent signalling routes of a quasiassociated signalling relationship may be associated-signalling in their own right (Fig. 11.2b), in which case the constituent routes carry the quasiassociated signalling traffic in addition to the associated-signalling traffic. Alternatively, the constituent signalling routes need not be associated-signalling in their own right (Fig. 11.2c). When the constituent signalling routes (AC and CB) in the quasiassociated signalling relationship are associated- signalling in their own right (Fig. 11.2b), the circuit labels for the speech-circuit group AB must be different from those of speech- circuit groups AC and CB. Thus for the necessary signalling discrimi- nation, the circuit labelling of signalling routes AC and CB must be increased to accommodate for this. When the constituent signalling routes AC and CB in the quasiassociated signalling relationship are not associated-signalling in their own right (Fig. 11.2c), the circuit labelling concerns the speech circuit group AB only in the typical case shown. Quasiassociated signalling may be adopted when a speech-circuit group is too small to justify economic application of associated-signalling The mode may also be used for signalling security backup for associated- signalling and for backup for another quasiassociated signalling relation- ship, but the circuit labelling tends to be complex in the latter case. With quasiassociated signalling, the number of signal transfer points in the signalling path for a group of speech circuits between two switch- ing centres should be as few as practicable to minimise the signalling time of those circuits, and to minimise the total signal processing load of the network. Normally one or two signal transfer points should suffice in a quasiassociated signalling relationship. Signal transfer point --------------------- In a nonassociated mode of operation, a signal transfer point is a signalling centre which forwards signal messages received on one c.c.s. link for onward transmission over another c.c.s. Iink. It follows from this that there is no need for a signal transfer point to have any con- nection with a switching centre (Fig. 11.2c). Alternatively, it may have (Fig. 11.2b). The main functions of a signal transfer point are: (i) to analyse the circuit label and the priority indication of every received signal message to determine the routing and, hence, to offer the message to the proper signalling link for onward transmission of the message taking account of the appropriate priority (ii) in doing so, it may be necessary to change (translate) the circuit label of the message according to some preset rules (iii) if for some reason, a signal transfer point is unable to transfer signal messages, a procedure is desired to notify the preceding signalling centre(s) so that the signal messages may be sent via an alternative route, if available. While c.c.s. has the potential for the various signalling modes discussed, it is thought that a combination of associated and quasi- associated signalling would meet the needs of most networks. The predetermined signal routings possible with these modes would be attractive in facilitating an orderly and efficient traffic-dimensioned c.c.s. network. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.4 Network centralised service signalling =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= Compared with wired-logic, processor (s.p.c.) control of switching has the potential for a far more sophisticated and greater degree of network-centralised services, and it is thought that this will be the undoubted trend. The combination of s.p.c. and c.c.s. will enable networks to be controlled and exploited to a greater extent than previously attainable. Many administrations are at present actively studying the extent and desired features of centralised services, typical services being: Speech-network management. Typically to initiate temporary changes of traffic routing patterns for reasons of congestion, catastrophic failure, etc. Signalling network management Network maintenance Centralised call accounting etc. Fig. 11.3 shows a possible basic arrangement in simplified form. Various implementations are possible, e.g. the various services may or may not be combined in the one centre, and the centres could be hierarchical (local, regional, national). The processors (CC) at relevant exchanges could be connected to relevant centralised service centres by data-signalling links, the signalling not being concerned with call handling. Should a hierarchical service-centre structure apply, further data links would interconnect the hierarchical centres as appropriate. ------------------------[See fig11-3.pcx in tadxf005]------------------------ Fig. 11.3 General arrangements network centralised services ----------------------------------------------------------------------------- The present interest is in the relevant signalling arrangements in the c.c.s. environment and it is thought that the switched-network c.c s. links should be capable of carrying centralised service in addition to the normal call-handling signalling to allow for the following pos- sibilities: (a) switched-network c.c.s. links used as security backup against failure of the centralised-service data links, e.g. on failure of the data link from exchange 1, the centralised service signalling passed on the c.c.s. link between exchanges 1 and 2 and then on the data link from exchange 2 (Fig. 11.3) (b) switched-network c.c.s. links utilised as data links to carry the centralised-service signalling en route to the service centres (c) when an exchange does not have direct data-link access to the service centres, access is obtained by means of a switched-network c.c s. link via an exchange having direct data-link access. It is clearly desirable that the switched-network c.c s. links should have adequate spare capacity to carry the centralised service in addition to the call-handling signalling, the centralised-service signalling con- forming to the switched-network c.c.s. signalling features (error control, signal unit size, etc.) in this situation. It would not be essential for the centralised service data links to be of same type as the switched network call handling c.c.s. links, but in certain circumstances there could be merit in uniformity if they were. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.5 CCITT No. 6 signalling system =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.5.1 Basic concepts. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= The CCITT 6 signalling system was the first c.c.s. system to be specified, designed and tested. While specified primarily for international use, it is equally suitable for national application and some administrations are presently programming its use in their national networks. Initially, system 6 was specified for 2.4 kbit/s analogue application. Subsequently, digital versions 4 kbit/s and 56 kbit/s were included in the specification but without change to the initial basic signalling arrangements formulated for analogue. Other signalling bit rates may be adopted as desired (e.g. 4.8 kbit/s analogue). It is convenient to present the pioneer system 6 solutions to the many problems arising in c.c.s. as a general basis before discussing improved solutions which may be adopted in some areas, improvements that arise from the greater knowledge now available in the c.c.s. art and from the natural evo- lution of the technique. Each signalling channel is operated synchronously but the two channels are not synchronised with each other, and a continuous stream of data flows in both directions. The data stream is divided into signal units (SUs) of 28 bits each, of which the last 8 are check bits, all SUs being of the same length for ease of synchronisation in particular. The SUs in turn are grouped into blocks of 12, the 12th and last SU of each block being an acknowledgment SU coded to indicate the number of the block being transmitted and carrying the ack- nowledgment i.e., the number of the block in the other direction being acknow- ledged, and whether or not each of the 11 SUs of the block being acknowledged was received without detected errors. Blocks are sequenced numbered but indi- vidual message SUs in the block are not, being identified by their position in the block. The first 11 SUs of a block consist of message or synchronisation SUs; the latter, transmitted only in the absence of other signalling traffic, facilitate achieving or maintaining SU and block sychronisation, and are coded to indicate the number of the position they occupy within the block to facili- tate locating the acknowledgment SU. A signalling bit-rate of 2.4kbit/s adopted for the analogue version is the maximum rate for transmission with acceptable error rate over a phase- equalised speech-band channel. At this bit-rate, one c.c.s. Iink can carry all the signals required by some 1500-2000 speech circuits in the telephony ser- vice without excessive delay to individual signals. Compared with the 2.4kbit/s analogue version, the 4kbit/s and 56 kbit/s signalling bit-rate digital versions allow, in theory, more speech circuits to be served per c.c.s. link due to the reduced emission time of the signals. In practice, the same size circuit label bit-field applies for both the analogue and digital versions in the international specification. National network variants of the specification may, of course, extend or reduce the size of the circuit label bit field as required. Optional 4.8 kbit/s analogue is presently being con- sidered. The signals, which may be LSUs or MUMs, are formatted in the processor func- tion and delivered to an output buffer, which delivers the highest-priority signal awaiting transmission to a coder in serial form in the next available time slot. Each SU is encoded by the coder by the addition of check bits in accordance with the check-bit polynomial. The signal is then modulated and transmitted over the link in serial form. At the receive end, serial data is delivered to a decoder where each SU is checked for error on the basis of the associated check bits, SUs received with detected errors being discarded. Message SUs which are error-free are transferred to an input buffer after de- letion of the check bits, the input buffer delivering the SUs to the processor function for appropriate action. A transmission path error rate of 1 in 10^6 to 1 in 10^7 was assumed and as the requirement of system 6 was to achieve an undetected error rate of 1 in 10^10 for significant signals and 1 in 10^8 for others, error control was adopted with error detection by redundant coding and error correction by retransmission. Message SUs are retransmitted on error detected, synchronisa- tion SUs are not. A data channel failure detector complements the decoder for the longer error bursts. On excessive error rate, or on complete failure, automatic procedures direct the signal traffic from a 'failed' link to an alternative signalling facility. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.5.2 Signal Codings =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= Basic philosophy The binary codings of bit fields in c.c.s. allows potential for a consider- able signal repertoire depending upon the bit field, and SU, size. Information is transferred in system 6 by means of one (LSU) or more (MUM) SUs, a SU being the smallest defined group of bits on the signal channel, and, in system 6, contains 28 bits (20 information, 8 check). A LSU may transmit either a single telephone signal, a system control signal (e.g. ACU) or a centralised service signal. Basically, a MUM, which conveys a number of related signals (e.g. address digit signals), may consist of any desired number of SUs, but in system 6 consists of 2, 3, 4, 5 or 6 SUs, the constituent SUs of a MUM being identified as ISU (the first) and SSU (the second and any following). In the c.c.s. art (but not 6) the FSU is the final SU of a MUM, the FSU being a SSU and is so called in system 6. The ISU, SSU (and FSU) facilitate MUM length indication in the signalling system. If the MUM is speech-circuit related, the circuit label is included in the ISU only. Basically, a defined c.c.s. system can be regarded as being merely a means of transferring bit-coded messages, individual administrations being free to adopt their own codings as appropriate. It is logical, however, to adopt some uniformity in the codings and constituent bit-field sizes, for ease of inter- working between national, regional and international applications of system 6, the approach that has been adopted. The CCITT specification details the signal codings for the international application, with adequate spare capacity allo- cated to cater for particular national network requirements not included in the international specification. As there is a high degree of commonality in international and national signalling requirements, considerable uniformity will apply which is the objective, but it must be accepted that national network variants may well arise. (a) Telephone signals LSU (lone signal unit): A number of bits within each SU must be allocated to distinguish the specific message being transmitted and in the telephony LSU nine bits perform this function, divided into two fields; heading field bits 1-5 and signal information bits 6-9 (Fig. 11.4a). The LSU Fig. 11.4a is also the basic format of the ISU. The two bit-fields permit ease of admini- stration of the signals, a heading being allocated to a class of signals, e.g. address digit signals, individual signals within each class being distin- guished by the relevant coding of the signal information field. These two bit- fields more than cover existing requirements (Table 11.1) and leave capacity for new signals and classes of signals not yet defined. Where appropriate, the coded bits 1-5 also give indication of the signalling direction (forward, backward) of the signal identified by the signal infor- mation bits 6-9 (e.g. heading code 11011 indicates that all the signals under this heading are backward signals--Table 11.1). While the heading generally consists of bits 1-5, there are two exceptions: ------------------------[See fig11-4.pcx in tadxf005]------------------------ Fig. 11.4 CCITT 6 system: typical message signal formats a Basic format of LSU and ISU of MUM b Format of SSU of MUM c Example of 3-unit MUM ----------------------------------------------------------------------------- (i) all SSUs of a MUM are identified by the same 2-bit heading code 00 (bits 1-2), Fig. 11.4b. (ii) the acknowledgment signal unit ACU is identified by a 3-bit heading code 011 (bits 1-3), Fig. 11.5a. The heading codes (bits 1-5) are allocated: 00 SSU 01000 -| 01001 | Spare 