Date: Sat Sep 29, 1990 8:38 pm CDT From: TELECOM Moderator EMS: INTERNET / NRI MBX: telecom@eecs.nwu.edu TO: * Jim Gorak / J.C. Penney Subject: TELECOM Digest Special: ISDN Introduction TELECOM Digest Sat, 29 Sep 90 18:45:00 CDT Special: ISDN Introduction Inside This Issue: Moderator: Patrick A. Townson An Introduction to ISDN From the CERFnet News [Excerpted by Jody Kravitz] ---------------------------------------------------------------------- From: Jody Kravitz Subject: An Introduction to ISDN From the CERFnet News Date: Sat 29 Sep 90 18:00:00 CDT The most recent issue of the CERFnet news contained a long and useful article on ISDN. I've excerpted the article from the newsletter: CERFnet News California Education and Research Federation Network August-September 1990 Volume 2, Number 5 AN INTRODUCTION TO ISDN by Dory Leifer Motivated by the ever increasing public need to send digital information in the form of voice, data or image, national governments along with private corporations have developed a scheme called Integrated Services Digital Network (ISDN). Although this concept dates back to the early 1970s, only recently have standards been developed. The standardization of ISDN has resulted in an emerging market of ISDN equipment and service plans. This technology will have widespread impact on both suppliers and users of network equipment and services. In the United States, all seven regional Bell operating companies have initiated limited testing and deployment of ISDN. General deployment is expected during the mid to late 1990s. Our European and Japanese counterparts are committed to the nationwide implementation of ISDN. This article introduces the basic concepts of telephone networks and ISDN and explores possible applications of ISDN technology. The telephone network In order to understand why ISDN evolved, let's look at the current telephone network. The basic telephone is an analog instrument connected to a pair of wires. The pair of wires from a subscriber's premises, a private home for example, is connected over approximately a mile of cable to a local telephone company's central office. This pair of wires is commonly called the "last mile" or local loop. Inside the central office, the pair is attached to a device called a switch. The switch converts the analog signal to digital by sampling it thousands of times a second. The switch also routes the call by examining the telephone number called. If the call is long-distance, it is routed by the local telephone company, Michigan Bell, for example, to an Interexchange Carrier (IEC) such as AT&T, MCI, or US Sprint. The IEC routes the call to the local telephone company at the destination, still preserving the digital nature of the signal. This conversion between analog and digital seems reasonable for voice since humans (even programmers) cannot hear or speak digitally. But what if we intend to exchange digital information by connecting two computers together? In that case, we must convert digital information from our computers into analog signals using a modem. When these signals reach the central office, they are converted back to digital. The reverse process is used at the destination switch to convert the digital signal back to analog and pass it to the destination modem which finally turns it back for the last time to a computer bit stream. This process is not only redundant, it is inefficient. When voice is converted from analog to digital, a bit rate of 56,000 bits-per-second (bps) is typically dedicated to carrying it. This rate is required to make sure that the voice will sound natural when it is converted back to analog. Since the telephone network treats modems the same way, a rate of 56,000 bps is also required to convey modem signals. However, most modems send and receive at or under 2400 bps. The rest of the capacity is wasted. Modems serve another purpose apart from digital transmission. Most modern modems incorporate automatic dialing and answer functions. We say that an autodial modem exchanges signalling information with the telephone network. The modem can be instructed to place a call and report its progress: examples of what it can report back are "ringing", "busy", and "no circuits available". Again in this case, because the telephone network is designed for voice, computer equipment is disadvantaged. The modem requires special hardware to detect (actually to listen and guess) the sound of a busy signal, ring, or call incomplete message (usually preceded by three tones). This type of signalling is not only analog but it is in band: that is, signals and real transmitted information use the same channel. Sharing a single circuit to convey both transmission and signalling information imposes serious limitations. ISDN relieves the limitations of both in-band signaling and analog transmission. The next section describes a standard ISDN interface which provides end-to-end digital transmission and separates the signaling functions from the transmission functions. ISDN basic rate interface. The ISDN basic rate interface is the standard interface to connect subscribers to the ISDN. This interface uses the existing telephone wire pair. Instead of using this pair for analog signaling and transmission, only digital information is conveyed. On this wire, three channels or digital paths exist. The channels are multiplexed by giving each a time slice on the wire. Since ISDN channels are half