







             COMMUNICATIONS PROTOCOLS FOR PERSONAL COMPUTERS
             










Hayes Microcomputer Products, Inc.
705 Westech Drive
Norcross, Georgia 30092



INTRODUCTION

        Since the beginning of human communications, man has defined 
"protocols," or sets of rules, to provide for the orderly interchange
of information.  These have included agreement on sounds, languages, 
alphabets, vocabularies, grammar, syntax, punctuation, and 
formatting.  The precision required in electronic communications 
needs similar, very carefully defined and precisely implemented 
protocols.

        Such protocols begin with agreement on the binary 
representation of data.  Standards such as the American Standard Code
for Information Interchange (ASCII) and the Extended Binary-Coded 
Decimal Interchange Code (EBCDIC), define specific bit patterns that 
are interpreted as particular characters.  These are the "alphabets" 
of computers, and provide the basis for the orderly use and exchange 
of information.

        Encoded characters can be transferred between computers using
two methods: parallel or serial.  Parallel communications requires a 
separate wire for each bit.  The ability to send all of the bits of a
character simultaneously makes parallel the preferred method of 
transfer when high speed is required.  When such speed is not 
required, or when it is not feasible to have eight individual 
circuits between the devices, serial communications is used.  In 
serial communications, each character is sent as a series of bits one
at a time on a single wire between the systems and the systems 
assemble or disassemble characters to create this stream of bits.


ASYNCHRONOUS AND SYNCHRONOUS COMMUNICATIONS

        There are two basic methods of communicating characters 
serially between computers: asynchronous and synchronous.  
Historically, they have been implemented separately.  Today, we bring
their application fields together in the personal computer 
communications environment.  In asynchronous communications, each 
character is transmitted as an individual entity, separated from 
preceding and following characters by an indeterminate period of 
time.  Special bits are sent preceding and following the actual data 
bit of each character to synchronize the receiver with the 
transmitter.  These additional bits result in at least a 20 percent 
overhead of non-information bits.

        In synchronous communications, data are grouped and sent as a
continuous series of characters called "blocks" or "frames." Rather 
than synchronize the receiver at the beginning of each character, a 
special bit pattern is sent at the beginning of each frame.  The 
following data can be separated into characters because of the fixed 
timing relationship between them. Using this method, overhead becomes
a smaller factor as block sizes increase, resulting in more efficient
use of the communications channel.

        Historically, the asynchronous method has been used for 
interactive terminal communications in which throughput is limited by
the capabilities of the terminal hardware and the operator's typing 
speed.  The synchronous method has been predominant in 
computer-to-computer communications where high volume of data is the 
rule and high speeds are required.  As corporations rely more on 
personal computers, and the need for sharing data between them and 
corporate mainframes increases, the trend will be toward higher speed
for microcomputer communications, and the greatest efficiency 
provided by synchronous communications. This separation of 
asynchronous and synchronous communications into exclusive 
environments has changed and, today, they have into one, more 
comprehensive communications environment.

        Computers are able to communicate a bit of information by 
asserting a voltage level on a wire connected to the intended 
recipient.  The most basic "physical" communications interface 
standards, such as RS-232, specify which voltage levels correspond to
which bit values, what type of connectors are to be used, and the 
ancillary control and timing signals and their meanings.  A direct 
connection of this sort is well suited to short distance between 
nearby systems.


MODEMS AND DATA COMMUNICATIONS

        When connecting to widely separated systems, direct wired 
connections are not feasible.  The cost of the physical circuit is 
prohibitive, and the incapability of sharing the circuit makes it 
impossible to spread this cost over multiple users. most common 
solution for long-distance data communication is to connect through 
the public telephone network.  Virtually any site can be reached, and
the cost is low compared to a direct wired connection because the 
facilities are shared by many users.

        The public switch telephone system represents a significant 
capital investment and connects to almost every part of the world 
with a steadily improving level of quality.  The telephone system is 
designed to carry sounds (voices) rather than voltage levels, and it 
is therefore not possible to communicate through that system as one 
would over a wire.  Instead, it is necessary to convert the digital 
voltage levels into sounds (modulation), and to convert them back to 
digital signals at the other end (demodulation).  The device that 
accomplishes this task is known as a "modem" which is a contraction 
of modulator-demodulator.