01010 | 01011 -| 011 ACU 10000 ISU of IAM or of MUM 10001 -| 10010 | 10011 | SAM (one-unit message or multiunit message) 10100 | 10101 | 10110 | 10111 -| 11000 -| 11001 | International telephone signals 11010 | 11011 -| 11100 Spare 11101 Signalling system control signals (Except ACU) and management signals 11110 -| Spare 11111 -| The signal information codes (bits 6-9) are allocated as shown in Table 11.1. MUM(multiunit message): A MUM, which may be address messages, centralised service messages, or messages conveying other types of information, consists of an ISU and a number of SSUs. The basic format of the ISU is the same as that of the LSU (Fig. 11.4a) the heading code identifying the ISU. SSUs are identified by the heading code bits 1-2 and have a 2-bit field (bits 3-4) coded to indicate the number of SSUs in the MUM (Fig. 11.4b), thus serving as MUM length indication. In system 6, an IAM MUM may be 3-6 SUs and all other MUMs 2-5 SUs, and each SSU of a MUM carries the same relevant length indicator coded as follows: Length indication in SSU (bits 3-4) Number of SSUs - - - - - - - - - - - - - - - - - - - - - - in MUM IAM MUM Other MUMs -------------------------------------------------------------------------- 1 - 00 2 01 01 3 10 10 4 11 11 5 00 - This leaves 16 bits (bits 5-20) in each SSU to carry information (Fig. 11.4b). The term MUM should not be confused with the term block in the block trans- mission of system 6. The block is simply a group of 12 consecutive SUs, which may be LSUs, a mixture of LSUs and SYUs, a mixture of MUM (or part MUM) and LSUs, two MUMs, etc. IAM (initial address message): The IAM is the first message of a call connection set-up. Unlike conventional speech path signalling systems, a dis- crete seizure signal is not required in c.c.s., the IAM performing this function. As the IAM conveys information (e.g. routing) in addition to address digits, more than one SU is always necessary and the IAM on any call is always a MUM, consisting of an ISU (Fig. 11.4a) and SSUs (Fig. 11.4b). As in SSUs of MUMs in general, bits 1-4 of SSUs of IAMs give SSU identification and MUM length indication. The information bit-field (bits 5-20) of SSUs 2-5 of IAMs in the international system 6 is subdivided into four 4-bit parts so that four address-digit signals can be conveyed in each SSU. The information bit-field (bits 5-20) of SSU 1 of IAMs is used for routing information required in the setting up of connections in the international system 6. This field could be used for other purposes in regional and/or national application of system 6 as required. Also as in SSUs of MUMs in general, SSUs of IAMs do not require the 5-bit heading or the circuit label bit-fields as these items of information are already contained in the ISU. In the detail, the codes used in the IAM are (Table 11.1): (i) ISU: The 5-bitheading code 10000 (bits 1-5) in combination with the signal information code 0000 (bits 6-9) identify that the ISU is the ISU of an IAM. (ii) SSU 1: The heading code 00 (bits 1-2) identifies the SSU and bits 3-4 are coded to give the appropriate length indication. In the interna- tional application, the special routing information required in the setting up of connections is as follows: Bit 5 Country code included or not in the IAM. This feature can be utilised to inform an incoming international exchange to function as terminal or as transit. Bit 6 Nature of circuit indicator. In particular, this bit is used in satellite routing restriction and records if a satellite link has been used. Bit 7 Echo suppressor control. Whether or not an echo suppressor is required. Bit 8 Spare (reserved for international) Bits 9-12 Spare (reserved for regional and/or national) Bits 13-16 Calling party's category, such as operator, ordinary sub- scriber, data service, etc. Bits 17-20 Spare (reserved for regional and/or national) (iii) SSUs 2-5 - telephone call: As in SSU 1, the heading code 00 (bits 1-2) identifies the SSU and bits 3-4 give the appropriate length indication. The four 4-bit parts of the signal information field bits 5-20 contain address-digit signals in sequence, bits 5-8, 9-12, 13-16, and 17-20, respectively, being coded as follows (Table 11.1): 0000 filler (no information) 0001 digit I 0010 digit 2 0011 digit 3 0100 digit 4 0101 digit 5 0110 digit 6 0111 digit 7 1000 digit 8 1001 digit 9 1010 digit 0 1011 code 11 operator 1100 code 12 operator 1101 spare 1110 spare 1111 ST (end of pulsing) The filler code 0000 is used where needed to complete the signalling infor- mation of the last SSU of the IAM if this SSU is partially used for carrying address digits. ------------------------[See tabl11-1.pcx in tadxf005]----------------------- Table 11.1 CCITT System 6 allocation of heading and signal information codes ----------------------------------------------------------------------------- Signal abbreviations, System 6: ACU Acknowledgment signal unit-error control ADC Address complete signal, charge ADI Address incomplete signal ADN Address complete signal, no charge ADX Address complete signal, coin box AFC Address complete signal, subscriber free, charge AFN Address complete signal, subscriber free, no charge AFX Address complete signal, subscriber free, coin box ANC Answer signal, charge ANN Answer signal, no charge BLA Blocking Acknowledgment Signal BLO Blocking Signal CB1-3 Clear Back signal No. 1 - No. 3 CLF Call Failure Signal COF Confusion signal COT Continuity signal CSSN Circuit state sequence number FOT Forward Transfer Signal IAM Initial Address Message ISU Initial Signal Unit LOS Line Out-of-service Signal LSU Lone Signal Unit MBS Multiblock Synchronisation Signal MRF Message-refusal Signal MUM Multiunit Message NMM Network Management and maintenance signal NNC National Network Congestion Signal RA1-3 Reanswer Signal No. 1 - No. 3 RLG Release guard signal SAM1-7 Subsequent Adress Message No. 1 - No. 7 SCU System Control Signal Unit SEC Switching Equipment Congestion Signal SNM Signalling Network Management Signal SSB Subscriber Busy Signal (Electrical) SST Subscriber Transferred Signal SSU Subsequent Signal Unit SU Signal Unit SYU Synchronisation Signal Unit UBA Unblocking Acknowledgment Signal UBL Unblocking Signal VNN Vacant National Number Signal Fig. 11.4c shows a typical three-unit IAM. In bothway operation of c.c.s., a double seizure is detected by an exchange when it receives an IAM for a speech circuit for which it has sent an IAM. At the analogue signalling bit-rate 2.4 kbit/s, each SU has an emission time of approximately 11.7ms and with a three-unit IAM, four address digits can be transmitted in some 35ms. Typically, a ten-digit national number, if transmit- ted en bloc, would require a five-unit IAM and transmitted in some 58.5 ms at a rate of 170 digits per second. This is much faster than any speech-path telephony-signalling system currently in use. Digital c.c.s. transfers address information at still faster speeds. SAM (subsequent address message): A SAM, which may be a LSU or a MUM, is used to transmit additional address information not available for transmission when the IAM is formed, or (but not in international system 6) when the trans- mission of address information after the IAM is controlled by backward request signals. The format of an LSU SAM and of the ISU of a MUM SAM, is the same as that of the normal LSU (Fig. 11.4a). The LSU SAM carries one address digit in the signal-information field-bits 6-9. The ISU of a SAM does not carry an address digit, bits 6-9 being coded 0000 filler (no information). The format of an SSU of a MUM SAM is the same as that of the SSU of any MUM (Fig. 11.4b) except that, unlike SSU 1 of an IAM (which does not carry address information in international system 6), SSU 1 of a SAM may carry address information in the information field bits 5-20. Thus all SSUs of SAMs may carry four address digits each, bits 5-20 being subdivided into four 4-bit parts. The 4-bit address digit fields are coded as for SSUs 2-5 of an IAM to transmit address information. The Code 11 and Code 12 operator codes are not used in SAMs, as this information will have been given by the IAM. The filler code 0000 is used, where needed, to complete the signal information field of the last SSU of a SAM. Heading codes (bits 1-5) in the range 10001-10111 are used in LSU, or the ISU of, SAMs, depending upon the sequence number of the SAM concerned (Table 11.1), i.e. 10001 first SAM 10010 second SAM 10011 third SAM, etc. This sequence numbering of SAMs is necessary in system 6 as messages can get out of sequence at the processor function in the working of the c.c.s. system, and the sequence numbering facilitates the restoration of SAMs to correct the sequence at the processor. It is preferred to limit the number of SAMs on a call connection set-up, but if more than seven are sent, the sequence is re- cycled so that the eighth SAM uses code 10001 (b) Management signals In international system 6, management signalling may include network manage- ment, network maintenance and signalling network management, the signals being transferred as LSUs or MUMs. The management signalling detail is not yet resolved for international system 6 and specific formats are not yet specified. The basic format of the normal LSU (Fig. 11.4a) will apply for management LSUs and ISUs of management MUMs, and the heading code 11101 (bits 1-5) is assigned. Appropriate coding of the signal information field (bits 6-9) distinguish the various categories of management signalling, i.e. network management maintenance, etc. (Table 11.1). Most management signals will require a band number (bits 10-16 Fig. 11.4a) to route the signal information on the switched network via appropriate signal transfer points. As management signals will not require a circuit number in the band, bits 17-20 (Fig. 11.4a) are used for management information. (c) Signalling system control signals Control signals in system 6, always LSUs, are necessary for the proper func- tioning of c.c.s. systems. They are not related to telephone signalling information and are thus not speech-circuit related. In system 6, the control signalling comprises: (i) acknowledgment ACU signals in the error control (ii) synchronisation (idle) SYU signal units (iii) multiblock monitoring and acknowledgment signals MBS for multiblock synchronisation (iv) system control signal units SCU, a group of signals concerned with signalling link failure and the various consequential procedures. The heading field (bits 1-5 Fig. 11.4a) is coded 11101 for signalling system control signals, except the ACU which is coded 011 in heading bits 1-3. Signal information bits 6-9 are coded to distinguish the category: 1100 signalling link failure procedure signals (SCU) 1101 SYU 1011 MBS Bits 10-20 give further signalling information in each category, bits that would otherwise be used for the circuit label in speech-circuit-related LSUs. SYU (synchronisation signal units): This consists of a 16-bit fixed pattern for bit (analogue c.c.s.) and SU synchronisation, and a 4-bit field for block synchronisation, the latter identifying the location of the SYU within the block (i.e. 1st, 2nd . . . or 11th unit). The SYU (Fig. 11.5b) completes with the eight check bits. The particular 16-bit fixed pattern of the SYU presents an easily recognisable pattern with a variation at the end to aid in its detection. It also provides six-bit transitions to aid the attainment of bit- synchronisation by the modems in analogue c.c.s. (Section 11.5.5(a)), bit- synchronisation being maintained by the transition between dibits. MBS (Multiblock synchronisation signal unit): The original system 6 specifi- cation, limited to 2.4kbit/s analogue c.c.s., allowed for a maximum of eight blocks in the error-control loop. This was adequate for a maximum delay of 740 ms (single-hop satellite) and accounted for the 3-bit (8 possibilities) block- completed and block-acknowledged fields in the ACU. The revision of the specification to include 4kbit/s and 56 kbit/s digital versions was coincident with a requirement to meet a maximum error-control loop delay of 1200ms (double hop satellite), requiring more than eight blocks in the errorcontrol loop at all the signalling bit rates specified i.e. 2.4, 4 and 56kbit/s. The revised specification treats eight consecutive blocks as a multiblock and allows for up to 32 multiblocks. Thus the maximum number of blocks in the error-control loop is 256 (3072 SUs), which is adequate for a 1200ms loop- delay signalling link at 56 kbit/s in the telephony service. The full 256- block capability need not be handled in all applications. Block memory may be limited to that required for the expected range of loop delays and signalling bit-rates at which the system is applied in particular situations. The concept of the multiblock gives rise to the requirement for two signals of a control nature (Fig. 11.5c) i.e. (a) multiblock monitoring signal, required on links where the number of blocks in the error-control loop exceeds eight, and sent to check multiblock synchronism (b) multiblock acknowledgment signal, sent in response to the multiblock moni- toring signal and used by the terminal receiving it to verify multiblock synchronism. Of the MBS as shown in Fig. 11.5c: Bits 10-12 are coded 000 for the monitoring signal and 100 for the acknow- ledgment. Bits 13-17 are coded to indicate the sequence number of the multiblock in which the multiblock monitoring signal is sent. Bits 10-20 are coded to indicate the sequence number of the block in which the multiblock monitoring signal is sent (or placed into the output buffer). ------------------------[See fig11-5.pcx in tadxf005]------------------------ Fig. 11.5 CCITT 6 system: typical control signal formats a Format of ACU b Format of SYU c Format of MBS ----------------------------------------------------------------------------- The above covers the main signals of CCITT system 6, the international specification details the format and codings of other signals included in the specification. Fig. 11.6 shows a typical signalling sequence with system 6. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.5.3 Errorcontrol =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= In system 6, provision is made in every SU to detect errors due to noise, line faults or transmission faults causing a mutilation of bits. ------------------------[See fig11-6.pcx in tadxf005]------------------------ Fig. 11.6; Typical signal sequence CCITT 6 system ----------------------------------------------------------------------------- Error detection is by redundant coding, a number of check bits being generated in the form of a cyclic code, each SU having an 8-bit check field. The gene- rator polynomial selected for system 6 to generate the check-bit pattern provides maximum protection for short noise bursts affecting up to four consecutive bits. To detect for longer noise bursts, a data channel-failure detector supplements the error detection by the check bits. This detects, and takes effect after a delay of about 5 ms, the following: (a) failure of the data carrier or an excessive noise level relative to the carrier level in analogue c.c.s. (b) loss of frame alignment in digital c.c.s. Not all errors will be detected by the error control, undetected errors will arise. Error detection: Error detection is performed by check bits (bits 21-28) in each SU, with coders and decoders equipped at the transmit and receive terminals, respectively, for the purpose. The principle of operation is that the digital sequence constituting the message is treated as a polynomial and is divided at the transmitter and at the receiver by a preagreed number. The remainder is transmitted from the transmitter to the receiver as check bits. The process is implemented using a modulo-2-division by a shift register with a number of stages equal to the number of check bits. For a given number of check bits C, there are 2^(C-1) different generating polynomials which can be chosen for the divisor, these polynomials having different characteristics which are indicated by their modulo-2 factors. Error burst patterns can also be considered as polynomials. Any error burst whose polynomial is the same as the generating polynomial is undetectable as the division process will change the quotient by one bit, but not alter the remainder. If the length of the error polynomial is less than the generating polynomial, the remainder will be changed and the error detected. When the error-burst polynomial has more terms than the generating polynomial, the errors will only be undetected if the generating polynomial forms a factor in the error-burst polynomial. In system 6, the coder generates eight check bits based on the polynomial: P(X) = (X+1)(X^7 + X^6 + X^5 + X^4 + X^3 + X^2 + 1) = X^8 + X^2 + X + 1 A characteristic of cyclic codes is their inferior protection in the case of a slip in frame synchronisation in p.c.m. Security is maintained by arranging for the check bits C to be inverted before transmission and reinverted before checking at the receiver. If there has been a slip, the reinversion is applied on an incorrect sequence of bits. When the decoder at the receive terminal has received all 28 bits of the SU after the check bits have been reinverted, it will indicate whether or not the signal has been checked as correct. This information is stored for inclusion in the acknowledgment indicator field of an ACU to be emitted in the return direction. The cyclic code for error detection is supplemented by a datachannel failure detector (Section 11.8). Indication of data-channel failure owing to unsatis- factory transmission conditions causes rejection of SUs in the process of reception, and during the alarm condition all SUs are rejected (i.e. acknow- ledged as corrupted) regardless of check. The system 6 specification requires that an undetected error rate of not more than one erroneous SU out of each 10^8 transmitted should result in a false operation and not more than 1 in 10^10 should cause malfunction such as false metering or false release. The specification is worded in this way as it was anticipated that most signals with undetected errors would result from the disturbance of idle units which predominate even during the busy hour. It was also evident that many false signals would cause no trouble as they would be addressed to nonassigned circuit labels, or would occur in conditions in which they were meaningless. Error correction: Error correction is by retransmission in system 6, which necessitates a retransmission store at the transmit end, correctly received SUs being cleared from the store. On error-detected, the SU must be discarded at the receive end and the information retransmitted. This requires a method of acknowledging signals and a means of identifying SUs. The latter is achieved in system 6 by transmitting SUs in blocks and allocating identifying numbers to the blocks, individual SUs can then be identified by position within a block. Blocks are of a 12-unit fixed length, comprising 11 SUs (which may be message or synchronising) and an acknowledgment unit (ACU) in the 12th position (Fig. 11.7). Positive/negative block acknowledgment is employed. A positive acknowledgment indicates that the relevant SU has been received error-free, a negative acknowledgment indicates error detected and is thus a request for a retransmission. Each ACU (Fig. 11.5a) contains: (a) a 3-bit fixed pattern (011) to identify the unit as an ACU (b) a 3-bit code to identify the block of 12 units completed by this ACU (providing for a count of 8) (c) a 3-bit code to identify the block being acknowledged by this ACU (providing for a count of 8) (d) the signal acknowledgment mechanism: eleven bits are allocated to identify the SUs of blocks received in the other direction on a one-to-one basis, zero (0) or (1) being written in the ACU depending on whether the corres- ponding SU was received error-free or in error (e) 8 check bits. As a MUM may spread over two blocks, break-in to a MUM may take place due to an ACU, whose position in the block is fixed. ------------------------[See fig11-7.pcx in tadxf005]------------------------ Fig. 11.7 CCITT 6 system: block signalling ----------------------------------------------------------------------------- With the block structure used, the block sequence numbers must repeat every 1120 ms at 2.4 kbit/s. In the original specification this was sufficiently long at 2.4 kbit/s to ensure that under normal conditions, there could be no ambiguity as to what is being acknowledged even when a single-hop satellite is included in the signalling link in the telephony service (but see Section 11.5.3 'Influence of digital versions system 6 on the error control'). Retransmission procedures: The following procedures apply in error conditions: (i) When a SU is corrupted, the appropriate acknowledgment indicator in an ACU is set negative. This procedure is also used if for any reason the receive terminal cannot handle a SU. (ii) When the corrupted SU is a LSU, that LSU is retransmitted (in another block). (iii) When a SU (or SUs) of a MUM is corrupted, the whole MUM is retrans- mitted. This avoids associating SSUs with a corrupted ISU. (iv) Corrupted SYUs (idle units) are not retransmitted. (v) It will be noted that a received ACU is not itself acknowledged. If an ACU, always known as such by its 12th position in the block, is corrupted, it is assumed that all its acknowledgment indicators are negative and all message SUs in the block waiting to be acknowledged are retransmitted. In the limit, a block of 11 message SUs could be retransmitted. This means that on occasions the same SU may be received error-free more than once. Processor reasonableness checks: The following irregularities on the same call may arise with system 6: (a) Unreasonable messages: (i) in incorrect sequence, as may arise on retransmission due to error detected (ii) an incorrect signal direction. (b) Duplicated (superfluous) messages, as may arise owing to corruption of an ACU, or to drift compensation. To resolve such irregularities, special processor procedures are defined and are included in reasonableness check tables which cover all possible stages in the signalling sequences. The reasonableness check tables for system 6 are based on the logical signal processes and sequences which should apply on a call. Typically, if two IAMs are received owing to corruption of an ACU or to drift compensation, the two are compared. If they are identical, one is discarded, and if they are different, a confusion signal can be sent. The action taken by the processor on unreasonableness detected depends on the nature of the irregularity, the various actions being: (i) Rejecting: Messages or SUs recognised to be unreasonable or superfluous are discarded. (ii) Waiting: Unreasonable messages or SUs which may become meaningful at a later stage of the signal sequence are provisionally held. The waiting time should be longer than the retransmission delay of the delayed message. The provisionally-held SUs are processed if the arrival of retransmitted signals within the waiting period makes them meaningful. Otherwise, if they are still meaningless at the end of the waiting period, they are rejected with the exception of the cases where the held signal is a clear forward. In this case, the release-guard signal must be sent. (iii) Clearing If owing to an abnormal signal sequence an ambiguous situation arises which would result in a circuit being held unduly for a prolonged period, the circuit is cleared by the release sequence. (iv) Confusion signal: If none of the above actions is suitable for resolving the situation created by the irregularity, the confusion signal is returned. On receipt of this, the distant exchange sends the clear- forward signal to release the connection and on certain irregularities an automatic repeat attempt is made to set up the call. The confusion signal will not be sent subsequent to sending the address complete signal or other signal causing the clearing from the store of address and routing information at the preceding exchange. The irregularities arise in the message-transfer process of the signalling system. Some reasonableness checking is always necessary at the processor regardless of the type of error control involved in the message-transfer control. It is a question of degree. Error-control methods which do not allow incorrect sequence and duplicated messages to be passed from the signalling terminal to the processor would permit relatively simple reasonableness- checking covering undetected errors in the main. The system 6 error-control method allows out-of-sequence and duplicated messages to be passed to the processor, resulting in significant irregularity in addition to that due to other causes, such as undetected errors. As a result, the reasonableness checks associated with system 6 are complex and load the processor. Drift compensation: As the two signalling channels of a signalling link are not synchronised with each other, the usual condition for telephony c.c.s., the two signalling terminals may get out of step. The following applies in system 6: (a) The faster terminal may be ready to send an ACU before it has a block to acknowledge. In this case the ACU of the previous block is repeated. (b) The slower terminal may find it has two blocks to acknowledge when it is ready to send an ACU. In this case it will omit to acknowledge one block altogether, the 12th unit being sent to complete the block but will not contain acknowledgment information. The far end then retransmits all message SUs of the unacknowledged block. Influence of digital versions system 6 on the error control The revision of the orignal analogue 2.4 kbit/s signalling specification to include 4 kbit/s and 56 kbit/s digital versions was coincident with a require- ment to meet a maximum error-control loop delay of some 1200 ms (double-hop satellite). This requires more than eight blocks in the error-control loop at all the signalling bit-rates specified (2.4, 4 and 56 kbit/s). With the retention of the 3-bit block-sequence number field in the ACU, an implicit approach was adopted, the block-completed and block-acknowledged indications being based on a so called 'dynamic indexing' plan. The plan takes advantage of the fact that after a signalling link is in synchronism, much of the information transmitted in the block-sequence number fields of the ACU is redundant. Once block counts have been established in the initial synchronisation, and as long as synchronism is maintained, local block counts kept at each end are entirely adequate. Block acknowledgment only, without block-sequence numbering, would suffice. As, however, drift may arise due to the two signalling channels not being synchronised with each other, some redundancy in block-number information is necessary in each block to permit compensation for drift and for detection of loss of block synchro- nisation. The 3-bit block-sequence number fields in the ACU performs this function. Each signalling terminal has two block counters, each of up to 8-bit capacity (256 possibilities): (i) Block completed counter: This indicates the sequence number of the last block transmitted by the terminal. The last 3 bits of this number are also sent in the ACU of the block in the 3-bit block completed field of the ACU. (ii) Block acknowledgment counter: This is updated by the 3-bit block-acknow- ledgment sequence number in the incoming ACU and thus indicates the sequence number of the block being acknowledged by the last received ACU. To keep it updated even when the ACUs are corrupted, the block acknow- ledgement counter is also incremented whenever the 12th unit of a block is received corrupted. Thus the signalling terminal and/or processor keeps track of the block numbers and identifies blocks in the error-control loop. The subsequent return in the block-acknowledged field of a received ACU of the last 3 bits of the maximum 8-bit block-completed number of the block-completed counter in the ACU in the opposite direction gives the block-acknowledgment indication to a transmit terminal. Should the 3-bit block acknowledged number in the received ACU be different from the last 3 bits of the block transmitted, drift or loss of block sychronism will have occurred. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.5.4 Analysis of the system 6 error-control method =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= The following main areas are analysed, many points of less significance being omitted: (a) Correct sequence of messages: Messages can get out of correct sequence on retransmission and would be passed in incorrect sequence from the signal- ling terminal to the processor. (b) Duplicated messages: Messages may be received correctly more than once for a number of reasons e.g. (i) corruption of ACU, resulting in all message SUs in the block being retransmitted, could result in the retransmission of messages al- ready received correctly (ii) drift compensation resulting in a block not being acknowledged (iii) loss of an ACU during failure conditions. Such duplicated messages would be passed from the signalling terminal to the processor. (c) Reasonableness checks: The incorrect sequence and duplicated messages passed to the processor require complex reasonableness checks at the processor to deal with the situation. This complicates the processor and increases the processor load. (d) Unnecessary retransmissions: These can occur owing to (i) complete MUMs being retransmitted on corruption of constituent message SU(s) of MUMs. (ii) Corruption of message SU(s) in unrequested retransmissions. SYUs are known to be such at the transmit end by virtue of their positions in the block and thus by their positions in the block store. Corrupted SYUs, while returning negative acknowledgments in the relevant indicator bits in the ACU, do not cause retransmission of SYUs, or any other signals. Thus unnecessary retransmissions do not occur on corrupted SYUs. (e) Unrequested retransmissions: These are said to arise when acknowledgments are corrupted. All message SUs in a block are retransmitted on corrupted ACU. Approximately 50% of all retransmissions will be unrequested due to this cause. (f) Drift compensation: While drift is always liable to occur when the two signalling channels of a link are not synchronised with each other, complex compensation procedures are necessary when the ACU is in a fixed position in a block. (g) Unique ACU signal unit. The use of a complete signal unit (ACU) for ack- nowledgment reduces the traffic handling capacity of a link by some 8.5%. (h) Fixed position of ACU in block: While the unique ACU has significant dis- advantage in regard to unrequested retransmissions on ACU corruption, it has advantage when it is in a fixed position in a block. The SU will always be known as being an ACU and thus corruption and undetected errors can never be interpreted as being a nommal SU. On the other hand, the fixed position of the ACU introduces complication when it splits up the SUs of a MUM. If a retransmission is requested for any of the message SUs comprising a MUM, then the whole MUM must be retransmitted. Thus even though the correct receipt of the first part of a MUM in a particular block may be acknowledged by the appropriate ACU, the possibility of a retransmission cannot be rulcd out until the receipt of the subsequent ACU in the next block which acknowledges the SUs forming the remainder of the MUM. (i) Delays: These are relatively long and are (i) up to almost two blocks delay when requesting retransmission (ii) up to one or two blocks delay in retransmitting. It is concluded that the system 6 error-control method is complex, and that far too much requirement is placed on the processor to enable the error control to function. It will be understood of course that the system 6 error control was formulated on the basis that the processor would be involved. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.5.5 Synchronisation =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= The transmit and receive terminals on a c.c.s. signalling channel must be in synchronism to enable transmitted information to be correctly received. It is not essential that the two signalling channels of a link be synchronised to each other and are usually not in telephony c.c.s. Four levels of synchronism are necessary in system 6: bit, unit, block and multiblock. The number of blocks (and multiblocks) in the error control loop at any one time will depend upon the signalling bit rate, the traffic load and the error control loop delay of the signalling link. (a) Initial synchronisation In analogue 6, bit synchronisation is provided by the data modem (modulator and demodulator) which uses a 4-phase, synchronous, differentially-coherent modulation method. The four phases each represent two bits of information (00, 01, 10, 11) called dibits. The modem then presents a continuous serial bit stream to the signalling equipment which then must identify units, blocks and multiblocks. In digital 6, modems not being required, bit synchronism is assured by the synchronised digital transmission system. The block completed and block acknowledgment counters are set to zero during the initial synchronisation procedure and are checked periodically using the multiblock monitoring procedure. On start-up of a link, each signalling terminal transmits blocks consisting of 11 SYUs and an ACU. The SYU (Fig. 11.5b) includes a 16-bit fixed pattern (for bit - analogue - and unit synchronisation) and a 4-bit field (for block synchronisation). The coding of the 4-bit field identifies the location of the SYU within the block (i.e. 1st, 2nd....or 11th unit). The particular 16-bit pattern of the SYU presents an easily recognisable pattern with a variation at the end to aid in its detection, and because the pattern provides six dibit transitions, to aid the attainment of bit synchronisation by the modems in analogue c.c.s. Bit synchronisation is maintained by the transition between dibits. The ACUs are transmitted initially with the 11 acknowledgment indicators set to 1, and the block-completed and block-acknowledged 3-bit sequence number fields set to 000. After bit-synchronisation has been established (in the demodulator of the modem for analogue c.c.s. and by the transmission system in digital c.c.s.) the incoming bit stream is monitored to find an SYU pattern. When found, and thus an SYU verified, its position within the block is known and the ACU posi- tion located. In due course, the ACU on the incoming channel should be cor- rectly received with its block number. At this time, the acknowledgment indi- cators in the next ACU sent back are set to reflect any detected errors in the SYUs of the associated receive block. Both block-sequence number fields in the ACU sent back remain at 000. The reception of at least two ACUs at the transmit terminal which check cor- rectly and acknowledge one (or more) SYU as having been received correct at the other end indicates that both terminals are in bit, unit and block syn- chronism. Block-sequence numbering is initiated by the block-completed counter, and the block-completed sequence number in the next outgoing ACU from the trans- mit terminal being set to 1. Thereafter the blockcompleted counter and the block-completed sequence number are incremented by 1 each time an ACU is transmitted. When the terminal receives an ACU having a block-acknowledged bit-field other than 000, the block-acknowledged counter at that terminal is set to this number received. Thereafter the counter is updated by the appropriate block- acknowledged number each time an ACU is received. When the block-acknowledged counter is advanced for the first time, the number of blocks in the error-control loop may be determined by subtracting the contents of the block-acknowledged counter from the contents of the block- completed counter. This gives the maximum number of blocks which may apply in the error-control loop of the signalling link concerned at the signalling bit- rate used. If the initial synchronisation procedure has indicated more than eight blocks in the error-control loop, the multiblock monitoring procedure is used once every cycle of the block-completed counter. In this case, the multi- block procedure is used for block synchronisation. On receipt of a multiblock acknowledgment signal, the multiblock and block numbers are compared With the contents of the block-acknowledgment counter. Multiblock synchronism is assumed to exist if the received number is within -4 to + 3 of the contents of the block-acknowledgment counter. (b) Resynchronisation Unit resynchronisation: Loss of unit synchronism results in continuous failure of SUs to check correctly, and when this occurs a signalling terminal takes unilateral action to resynchronise to the incoming bit stream. In any ACUs transmitted during this procedure, all the 11 acknowledgment-indicator bits are set to 1 and the block-acknowledged and block-completed numbers in- cremented as in normal operation. When synchronisation is re-established on the incoming channel, which is detected by SUs being checked correctly, the acknowledgment indicators are set according to the incoming SUs, i.e. normal operation is resumed. The SU error rate monitor continues to count SUs in error during this procedure, and changeover results if synchronisation is not re-established before the changeover criteria becomes operative. Block resynchronisation: Loss of block synchronism is recognised when (i) a valid SU, which is not an ACU, is received in the 12th position in a block (ii) an ACU is received in other than the 12th position in a block (iii) the block-completed number is not the one expected. On recognition of any one of the above, the terminal will stop sending message signals and send only SYUs and repeated ACUs, the latter meaning that the acknowledgment indicators and the block-acknowledged number from the previous blocks are repeated. When the terminal has identified the unit position in a block, either by recognising the SYU number or by identifying an ACU in the 12th position, and has also received an ACU which checks correctly, resynchronisation has been achieved. After successful block resynchronisation, the block being trans- mitted is completed with SYUs and an ACU. At least one complete block of 11 SYUs is sent before normal operation is resumed. The first ACU sent after resynchronisation has been achieved has: (i) the 11 acknowledgment indicators all set to 1 (ii) the block completed number set to the next in sequence (iii) the block acknowledged number corresponding to the latest ACU received After the completion of block resynchronisation, multiblock synchronism should be checked if the multiblock condition is applicable, i.e. more than eight blocks in the error-control loop. Multiblock resynchronisation: If the multiblock numbers in a multiblock ack- nowledgment SU are not within -4 to +3 of the contents of the block-acknow- ledged counter, a new multiblock monitoring signal is sent. If the result of the second measurement is not within the above limit, multiblock synchronism has been lost, and can be regained by updating the contents of the block- acknowledged counter to the obtained result. When the second multiblock monitoring signal is sent, the terminal will send only SYUs and ACUs for three blocks. Normal operation is then resumed and all messages transmitted in the interval between the two multiblock monitoring signals are retransmitted. If multiblock sychronism cannot be regained, changeover is initiated. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.5.6 System 6 analogue and digital versions =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= Signalling in the digital environment, either point-to-point p.c.m. or inte- grated digital-switching and transmission, can be achieved by the built-in, channel-associated, p.c.m. signalling technique (Sections 6.2 and 6.3). Such signalling is underutilised in the telephony service. Improved utilisation can be achieved by treating the signalling facility on speech-circuit group rather than on a per-p.c.m. system basis and c.c.s. is preferred when s.p.c. applies. The c.c.s. technique has potential for rationalised signalling for analogue and digital application and this concept is adopted. The basic features of the c.c.s. system apply on a common basis, the analogue and digital differences being limited to the signalling bit-rates. Ideally, the common signalling features should be optimum for both analogue and digital, but as this is difficult to achieve in all the feature areas, some penalty may well arise in either application. The pioneering CCITT system 6 was produced primarily for analogue applica- tion and its digital versions are far from optimum. The evolution of networks to integrated digital has shown the need for a new rationalised c.c.s. system optimised for digital rather than analogue, and this new system is currently being studied by the CCITT (Section 11.11). Derivation of signalling data-bit streams 2.4 kbit/s (analogue version) and 4 kbit/s and 56 kbit/s (digital versions) signalling bit-rates are presently specified for system 6. Optional 4.8 kbit/s analogue is presently being considered, The data-bit streams are derived from the modem (analogue) and from the multiplex (CCITT specified p.c.m. systems) for digital, as follows: (a) Analogue 2.4 kbit/s. A 2.4 kbit/s modem (modulator/demodulator) is used. The modulation technique uses phase-shift-keying to transmit serial binary data over the analogue signalling channel. The binary-data signal is encoded by first grouping it into bit pairs (dibits), each dibit being represented by one of four possible carrier phase shifts. Thus, the output from the phase modulator consists of phase-shifted carrier pulses at half the data bit-rate. The phase shift between two consecutive modulation elements contains the information to be transmitted, The principal requirements of the modem used for system 6 are: (i) use of differential 4-phase modulation (ii) use of differential coherent 4-phase demodulation (iii) full duplex operation over a 4-wire signalling link (iv) a modulation rate of 1200 bauds (v) a bit rate of 2.4 kbit/s. (b) 1.544 Mbit/s digital transmission 4 kbit/s signalling bit rate. As now specified, 24-channel p.c.m. systems (1.544 Mbit/s) do not permit a 64 kbit/s channel bearer for signalling purposes. Here, for signalling purposes, a channel is derived over which a stream of pulses is trans- mitted at 4kbit/s. The binary data from the signalling terminal is trans- ferred serially at a data transmission rate of 4kbit/s to the primary multiplex. Here, each bit of the data stream is successively inserted into the S-position (Section 6.2.3). In the receive direction, the primary multiplex extracts the bits from the S-position and transfers them serially to the signalling terminal. (c) 2.048 Mbit/s digital transmission. A 64 kbit/s channel bearer is available for signalling in 2.048 Mbit/s 30-channel p.c.m. systems (Section 6.3) and signalling bit-rates of 4 kbit/s or 56 kbit/s may apply. A digital inter- face-adapter is provided between the multiplex and the signalling terminal. 