duplex or uni-directional, a "ping-pong" method is used so that when one end transmits, the other listens. The ping pong happens with every tick of some central clock so the link appears to be bidirectional. Each ISDN circuit includes three channels: * 2 B or Bearer channels for data or voice (each 64,000 bps) * 1 D or Data channel for signaling or packet data (16,000 bps) These channels provide both signaling and transmission. Notice that there is no distinction between voice and data on the B-channel. The ISDN treats both as a stream of bits. The bits have significance only to the terminating equipment such as a telephone for voice or a computer for data. When a subscriber wishes to place a call, the terminating equipment sends a packet on the D-channel containing the information needed by the network in order to establish the call. Assuming that the call succeeds, the subscriber may then send either voice or data on a B-channel. To end the call, a take-down packet is sent. This is analogous to hanging up. Bearer channel transmission The B-channel is referred to as a clear channel because of its ability to pass an arbitrary bit stream transparently. In reality, arbitrary bit patterns have limited uses since the B-channel must adhere to the disciplines of existing voice and data networks. Sending voice using some non-standard encoding would preclude placing calls between the ISDN and the existing telephone network. A standard Pulse Code Modulation (PCM) scheme has been standardized for digitized voice because it is compatible with the existing voice network. Correspondingly, a data protocol must be employed on the B-channel if the subscriber is to reach hosts on the existing packet services which are not yet on the ISDN. Even if the host is on the ISDN, the network provides no guarantee that the data will be transmitted without errors. This is not a serious problem with terminal sessions (we live with error-prone modems), but for computer to computer connections (for example, performing a file transfer) an error-correction protocol may be required. The B-channel itself provides services that comply with layer one of the Open Systems Interconnection (OSI) Reference model (the physical layer). That is, it offers a medium through which bits may pass. If a subscriber uses the ISDN to call another computer directly, a minimum of a layer-two protocol is involved for error correction and flow control. In many cases, the subscriber will wish to access a host on a packet network like Telenet. In this case, both a link layer (OSI layer two) and network layer (layer three) are required. The subscriber then uses the X.25 protocol between the ISDN and his or her machine. An interworking unit acts as a gateway between the ISDN and the packet network, using the X.75 protocol. A somewhat similar service could be deployed by Merit in the future to provide Internet access for ISDN subscribers. Off-campus users could place an ISDN call to an Internet gateway. They could then access TCP/IP applications like file transfer, remote terminal, and mail. ISDN provides added support in this case: since the ISDN would report the caller's address, a unique Internet address could be associated with a particular calling address. Other services which require authentication of the caller would also be facilitated by this feature. The data channel The Data or D-Channel was originally specified by the CCITT for signaling but later was re-specified to include both signaling and transmission of packet data. Unlike its sister B-channel, the D-channel is not designed to carry an arbitrary bit stream. The D-channel uses both a link layer, Link Access Protocol-D (LAPD), similar to HDLC, and a network layer, Q.931, similar to X.25. The D-channel may be used for packet data when data throughput is not of high priority. No call set-up or take-down is required when using the D-channel to interface in packet mode. The signaling protocol on the D-channel is based on the set of signaling messages needed to establish and release a simple 64,000 bps B-channel voice or data connection. Included in call set-up are: * Flexible addressing compatible with many standard network * Required data rate * IEC (long distance carrier) selection if applicable * Notification if line forwarded to another address * User information text Signaling information is exchanged between a subscriber and the ISDN. But this information must also be passed within the ISDN to assure timely circuit establishment, efficient allocatmon of resources, and accurate billing and accounting between various service providers. A protocol called Common Channel Signaling Number Seven (CCS7) performs these functions. CCS7 was designed by AT&T and is based on the international standard CCITT Signaling System Seven (SS7). CCS7 is already used on a wide scale for signaling in the non-ISDN world but will be essential to support ISDN. Equipment Compatibility with existing equipment is extremely important to most of the users who will migrate from switched and private networks to ISDN. Therefore, most of the early ISDN equipment which users will purchase will be adapters for non-ISDN devices such as asynchronous terminals with RS-232 interfaces, 3270 style terminals with IBM SDLC and coax interfaces, and various LANs. An interface to connect common analog telephones will surely be a hot seller. Many of these devices are quite complex because they have to