        Early modems required total manual control - the human 
operator had to physically dial the telephone, and switch to "data 
mode" or place the telephone handset into a device that transferred 
the sounds to and from the telephone.  As modems came into wider use,
and use by non-technical personnel demanded simplication of the 
communications process, Hayes Microcomputer Products, Inc. introduced
its innovative modem series.  A Hayes modem can be directly connected
to the telephone system, eliminating the error-prone acoustic 
connection.  It also permits the terminal operator or computer to 
control modem functions, such as dialing a number or switching from 
command to data mode from the keyboard or with software.  The Hayes 
Standard "AT" Command Set has since become the standard method of 
controlling modem functions from software.  The "AT" refers to the 
letters that precede each command string that allow the modem to 
detect the baud rate and parity the computer is using.  The Hayes 
Standard "AT" Command Set has been adopted by many modem 
manufacturers, thus claims such as "Hayes compatible" are frequently 
seen.

        The Hayes Standard "AT" Command Set specifies a method of 
controlling modem functions from the attached equipment, but does not
define the means of signalling between modems.  Protocols such as the
Bell 212 standard established by AT&T for use on its domestic 
circuits, and the V.22 standard established by the CCITT for 
international circuits ensure that the operating capabilities of the 
telephone network are not exceeded, and provide a standard modulation
method so that the receiving modem can decode the transmitted 
signals.  These standards, among others, are implemented by Hayes so 
that Hayes modems can be used to communicate with those produced by 
other manufacturers that follow the same standards.

        While standards such as Bell 212 and CCITT V.22 provide for 
signalling in such a way that the receiver can usually decode the 
sounds accurately, it is possible for the sounds to be corrupted by 
interference (noise, background hiss, or pops and clicks) so that the
received data does not match the transmitted data.  Noise is a 
characteristic of the public switched telephone network and system 
implementers have to be aware of the existence of transmission 
errors.

        In interactive applications, where the typist sees an echo of
the results of the communications, these problems can be corrected 
immediately.  Even so, the frequent introduction of errors can be 
very frustrating and confusing.  In file transfer applications, the 
user does not see the data directly, and it is not feasible to 
examine it for errors.  Errors might go uncorrected, resulting in the
data being useless as received.

        Additional protocols are used to ensure correctness of data 
delivered by the physical devices.  The next section describes the 
protocols that Hayes has selected for this higher-level 
error-control, and our justification for this decision.

        
ERROR-CORRECTING PROTOCOLS

        Various protocols have been defined by computer users, 
manufacturers, and international standardization bodies.  These 
include protocols for transferring files between microcomputers (such
as Hayes Verification and XMODEM), for block-mode communications 
between terminals and mainframes, between modems, between mainframes,
to public data networks, and between public data networks.

        Hayes selected the protocols it feels are best suited to 
simplify migration to the communications environment that will exist 
through the eighties and beyond.  The following points were critical 
to the decision:

        - For speed, efficiency, and integration with mainframe 
communications networks, synchronous communications must be 
supported.

        - An easy migration path for current users of asynchronous 
equipment to the synchronous environment must be provided.

        - Bidirectional, error-free communications must be provided 
with minimal overhead or loss of performance.

        - Interactive, file transfer, and other applications must be 
supported.  - Considering the worldwide nature of communications, and
the desire to minimize barriers to free flow of information, the 
protocol must be recognized by international standards organizations 
and accepted in all countries.

        - Information regarding the protocol must be publicly 
available so that implementation will be compatible and supportable, 
further minimizing barriers to data interchange.

        - The protocol must be compatible with the planned Integrated
Services Digital Networks (ISDN) of the future, again providing an 
easy migration path for users.


THE X.25 PROTOCOL

        The International Telegraph and Telephone Consultative 
Committee (CCITT) in 1976 established Recommendation X.25 as a 
standard for communications between terminals and public data 
networks.  X.25 uses bit-oriented synchronous communications to 
provide an error-free, bidirectional communications service to 
communicating applications.  The CCITT enhanced X.25 in 1980 and 1984
to provide increased flexibility and to address technology such as 
satellite communications, proving the intention of maintaining X.25 
as a "living" specification.  ISDN protocols compare closely with 
X.25, making adaptation of X.25 products to ISDN a simple task.  X.25
is public information, and, being an international standard, all 
implementations can therefore be compatible.

        For these reasons, Hayes has selected X.25 as its standard 
for error-correcting protocols in future hardware and software 
products.  Companion standards, such as X.32 (defining the method by 
which a dialed connection can be made with a network) will also be 
adhered to.  The protocols will be implemented both in modems (so 
that existing applications can take advantage of an error-free 
connection) and in software (so that new applications can take 
advantage of advanced X.25 features such as multiple simultaneous 
connections).  Other advanced products are planned that will 
implement X.25 and deliver its advantages to the communications user.