4 kbit/s signalling bit rate: The binary data stream from the signalling terminal is transferred serially to the interface-adapter. Here, the 4 kbit/s data stream is modulated on a 64 kbit/s bearer channel such that 16 bits of bearer channel correspond to one bit of the 4 kbit/s channel. The 64 kbit/s data stream is transferred serially to the 2.048 Mbit/s primary multiplex in alignment with an 8kHz clock (octet timing). At the primary multiplex, the 16 bits corresponding to one signalling information bit are inserted into the designated channel time slot of two successive frames. In the receive direction, the primary multiplex extracts the bits from the designated time slot and transfers them serially at 64 kbit/s to the inter- face-adapter. Here, the 16 bits corresponding to one signalling information bit are extracted after detection and the binary data is transferred serially to the signalling terminal at 4 kbit/s. 56 kbit/s signalling bit rate: The system 6 signal unit is 28 bits. As this is not a multiple of the 8bit octet it has weaknesses in digital transmission. Four bits are added to the signal unit to increase its size from 28 to 32 bits, for the reasons of: (a) octet timing The signal unit and padding bits together occupy exactly 4 octets. This simplifies the synchronisation and resynchronisation proce- dures. (b) slip detection: With appropriate coding, the four padding bits may be used to detect octet slips. No signalling information is carried by the four padding bits. Thus with the system 6 28-bit signal unit, the signalling bit-rate is 56 kbit/s on the 64 kbit/s bearer. The bit pattern for the padding bits is 1010...1010, which allows for detection of octet slips. As the pattern and location of the padding bits interfere with check-encoding, the padding bits are added after encoding and removed prior to decoding. The binary data from the signalling terminal is transferred serially to the interface-adapter. Here, the 28 bits of the signal unit are placed in bit positions 1-7 of four 8-bit bytes. These four bytes are transferred serially at the data transmission rate of 64 kbit/s to the 2.048 Mbit/s primary multi- plex, in alignment with an 8kHz clock (byte timing). At the primary multiplex, the four bytes are inserted into the designated time slot of four successive frames. In the receive direction, the primary multiplex extracts the bits from the designated channel-time slot and transfers them serially at the data trans- mission rate of 64 kbit/s to the interface-adapter, in alignment with an 8 kHZ clock. In the interface-adapter, the bits 1-7 of each 8-bit byte are transferred serially to the signalling terminal at the 56 kbit/s rate. The padding bits do not apply for the 4 kbit/s digital version. They would not be necessary in signalling systems having a signal unit size a multiple of the 8-bit octet. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.6 Common channel signalling loading =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= With c.c.s., the signalling is time shared and a speech circuit does not have exclusive use of a signalling facility, thus a queue is built up from which signals are transmitted in order of their time of arrival and of their priority. Thus the loading in terms of number of speech circuits served per c.c.s. link must reflect acceptable queueing delays. This loading will be influenced by a number of factors, in particular: (a) the number of busy hour calls per speech circuit (b) the number of signals involved per call (c) the mix of LSUs and MUMs; the greater the number of MUMs for a given total signal information transfer per call, the less the signalling load (d) the signalling bit rate: the faster the bit rate, the less the signal emission time and the greater the permissible number of speech circuits served. (e) the error-control method: typically, a noncompelled error control method permits more speech circuits served per c.c.s. link than a compelled method, other factors being equal. It is thus not practicable to specify a general maximum limit of speech circuits served per c.c.s. link. An adopted limit must take account of the various conditions applying so that the total signalling load per link is held to a level which will maintain an acceptable queueing delay. It is generally accepted that the signalling load per link should not exceed of the order 0.4 Erlang in normal operation. This allows temporary signalling overload in normal operation and for increased load in abnommal conditions such as security backup for a failed regular signalling link, when the load of the failed link is transferred to the backup link, the queueing delay increasing in the abnormal condition. It is also generally accepted that the maximum number of circuits served per c.c.s. link adopted in practice for a particular network should reflect the probable size of the speech circuit groups applying, and the signalling network security philosophy adopted, for that network. Typically, one admi- nistration may adopt a maximum loading of, say, 1500 circuits per c.c.s. link in normal operation, whereas another may wish to adopt a smaller number, say 500 circuits, which reflects the different network conditions. Indications of the maximum number of speech circuits served per CCITT system 6 c.c.s. link in the telephony service with acceptable queueing delays, assu- ming 15 busy hour calls per speech circuit, 0.4 Erlang signalling link loading, and representative conditions, are: 2.4 kbit/s signalling bit rate 1500 circuits served 4 kbit/s 2500 56 kbit/s 20000 It is stressed that these values must be regarded with some reserve in view of the various assumptions adopted, nevertheless the broad indication is reasonable and is of interest. The values for the new CCITT optimised digital c.c.s. system with signalling bit rates 2.4, 4 and 64 kbit/s (Section 11.11) will be different as much will depend upon the finally resolved method of transferring the signal information - by messages of variable bit length or by fixed bit length SUs. In either event however, there are indications that the number of signalling bits transferred per telephone call will be somewhat greater than in system 6, and with noncompelled error control the maximum number of speech circuits served per c.c.s. link may well be somewhat less than for system 6 at the same signalling bit rate and for the same assumed conditions. Nevertheless, at 64 kbit/s, it is thought that some 20000 speech circuits would apply. Compelled error control, should this apply, will signi- ficantly reduce the values relative to non compelled. While in some circumstances some 2500 or 20000 speech circuits could be served per c.c.s. link in the telephony service in normal operation, it is extremely unlikely, particularly for signalling network security reasons, that such high values would be adopted in practice. It is thought that a more realistic maximum adopted would be of the order 1500 circuits in normal operation. This implies that a c.c.s. link would be very lightly loaded in the telephony service in normal operation at the higher signalling bit rates 56 kbit/s and 64 kbit/s. The maximum number of circuits served per c.c.s. link for the switched circuit data service would very likely be significantly less than for the telephony service in view of the shorter call durations. Circuit label capacity The size of the circuit label bit field should allow for greater possibili- ties than the requirement for the maximum number of circuits served per c.c.s. link in normal operation. This is to allow for the increased load in the ab- normal condition when a link functions as security backup for a failed link. The CCITT specification for system 6 provides for an 11-bit (7-bit band, 4-bit circuit number) circuit label bit field (potential to identify 2048 speech circuits). This may be varied in particular national applications of the system, typically, the Bell system national version of system 6 proposes a 13-bit (9-bit band, 4-bit circuit number) circuit label field (potential to identify 8192 circuits), which reflects the large size speech circuit groups applying in the N. American network. It is considered advisable that the circuit label bit field be of size giving flexibility to cater for possible future requirements not yet defined. Thus the potential for a larger field than that provided by the CCITT speci- fied system 6 is preferred, which approach is proposed for the new optimised digital c.c.s. system (Section 11.11). A national network not requiring the large number would have possibility to use spare circuit label bits for other suitable national purposes. The circuit label bit field is divided into band number bits and circuit number bits. The band number identifies a group of circuits (typically a p.c.m. system), the circuit number identifies a particular circuit in a group. The banding concept minimises the memory required at a signal transfer point (s.t.p.) in the quasi- and dissociated-c.c.s. signalling modes. The s.t.p. processor translates on only the band number bit field and not on the complete circuit label field, as signalling routing only is required at s.t.ps. A dis- advantage of banding is the loss of some label capacity since all speech circuit group sizes are not an integer multiple of band size. Usually the circuit number field is 4 bits, which identifies a circuit within a group (band) of up to 16 circuits. Should spare bits from a large circuit label field be used for other national purposes, these should be spared from the band, and not from the circuit number, bit-field. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.7 Signalling link security and load sharing =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= Various security and load-sharing arrangements are possible within a parti- cular c.c.s. system specification and in national application would be matters of individual administration's choice. Bilateral agreement on particular arrangements may apply in international application. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.7.1 Security =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= The basic concept of securing a signalling route is to supplement its regu- lar signalling facility with a readily accessible alternative brought in by automatic procedures. This requires the provision of at least two signalling facilities, termed 'regular' and 'backup' respectively for convenience. The backup may be: (a) A standby nonsynchronised link switched into service when required. (e.g. a speech circuit nominated as the signalling backup). (b) A synchronised signalling link in the same signalling relationship (e.g. regular and backup signalling links on an associated signalling route). (c) A separate signalling relationship (e.g. quasi-associated signalling as a backup for associated signalling). There are other possibilities. Further signalling security can be achieved by fabricating a backup under the control of network management. As this would not be an automatic proce- dure, it would not be a function of the signalling system. It is clearly preferable that the backup link(s) should have immediate signalling capability on failure of the regular link. Thus while (a) is a valid approach, the procedures required to prove the signal carrying capabi- lity of the backup takes time and adds to the complexity. Approaches (b) and (c) are preferred. Inbuilt associated signalling security: This implies that an associated- signalling route has inbuilt features to achieve, by automatic procedures, the degree of signalling security required. The concept requires the provision of at least two signalling links in that associated signalling relationship, preferably diversely routed. The provision of more than one signalling link to achieve the security can be justified on the relative cost insensitivity of providing additional link(s) due to: (i) the inherent 'common equipment' feature of c.c.s. (ii) the application of such signalling to concentrated traffic (iii) in the digital environment, the low cost of signalling time slots in digital transmission systems due to the cost sharing in multiplex trans- mission. Associated signalling module security: There is an interplay between the maximum number of speech circuits to be served per c.c s. link in normal ope- ration and the size of the circuit label bit field to determine the number of signalling links which may apply in an associated signalling relationship. The term 'signalling module' is adopted in this regard. Typically, with a 6000 circuit label capacity, the maximum size signalling module would be four links for a maximum loading of 1500 speech circuits per c.c.s. link, and six links for 1000 circuits per link. In practice, not all modules in a network would be of maximum size, they would be of size dependent upon the sizes of the speech circuit groups served. Many associated signalling routes in many networks would consist of two signalling links only (1 + 1), which could be smaller than the maximum size signal module. The larger speech circuit groups would require more than two signalling links and the very large groups would require more than one signalling module. Various security approaches may apply with the signalling module concept, but the following are thought logical: (a) Any one signalling link in the module should be capable of being a backup for any other links in the module. This precludes a particular link being the single nominated backup link. (b) When the module is bigger than 1 + 1, the potential to allow for more than one failed link in the module should be admitted in the automatic security procedures. In theory, failure could be allowed to the limit of one sig- nalling link carrying the full load of the module, which approach could result in significant increase in queueing delay. In practice, an admini- stration may wish to limit the extent of permitted failure within a module to be covered by the automatic security procedures. In a two-link module of course, one link must carry the total signalling load on failure of one link. (c) When a number of signalling modules are provided on a signalling route, each module should be autonomous, being self-contained in the sense that no traffic assigned to a module would ever be handled by another module. Each module would be inbuilt secured and would not rely on another module for signalling security. The circuit label bit field of the international system 6 is of modest size (11 bits, 2048 circuits). This is mainly suitable for a 1 + 1 signalling concept and would restrict the module size in a module concept. If necessary of course, national variants of the international system 6 could arrange for additional circuit label bit(s) within the 6 SU size, as instanced by the 13-bit circuit label field of the Bell system version of system 6 for application in N. America. Quasiassociated signalling security: A number of possibilities arise, typically (a) Assuming the quasiassociated signalling route to have associated signal- ling on the relevant constituent routes (Fig. 11.2b), and the associated signalling modules to have inbuilt security, the quasiassociated signal- ling route is then securcd in the inbuilt concept without the necessity for further arrangements. (b) Quasiassociated signalling without associated signalling on the consti- tuent routes (Fig. 11.2c). Assuming quasiassociated signalling to be the principal signalling mode in a network, situations arise where it would not be of the type having associated signalling on the constituent routes of the quasisignalling relationship. A signalling module concept could apply for quasiassociated