support both signalling and transmission. For example, an adapter which allows RS-232 attachment for terminals needs to interface with both the B- and D- channels. Under development by several manufacturers are integrated terminals that combine voice, data, and signaling into a compact desktop package. Initially, these terminals will function as expensive desktop space savers, replacing a separate phone and terminal, but later they will provide access to truly integrated services. What is an integrated service? An integrated service is one that is capable of providing a wide assortment of information well organized into a single package. This information may be, for example, in the form of voice, computer data, video, or facsimile. Initially, services available on ISDN will not be integrated. Voice and data, although they may be accessed together on an integrated terminal, have little to do with one another. Voice calls will involve only voice and data calls only data. We speak of this relationship as Service Coexistence. The second generation of ISDN services will be integrated. For example, consider a future bank credit card service. A card holder who disputes an entry in the credit card bill places an ISDN call to the bank. At the bank, a customer representative equipped with an ISDN terminal answers the call. The bank representative immediately has access to the caller's name and records since the ISDN passes the customer's originating address. The bank uses this address as a key into its customer database. The representative can address the customer by name when answering the phone. When the customer explains the nature of the problem, the bank representative retrieves the previous month's bill, which appears simultaneously on both screens. If the statement is in error, the balance can be recomputed before the customer's eyes. Integrated services can also facilitate research collaboration via multi-media voice, image, and control functions between scientists. Applications which require exchange of only short, infrequent messages can use services offered by the D- channel. Applications such as burglary alerting, energy control, credit card verification, cable TV requests for service, and home shopping can be accomplished using the D-channel packet facilities. Advantages of circuit switching Although the data rate of 64,000 bps may be too slow for bandwidth-intensive applications like real-time high definition imaging, ISDN's circuit-switched capabilities do offer several advantages to the research community over packet-switched networks like Merit, NSFNET or ARPANET. Certain real-time applications which require cross-country connectivity can be run over ISDN. Although the individual circuits which comprise modern packet networks may be much faster than 64,000 bps, the overhead involved in packet switching and queueing is far in excess of similar circuit switching functions on an established call. Packet networks try to optimize aggregate performance across the entire network. Real-time applications are usually interested not in averages but rather in worst cases. If you get a 64,000 bps ISDN circuit, you will be guaranteed 64,000 bps service for the duration of the connection. Throughput on a packet network might average 150,000 bps, for example, but might fall below 64,000 bps 10% of the time, causing serious problems for a real-time system. Another advantage ISDN has over packet networks is its potential ability to interface to a wide variety of digital laboratory equipment. The ISDN B-channel offers clear channel transmission. There is no protocol overhead involved in order to exchange information. This bit pipe can be used, for example, between detector/collector paired devices without the complication and expense of packet protocol gateway machines at each end of the connection. ISDN interfaces will eventually be readily available in VLSI, which will allow them to work with a wide variety of equipment at minimal additional cost. High speed (broadband) ISDN Many argue that 64,000 bps, based on the transmission capacity of the existing telephone system, is too slow to provide a wide assortment of integrated services. High-definition television, computer-aided design, medical imaging, and high-quality audio all require far more bandwidth than available in the current ISDN. An evolving standard for broadband ISDN (B-ISDN) may include 150 Megabit-per-second subscriber lines over fiber optic local loops. Conclusion ISDN will extend the capabilities of today's telephone networks, thus providing a market for new services. Most introductory services will apply service co-existence; services will be described as "running over" ISDN. ISDN will do for data networks what the Communications Act of 1934 did for voice -- provide a ubiquitous method for public transmission. Pioneer users of this technology will have both the opportunity and the challenge of helping to shape the future of telecommunications. * (Dory Leifer is a programmer for the Merit Computer Network, located in Michigan. This article was originally published in the Merit Network News, Vol 3 # 3, October, 1988). -------------- CERFNET NEWS AVAILABLE IN HARD COPY Send a request to help@cerf.net if you would like to be added to the hard copy distribution of CERFnet News. Postscript versions are also available via anonymous ftp to NIC.CERF.NET in the subdirectory cerfnet_news. ------------------------------