        Hayes will continue to support the Hayes Verification 
Protocol, XMODEM, and other accepted file-transfer protocols for the 
microcomputing community.  We will support and contribute to the 
development of international standards in the area of file-transfer 
protocols (such as the proposed FTAM standard) and intend to 
implement protocols in our products.


LAYERS OF CCITT PROTOCOLS

        X.25 is a "multi-layer" standard, meaning that it consists of
separate distinct parts each designed to perform a specific function.
The "packet layer" provides multiple circuits (the capability for one
terminal to communicate with several hosts simultaneously) or for 
several terminals to share a single physical circuit to a network and
then on to the same or different hosts.  Information at the packet 
layer is transferred in "packets," which contain the user data plus 
routing information necessary to get the data to its proper 
destination.

        The packet layer uses the services of the next lower layer, 
the "link layer." The link layer establishes a single communication 
path between adjacent nodes.  The basic unit of information transfer 
between link layer entities is the "frame." A portion of an "I" 
(Information) frame contains the "packet" (if the packet layer is 
being used).  The link layer protocol used in X.25 is known as 
"LAPB," which stands for "Link Access Procedure-Balanced." It is 
"balanced" because the stations on each end of the link have equal 
rights to send an appropriate frame type at any time, unlike a 
"master-slave" relationship in which the slave can only transmit when
"polled" by the master.

        In situations where only one connection exists between two 
stations, and there is no need to route information to other stations
beyond those two, it is possible (and desirable) to use only the link
layer of X.25 (LAPB) rather than the entire packet layer, in order to
reduce protocol overhead.  The new Hayes Vseries system modems, 
which provide a single-channel error-control link, use only LAPB 
without the overhead of the X.25 packet layer.  Hayes future V-series
system products will implement the packet layer as well and provide 
multiple connection capability to the user.

        The upcoming Integrated Services Digital Networks (ISDN) use 
a protocol very similar to LAPB for communication on the "D" channel.
This protocol, known as "LAPD," provides multiple virtual circuit 
capability at the link layer, in addition to supporting the X.25 
packet layer.  Hayes has implemented LAPD in our prototype ISDN PC 
card, and it will become increasingly important to other ISDN-related
products in the future.  LAPD has also been discussed in the CCITT as
a possible standard for modem error-correction, and Hayes would 
support this standard, if adopted, by designing modems which would 
support both the new CCITT standard and the present V-series system 
LAPB protocol.


ASYNCHRONOUS FRAMING TECHNIQUE

        Although the overwhelming trend is toward synchronous 
transmission, asynchronous hardware and software currently dominate 
the personal computer marketplace.  X.25 as it is currently defined 
requires a synchronous data transmission link, and thus cannot be 
used with most of the existing equipment.  To address this need, 
Hayes has proposed (in the CCITT and the International Standards 
Organization) Asynchronous Framing Technique (AFT) which can be used 
with any bit-synchronous protocol such as LAPB, LAPD, or IBM's SDLC. 
AFT replaces the lowest-level framing and error detection portions of
these protocols, adapting them for operation over asynchronous 
transmission links while retaining all of the formats and 
error-correction and frame sequencing procedures.  Higher layer 
protocols, such as the X.25 packet layer, can be used without 
modification on an AFT-adapted link layer.

        The advantages to this approach, as opposed to adoption of an
entirely different non-X.25-compatible protocol, are many.  AFT 
provides an easy migration path to synchronous X.25 and can be used 
as a stopgap measure until synchronous becomes more widespread.  It 
requires minimal modification to existing implementations, making it 
easy for a vendor who already provides X.25 to add support for 
asynchronous transmission.  AFT takes advantage of the wide knowledge
and experience with now present in the worldwide data communications 
industry.  Existing certification procedures and equipment for X.25 
are easily adapted to AFT.  Familiarity with the protocol will 
shorten the development cycle for adoption and implementation of 
X.25/AFT.

        Hayes will support X.25/AFT, in addition to X.25, in those 
future products in which transition from asynchronous to synchronous 
communications is important, including both software and advanced 
hardware products.  The goal is to move to the speed and efficiency 
of synchronous communications as quickly as possible, and X.25/AFT 
facilitates this by providing a compatible interim asynchronous 
standard.


THE HAYES COMMITMENT

        As the leader in communication hardware and software products
for microcomputer systems, Hayes takes seriously its responsibility 
to analyze present and future requirements, and to provide products 
for our customers to accomplish their communications and data 
management tasks.  As we grow into a world of high-speed data 
communications between systems produced by various manufacturers on 
many continents, the adoption of and adherence to international 
standards becomes increasingly important.  Our commitment to quality,
service, and value will allow our customers to continue to benefit 
from Hayes products in the future.