signalling of this type. The module size would be determined by the same considerations as apply for an associated signalling module, the module would have inbuilt security and all the considerations postulated for the associated signalling module would apply. The smallest quasisignalling module would be two links 1 + 1. Unlike the associated signalling module case, s.t.p. security considerations arise in quasiassociated signalling of the type being considered and the s.t.ps. may or may not be duplicated depending upon an administration's policy. If duplicated, the operation of the quasiassociated signalling module would need to take account of the duplicated s.t.ps., but the basic philosophy, and the basic working arrangements of the quasiassociated signal- ling module would not be disturbed with duplicated s.t.ps. An example of this type module is the proposed Bell system quasiassociated signalling in the N. American network (Fig. 11.8). Here it is proposed that the complete network comprises a number of signalling regions, quasiassociated signalling being applied between the regions and thus in an organised manner on a complete network basis, as distinct from quasiassociated signalling applied on parti- cular signalling route situations and thus not as the principal mode for the network. Quasiassociated signalling is proposed as the main c.c.s. mode in the N. American network. In some situations, associated signalling may apply on particular speech circuit groups and when applied, the basic capacity of an associated signalling link is 1500 speech circuits maximum in normal operation at the 2.4 kbit/s signalling. Under fault conditions, an associated signalling link may carry twice this load, but with an increase in signalling delay. This same capacity, 1500 speech circuits maximum, applies to the quasi- associated A-links (Fig. 11.8). A toll office of 3000 speech circuits or less would connect with the quasiassociated signalling network by providing a single pair of A-links, one link to each regional s.t.p. An additional A-link pair would be provided for each increment of 3000 speech circuits terminated in the toll office. Each A-link pair functions as an autonomous pair, having no operational interaction (such as failure backup) with other A-link pairs. The interregional quasisignalling quad (four-signalling link module fully inbuilt secured) forms a functional unit serving up to 6000 speech circuits between a pair of signalling regions. Additional autonomous quads would be provided for each increment of 6000 interregional speech circuits. As mentioned, this arrangement requires a 13-bit circuit label field (9 bit band, 4-bit circuit number). Exploitation of c.c.s. modes for signalling security: Signalling security can be achieved by employing a backup signalling mode of a different type, or another mode of the same type, from the regular signalling mode. Both inbuilt security and regular modes secured by backup modes can be applied in the one network. The following are possibilities: (a) When it is not possible, or not economic, to inbuilt secure an associated signalling route (i.e. when a single associated signalling link applies), such a route could rely on a quasiassociated signalling backup for its security. (b) A quasiassociated signalling backup for a regular quasiassociated signal- ling route, the associated signalling constituent routes of the regular route not being inbuilt secured. ------------------------[See fig11-8.pcx in tadxf005]------------------------ Fig. 11.8 Bell system c.c.s network configuration ----------------------------------------------------------------------------- (a) is liable to arise in practice. (b) introduces circuit labelling problems, and routing complexities could arise should the quasiassociated backup be a regular quasiassociated signalling route in anothcr relationship, with possible further quasiassociated backup. While not preferred, (b) could be applied under strictly controlled conditions within the circuit labelling capacity applying. It is thought that the associated and quasiassociated signalling modes would be suitable for most national networks. Secured quasiassociated signalling could be applied in particular situations where associated signalling is not justified, or as in (a) above. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.7.2 Load sharing =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= The technique of distributing the total module signalling traffic over the individual links is termed 'load sharing', it being understood that all the links, regular and backup, carry signalling traffic in normal operation. There are two main categories of load-sharing mode: (i) random mode This may be either: (a) messages distributed randomly on the links of the module (b) all messages for a call transferred over the same signalling link, which link is randomly selected from the links in the module. (ii) Predetermined mode This may be either (c) predetermined on a call basis. There are a number of variants of this basic approach, but all aim in principle that all messages for a call be transferred over the same signalling link. There is little dif- ference between this and (b) above. (d) predetermined on a speech circuit basis. All the speech circuits served by a module are distributed over all the signalling links in the module on a predetermined basis in normal operation. The circuit labels of a number of speech circuits are allocated to a particular link, and all messages for all calls carried by the allocated speech circuits are transferred over the same signalling link. With (a), SUs or messages may get out of correct sequence on message transfer. With (b)-(d), all messages for a particular call are transferred over the same signalling link, which has potential to avoid out of sequence due to the load-sharing technique. SUs or messages may get out of correct sequence with certain error-control methods (e.g. system 6). With such methods, there is no compelling reason to adopt load-sharing techniques which aim to avoid out of sequence. Thus any one of (a)-(d) could be applied. With error control methods purposely designed to avoid out of sequence on message transfer, which is preferred, it would be illogical to adopt load-sharing technique (a), and here it is considered that the choice rests between (b) and (d). Technique (b) has merits: (i) The warking links in a module would automatically take the load on signalling link(s) failure. Similarly when failed link(s) are restored to service. (ii) Would automatically achieve the limit of one signalling link taking the whole load of a module. (iii) Avoids a predetermined nominated link for security. (iv) Meets the preferred principles of module signalling link security as discussed without requiring procedures beyond busying out failed link(s) and automatically diverting traffic from these link(s). (v) Avoids a set program to allocate speech circuits to signalling links on a predetermined basis, which also avoids updating the allocation when speech circuit groups are extended. (vi) Has the possibility of equalised distribution of signalling load on the working links in a module on signalling link(s) failure. On the other hand (b): (i) Necessitates an arrangement to ensure that all messages for a call are transferred over a randomly selected signalling link. This could be some- what complex with certain approaches to processor system architecture. (ii) On signalling link failure, requires a link to be nominated for the retrieval procedure if messages are to maintain correct sequence, with consequent complexity. For practical arrangements, it is often preferred to avoid nominated retrieval links, and retrieval on the link changed over to is considered preferable. The features and merits of(d) are: (i) Changeover to another link in a module on signalling link failure. The circuit labels of a failed link are transferred in total to another link (or alternatively, distributed over the remaining working links). (ii) On restoration of a failed link back to service, all the circuit labels allocated to the failed link prior to the failure are transferred back. (iii) Would automatically achieve, in potential, the limit of one signalling link carrying the whole module load on multilink failure. (iv) Avoids requiring a nominated link for retrieval. (v) Meets the principles of module signalling link security as discussed under the control of the changeover and changeback procedures. (vi) Avoids any further arrangement in the processor to ensure that all messages for a call are transferred over the same signalling link. This feature is assured by the predetermined allocation of circuits labels to signalling links, which arrangement may be more capable of accom- modating various approaches to processor function system architecture. On the other hand (d): (i) Necessitates an administrative procedure to allocate speech circuits to particular signalling links and to update the allocation on speech circuit group extension. (ii) Compared with random selection, could result in a signalling link in a module carrying a significantly heavier load than others on signalling link(s) failure when all the circuit labels of a failed link are trans- ferred to one working link. Depending upon the signalling load in normal operation, this could result in queueing delay problems in the abnormal condition of signalling link(s) failure, particularly at slow signalling bit rates. On balanced assessment, there is probably little between load sharing techniques (b) and (d) and either could be applied when it is required to avoid out of sequence, which is preferred. Should simple arrangements be the main consideration, it is thought that this would be achieved by (d) in combination with the transfer of all the circuit labels of a failed link to one backup link. This could result in a significant increase in queueing delay in certain circumstances with certain size signalling modules depending upon the extent of multilink failure in a module permitted by the automatic security procedures. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.8 Changeover, retrieval and changeback =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= The changeover, retrieval and changeback procedures consequent on signalling link failure are detailed and the principle only will be discussed. At this time, the procedures for international system 6 are the only ones available. These could form the basis for procedures required for any future c.c.s. system(s). Administrations are free to adopt the CCITT international proce- dures for national application of international c.c.s. system(s), or, if desired, could adopt procedures more suited to the conditions of individual national networks, which approach would not disturb the basic specification of international c.c.s system(s). Changeover Signalling traffic is diverted from a regular link to an alternative signal- ling backup when the regular link is recognised as having failed. Failure may be caused by (Section 11.5.3): (a) loss ot the analogue data carrier or loss of the digital frame alignment (b) continuous failure of SUs (or messages) to check correctly (c) unacceptable intermittent failure of SUs, or messages, to check correctly (d) loss of relevant synchronism for a certain duration. Each signalling terminal in c.c.s. is equipped with appropriate monitoring equipment to recognise failure due to any one of the above causes. In system 6, the error rate monitor initiates changeover when: (i) X consecutive SUs are received in error for 350ms, or (ii) 2% of SUs are received in error out of Y SUs received. X and Y are assessed as follows for the telephony service: Signalling bit rate Number of SUs (kbit/s) X Y ----------------------------------------------------------- 2.4 31+/-1 2500 4 50 4200 56 700 60000 64 800 68500 On detection of failure, each terminal starts sending 'faulty link infor- mation' of appropriate form on the link just failed. Typically, in system 6, this could consist of alternate blocks of 11 changeover and 11 SYU signals plus ACU. If the terminal has lost synchronism, the normal synchronising procedure is started. Where applicable, the synchronising procedure on the backup signalling link is initiated. retrieval Messages and acknowledgments may be lost in the transfer process on signal- ling link failure. Thus a terminal would not be aware whether or not all or some of the messages in the retransmission store had been received correct at the receive terminal. Retrieval is the process of recovering from this situa- tion to avoid loss of messages on changeover. For simplicity, and also to avoid out of sequence messages, retrieval should be performed over one link, and for further simplicity, preferably over the backup link. There are a number of retrieval possibilities, typically: (a) Retransmission of all the contents of the retransmission store. This could result in duplicated messages, as arises in system 6. (b) Retransmission only of messages known to have been received corrupted. This requires the receive terminal to inform the transmit terminal, by backward signalling over the retrieval link, as to the last message received correct over the failed link should cyclic retransmission be used (not in system 6). (c) With SU (or message) sequence numbering, the retrieval retransmission carrying its original sequence numbering, (a) or (b) could apply and retrieval start and stop indicators would be necessary if retrieval is on a working link due to the differences in number sequences on the failed and backup links. (d) With sequence numbering, the retrieval retransmission carrying the sequence numbering of the working backup link over which retrieval is performed. Assuming SU (or message) sequence numbering, (b) would apply and retrieval start and stop indicators would not be essential if retrieval is on a message, LSU, or MUM basis. A start retrieval indicator could be of value if retrieval is on an SU basis to cover the situation of a partial MUM having been received correct over the failed link. There are other possibilities. The retrieval method adopted would depend upon preferences and circumstances. Changeback Once the backup facility has been taken into service, the regular signalling link should not be brought back into service for signalling traffic until it has been checked to give satisfactory performance for a period of about one minute. The proving period begins when either terminal has regained synchro- nism on the failed link. End-of-failure monitoring equipment is provided at each terminal. In system 6, the failed link is not restored to service until a SU error rate of 0.2% or less has been achieved in the proving period. In the telephony service, the end-of-failure is assessed to have been achieved when not more than: 10 SUs at 2.4 kbit/s 16 SUs at 4 kbit/s 240 SUs at 56 kbit/s 266 SUs at 64 kbit/s are received in error in the one minute proving period. On end-of-failure recognised, a terminal will cease sending faulty link information. In system 6: (a) The faulty link information is replaced by continuous blocks of SYUs (plus ACUs). (b) To return to the regular link, an exchange, say A, initiating the change- back sends two load transfer signals on the regular link. Exchange B responds with a load transfer acknowledgment signal on the regular link and transfers signalling traffic from the backup to the regular link. Receipt of one load transfer acknowledgment causes A to transfer signal- ling traffic from the backup to the regular link. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.9 Continuity check of the speech path =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= Unlike speech path signalling, c.c.s. does not monitor the speech path and other arrangements must be made to assure speech path continuity (line plus switching), otherwise there could be possibility of a connection being set up on the signalling path but without the speech transmission facility. Speech path continuity assurance may be by: (a) a per-call continuity check, or alternatively (b) a statistical method of routine testing of idle transmission circuits and idle switching paths (and no per-call continuity check). The per-call continuity check is made prior to the conversation. It may be loop link-by-link or end-to-end and consists of the transmission and detec- tion of receipt of an audio frequency on the speech path. Typically, in system 6, the per-call check is line loop link-by-link with a 2000Hz check tone frequency. A separate cross-office check is made. To admit the possibility of 2-wire circuits, which may arise in national networks, the per-call check could consist of different forward and backward check frequencies. On 'check not OK' a repeat attempt is made to set up the call on an alter- native speech path. To cater for intermittent faults, the original speech path is rechecked and passed to maintenance attention if still found to be faulty. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.10 Signal priority =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= Transmission (queueing) delays can arise with c.c.s. As c.c.s. may have a considerable signal transfer requirement, embracing a number of different services and signals within the different service categories, delay to time sensitive signals or to time sensitive service categories could give rise to system problems and complexity. For this reason, some signals and some categories may be given priority over others for transfer purposes and the processor function is so programmed. The priority program will depend upon the characteristics of the c.c.s. system, typically, with representative loadings a high speed 64 kbit/s c.c.s. system would transfer information at a faster rate and the queueing delay would tend to be less, relative to a slower speed system, and thus the extent of the priority requirement would tend to be less. As the arrangements for signal priority complicate c.c.s., it is logical that all efforts be made to minimise the priority requirement. The following priority rules apply for international system 6: (a) ACUs (12th position signal unit of each block) have absolute priority for emission at their fixed predetermined position. (b) Faulty link information has priority over all signals except ACUs. (c) The answer signal has priority over other waiting telephone signals and signalling system control signals except those in (a) and (b). (d) All other telephone signals (LSUs and MUMs) and all other system control signals, except synchronisation signal units, have priority over manage- ment or other signals concerned with the bulk handling of information. (e) Any signal which is to be retransmitted will take precedence over other waiting signals in the same priority category. (f) Management signals have priority over synchronisation signal units. (g) Synchronisation signal units have no priority. It is the intention to minimise the priority requirement in the new opti- mised digital 64 kbit/s c.c.s. system. Typically, it could be reasoned with some logic that due to the high speed of information transfer, there would be no need to give priority to the answer signal. In slower systems, answer signal priority has been considered desirable to avoid speech clipping due to speech transmission path line splits on interworking v.f. signalling systems. Signal priority may be associated with the 'break-in' feature, which allows a priority message to break into the emission of a lower priority MUM to avoid delay in the emission of the higher priority message. The following break-in rules apply for international system 6: (a) Potential for a priority LSU to break into a MUM is provided for in the specification of the system, but initially this will not apply except for ACUs. (b) All telephone signals to have potential to break into a network management MUM or other MUMs concerned with the handling of bulk information. The break-in feature gives rise to significant complication in c.c.s. systems and it is not proposed to adopt it for the new optimised 64 kbit/s digital c.c.s. system. To facilitate this, the maximum bit length of messages in the new system may well require to be limited. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.11 CCITT No. 7 optimised digital common channel signalling system =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.11.1 Basic concepts =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= The pioneer CCITT 6 c.c.s. system was produced primarily for analogue appli- cation, the digital versions included in the specification being produced by subsequent study, but retaining the basic features of the analogue system. In the result, system 6 has limitations in the digital environment. In the back- ground of experience gained in the production of system 6, and with recogni- tion of the evolution of many networks from analogue to digital (integrated switching and transmission), the CCITT is presently engaged on the specifi- cation of a new c.c.s. system. This system will be optimised for the 64 kbit/s digital environment, will include other digital signalling bit rate(s), and analogue bit rate(s). Particular regard is being paid to national network requirements, and as it is aimed to produce a simpler and more flexible system than 6, a main desire for national application, there is little doubt that many administrations, programming c.c.s. application, will adopt this new system for their national networks in the s.p.c. digital (or analogue) environment. This CCITT study is not yet concluded, but it is clear from the approaches emerging that many features of the new system will be different from those of system 6, which, in part, reflects further knowledge in the c.c.s. art since system 6 was specified. The c.c.s. technique has significant potential for signalling rationalisa- tion and an objective of the new system is to use the same basic system for international, regional and national for a variety of services, digital and analogue. This would allow the potential, from the signalling aspect, for dedicated digital service networks (telephony, data, etc.) to evolve to a single multipurpose network, should this ever be a future requirement. While the new system will be optimised for digital, there is little doubt that it will be a superior system to 6 for analogue, as well as for digital. UK pioneered much of the new system (CCITT No. 7) and has adopted it for its System X digital network. Rationalisation implies a flexible signalling system and it is proposed that the new system be based on a number of concepts to achieve this (Fig. 11.9). These concepts cater for various services and applications without mutual interaction, and give potential to cater for future requirements not yet defined. Commonality concept: Options incorporated in the same signalling system would permit reasonably optimum arrangements for different services and appli- cations (international, regional and national) of the same service. A single c.c.s. system for various services within a particular network could also be realised by appropriate choice of options incorporated. Desired simplicity of national network c.c.s. is an example of the merit of the option approach, as here advantage could be taken of national network conditions, not present in, say, international, which could contribute to signalling simplicity. Typical options concern the signalling bit rate, the error control method, and message bit length, formats and codings for different services. Functional concept: A functional concept embracing processor function, user function, and message transfer function, allows change to any one function without significant impact on the others. It involves, for example, flexible arrangements in the message transfer function to enable an error control option to be exercised without involving the processor. This implies that the error control should be associated with the signalling terminals, the processor never being involved. User subsystem concept: This is part of the functional concept. User sub- systems (separated parts of the processor) are functional parts which control the message requirements for particular services, typically, telephony call processing, data call processing, network management, network maintenance, centralised call accounting, etc. Such subsystems (Fig. 11.9) may be regarded as being the service dependent parts of the signalling system. They define message, signalling and call control procedures, and the function and coding of the signalling information for different services. Each user subsystem module deals with a particular service, and the concept allows the system to evolve in that additional services may be catered for by the addition of appropriate subsystem modules. The message transfer function (the c.c.s. system) allows user subsystem modules held on different processors to communicate with each other by means of addressed messages. This processor subsystem addressing (type of service) directs the message to the appropriate subsystem module, the subsystem addressing identifying the service for which the message is intended. Should corresponding user subsystems communicate with each other for the interchange of information not related to call handling, such messages will be conveyed by the message transfer function. ------------------------[See fig11-9.pcx in tadxf005]------------------------ fig 11.9 Generalised functional arrangements common channel signalling A = Telephony call processing ---| B = Data call processing | C = Viewphone |-> Typical User Subsystems D = Network Management | E = Network Maintenance | F = Call Accounting ---| ---------------------------------------------------------------------------- =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.11.2 Signalling bit-rate =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= A 64 kbit/s bearer is conveniently available for c.c.s. in 30-channel digi- tal transmission, and with a message bit length a multiple of the 8-bit octet, the signalling bit rate can be this bearer bit rate. There is no reason to depart from this rate for telephony c.c.s., indeed, at this rate the message signalling activity will be very light with the speech circuits per c.c.s. link loading likely to apply. Commonality c.c.s. for data services introduces further considerations as such services wish a faster connection set-up time than that usually acceptable for telephony. Relative to telephony, the data service may require a greater signalling load in terms of calls per second handled per c.c.s. link, which could slow data call connection set-up. Study concluded however that signalling bit rates greater than 64 kbit/s would not significantly reduce queueing delay, nor connection set-up time in the data service, and the new c.c.s. system will be optimised for 64 kbit/s. Optional signalling rates 4 kbit/s (for 24-channel digital) and 24 kbit/s (for analogue) will be used. Other rates could apply as appropriate (typically optional 4.8 kbit/s analogue). =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.11.3 Error control =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= The combination of the error control method and the various consequential procedures constitute perhaps the most complex part of c.c.s., and as a simple system was desired, the error-control problem was reassessed. Forward error correction: Here, the receive end performs both error detection and correction, and error correcting codes ('Hamming' codes) used, sufficient check bits being required to enable the original message to be reconstituted on error detected. Retransmission of messages does not arise, and as the method has some potential for simple arrangements, the following analysis is made in regard to c.c.s.: (a) It is independent of the propagation time of signalling links. (b) The procedure is simple; acknowledgment signalling and sequence numbering of messages not being required. (c) Messages cannot get out of correct sequence, nor be duplicated. (d) Error correction is reasonably rapid relative to correction by retrans- mission, which has merit on long propagation time signalling links. (e) No retransmission store required. (f) The automatic changeover procedure is simple and retrieval not required. This, however, is valid only when the possibility of loss of messages is acceptable when diverting signal traffic to an alternative signalling facility. On the other hand, forward error correction: (g) Requires far more redundancy in check bits relative to that required for error detection only (or for a given number of check bits it is possible to detect more errors than it is possible to correct). The coding and decoding systems for the check bits are relatively complex. (h) Cannot request retransmission of a message which has been error detected but not corrected. (j) Complicates the receiving terminal. The following points are made in the consideration of the above: (i) As c.c.s. is error-free for most of the time, penalties (g) and (j) in particular are not justified. (ii) Error-free messages are slowed. (iii) In any c.c.s. system there is the need to safeguard against major break- down of a signalling link. Correction by retransmission has a signi- ficant merit relative to forward error correction in that it has an inherent ability to direct signalling traffic to an altemative signal- ling facility without loss of signal information. (iv) With reasonable arrangements, correction by retransmission is superior to forward error correction in both throughput and undetected error rate. It was concluded that forward error correction would have limitations in c.c.s. application and that correction by retransmission be adopted for the new c.c.s. system. This supported a previous conclusion in regard to system 6, but not the system 6 errorcontrol method, which would not be suitable for the various reasons discussed in Section 11.5.4. Guidelines for a preferred error-control method: From the analysis Section 11.5.4, it is considered that the main guidelines are: (a) Messages should not get out of correct sequence in any working situations of the error control. (b) Duplicated messages should not arise, but if they do they should be detec- ted and dealt with by simple means at the signalling terminal, the processor function never being involved. (c) Unnecessary and unrequested retransmissions should not arise. (d) The processor function should not be involved in the error control function. Guideline (d) requires all messages passed from the message transfer function to the processor function to be valid in all respects, except for errors undetected by the error control. Ideally, to achieve this, the error control method itself should be such that neither out of sequence nor dupli- cated messages occur, but this may not be conveniently realisable. It would be illogical to require a signalling terminal to detect and correct for messages in incorrect sequence (a) as this would necessitate a measure of reasonableness checking and correction logic at the signalling terminal, to complicate the system. It is thought that the errorcontrol method itself should be such that out of sequence messages do not occur. With sequence numbering (or the equivalent) of messages, it would be a simple matter for a signalling terminal to detect for duplicated messages (b) and to discard messages already received. Thus, if necessary, duplicated messages could be admitted on the message transfer function. A philosophy of 'ignoring corruptions' in the error control could eliminate both unnecessary and unrequested retransmissions (c). Here, no action is taken on error detected, the system merely awaits the receipt of the next error free SU (or message) before taking decision as to the appropriate action to be taken. With this philosophy: (a) corrupted SYUs (and corrupted messages which would have been unacceptable if error free) would not cause unnecessary retransmissions. (b) to eliminate unrequested retransmissions, the acknowledgment indications are required to be in a continuous stream. An error-control method based on the above philosophy precludes adoption of the system 6 error-control method because of the block transmission and the single acknowledgment (ACU). 'Ignore corruptions' error-control method: The error-control method for the new CCITT No. 7 c.c.s. system is not yet finalised by the CCITT, but present indications are that it will be based on the ignore corruptions philosophy. The following is a possible realisation of the philosophy (Fig. 11.10), is presented to demonstrate the principle, and should not be regarded as being the final agreement in the detail. Basic features are: (a) Cyclic retransmission on error detected, meaning that the corrupted message SU (or message) and all the message SUs (or messages) following it in the retransmission store are retransmitted, Corrupted SYUs are not corrected and do not cause retransmission. (b) Each SU (or message) contains both a forward sequence number (FSN) and a backward sequence number (BSN). (c) SYUs transmitted between message SUs (or messages) all carry the same forward sequence number of the next message SU (or message) to be trans- mitted. (d) Each SU (or message) contains a forward indicator bit (FIB) and a backward indicator bit (BIB), used to control retransmissions. The philosophy may be applied with message SUs (LSUs and constituent SUs of MUMs) of fixed bit length or with messages of variable bit length. The FSN, BSN, FIB, BIB and check bits are 'housekeeping' bits, concerned with the con- trol of message transfer, and are not passed to the processor. Drift problems do not arise. ------------------------[See fig11-10.pcx in tadxf005]----------------------- Fig. 11.10 Typical realisation ignore corruptions error control ----------------------------------------------------------------------------- FSN: Message SUs (or messages) are numbered sequentially to enable the correct sequence to be preserved during the message-transfer process, and to identify message SUs (or messages) when retransmission is required. The range of sequence numbers must be such that a retransmission store never contains two message SUs (or messages) bearing the same FSN and reflects the maximum number of message SUs (or messages) liable to exist in the error-control loop. BSN. The BSN contained in a unit (or message) transmitted from a terminal indicates the FSN of the next message SU (or message) that terminal is pre- pared to accept. If a terminal has just accepted a message SU (or message) with a FSN of n, the next unit (or message) to be transmitted from that ter- minal will have a BSN of n + 1. When accompanied by a retransmission request (change in polarity of the BIB), the BSN indicates the point in the transmit- ted sequence at which the retransmission should start. BIB: Is used by a receive terminal to signal back to the transmit terminal that a retransmission is required, the request being made by reversing the polarity of the next BIB to be transmitted. Once a receive terminal has requested a retransmission, no further reversals of polarity are made until a new retransmission is required. No action is taken by a terminal when a received unit (or message) is corrupted. The system waits (ignores corrupt- ions) until an error-free unit (or message) is received. The polarity of the BIB is reversed to request retransmission when the FSN of this error free received unit (or message) is not one greater (cyclically) than the last correctly received, and accepted, message SU (or message). After requesting a retransmission, a terminal takes no further action until a change in polari- ty of a received FIB indicates the retransmission, the terminal then rever- ting to normal operation. FIB: The polarity of the FIB in any particular unit (or message) transmitted by a terminal is always the same as that of the BIB contained in the last unit (or message) correctly received by that terminal. Since a change of polarity of the BIB is used to request a retransmission, a change of polarity of the FIB will indicate that the retransmission is taking place. The polarity of the FIB is reversed when a retransmission is started, and after a reversal the polarity is unchanged until the next retransmission is started. The FIB and BIB thus perform a 'handshaking' procedure. Operation Information is transferred in the two directions of a signalling link, but as the two directions are independent from the error-control point of view, it is only necessary to consider one of them (A-B). (a) Error-free: A message SU (or message) is accepted at B provided it has a FSN which is one greater (cyclically) than the previous one accepted. The BSNs returned to A signal the progress made in the process of accep- ting correct message SUs (or messages). When received at A they enable the message SUs (or message) to be cleared from the retransmission store. (b) Error: The corrupted unit (or message) is ignored. The system waits the receipt of the next error free unit (or message), the FSN of which will serve to indicate whether or not the corruption was a message SU (or message) or a SYU. If the FSN is the next in the sequence expected by B, the corruption must have been a SYU, in which case no further action is necessary and normal operation applies. The receipt of any other FSN serves to indicate that a message SU (or message) was corrupted and a retransmission necessary. B requests this by reversing the polarity of the BIBs sent back B to A and does not accept further message SUs (or messages) until it has detected that a retransmission has taken place. The retransmission takes place when the BlB polarity is reversed and the reversal recognised at A, the first message SU (or message) of the retrans- mission being identified by the latest BSN received at A. The fact that the retransmission has taken place is indicated by reversed polarity of the FIBs transmitted A to B, commencing with the first message SU (or message) of the retransmission. B detects this FIB reversal to recognise the retransmission. If there are no further errors, the first message SU (or message) of the retransmission is accepted by B, which then reverts to normal operation. If the first one (or more) message SU (or message) of the retransmission is corrupted, when B eventually receives an error free SU (or message) it will detect from the FIB reversal that a retransmission has taken place, but the FSN will be different from that B will accept. A second request for retrans- mission is made by again reversing the polarity of the BIBs sent back. B then rejects all further message SUs (or messages) until the next retransmission is detected by the reversals of the FIBs received. Noncompelled error-control mode The ignore-corruptions error control as described above is an example of this mode, messages being transmitted in a continuous stream. A number of messages may be present on the signalling link at any one time and the error- control method is required to cater for this, which accounts for the sequence numbering in the example described. The mode is not dependent upon the magni- tude of the error control loop delay. With noncompelled, the c.c.s. link will be very lightly loaded at high signalling bit rates in the telephony service, which gives possibility of repeat transmission of the information (typically by consecutive multiple transmission of a message SU (or message), or by repeated transmissions of the contents of the retransmission store) to mini- mize, but not replace, correction by retransmission. Compelled error-control mode Here, one message only is transmitted at a time, a following message not being transmitted until the previous message had been correctly received and the transmit end is aware of this, either implicitly or explicitly. The error control loop delay of the signalling link is occupied for the complete trans- mission of a single message, and relative to noncompelled, the c.c.s. link occupancy is significantly increased for a given signalling requirement. Compelled signalling is interleaved in the two directions. The mode reduces the number of speech circuits which may be served per c.c.s. link relative to noncompelled under otherwise equal conditions, the loading being dependent upon the error control loop delay and thus on the propagation time of the signalling link. The compelled mode has potential for simplicity (one message only dealt with at a time, no sequence numbering, retransmission store limited to one message), and has possibility of being optional to noncompelled in the common- ality concept of the c.c.s. system. While the mode may be suitable for the telephony service in certain restricted national network conditions, it is not thought suitable for the switched circuit data service in any conditions due to the excessive queueing delays which would arise, the data service having a high proportion of short duration calls, with consequential high c.c.s. link signalling requirement. It is thought that the application flexibility merit of the noncompelled mode in any service may well outweigh any merit which may be realisable with the compelled mode in restricted application. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 11.11.4 Message structure =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= The message size, formatting and codings for the new No. 7 system are not yet finalised by the CCITT. As the system is to be optimised for 64 kbit/s, and thus a c.c.s. link very lightly loaded in the telephony service, the bit length of the constituent bit fields of a message, and the total message bit length, can be reasonably long, and significantly longer than those in system 6, without penalty. The flexibility of the new c.c.s. system will permit, optionally, messages of fixed bit-length SUs, or messages of variable bit- length, as desired. For compatibility with the digital environment it is logical that the message length should be a multiple of the 8-bit Octet, and to simplify the processing procedures, a byte structure adopted. =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= References =컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�= 1. LUCAS, P., LEGARE, R., and DONDOUX, J.: 'Principes nouveaux, pour la signalisation telephonique', Commun. & Electron., 1967, 18, pp. 7-23 2. CCITT: Green Book, 6, Pt. 3, Recommendations Q252-295 'Specification of signalling system No. 6', ITU, Geneva, 1973, pp. 427-521 3. CREW, G.L.: 'CCITT system No. 6 - a common channel signalling scheme', Telecommun. J. Australia, 1968, 18, 3, pp. 251-256 4. AKIMARU, H., TEKEDA, H., and ABU, M.: 'Common channel signalling system for DEX2 electronic switching system', Rev. Electr. Commun. Lab. (Japan), 1969, 17, p. 11 5. DAHLBOM, C.A.: 'Common channel signalling - a new flexible interoffice signalling technique'. lEEE International Switching Symposium Record, Boston, USA, 1972 6. PETERSON, W.W., and BROWN, D.T.: 'Cyclic codes for error detection', Proc. Inst. Radio Eng., 1961, 49, Pt. 1, pp. 228-235 7. CCITT: Green Book, 6, Pt. 3, Recommendation Q277 'System No. 6 error control', ITU, Geneva, 1973, pp. 497-499 8. CCITT: Green Book, 6, Pt. 3, Recommendation Q267 'Unreasonable and super- fluous messages', ITU, Geneva, 1973, pp, 477-480 9. CCITT: Green Book, 6, Pt. 3, Recommendation Q274 'System No. 6 modulation method and modem requrement~', ITU, Geneva, 1973, pp. 491-495 10.CCITT: Green Book, 6, Pt. 3, Recommendations Q291 and Q293 'Security arrangements', ITU, Geneva, 1973, pp. 508-510 and 512-518 11.CCITT: Green Book,6, Pt. 3, Recommendation Q271 'System No. 6 continuity check of the speech path', ITU, Geneva, 1973, pp. 484-487 12.WELCH, S.: 'Common channel signalling - a flexible approach'. International Switching Symposium Record, Kyoto, Japan, 1976 13.HAMMING, R.W.: 'Error detecting and error correcting codes', Bell Syst. Tech. J., 1950, 29, p. 147 컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 컴컴컴컴컴컴컴컴 Remember Where You Saw This Phile First 컴컴컴컴컴컴컴컴 컴컴컴컴[=] ARRESTED DEVELOPMENT +31.77.547477 The Netherlands [=]컴컴컴� 컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