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X.25 and V.42 explained
doc: x25v424.txt
X.25 AND V.42 SOLUTIONS FOR DATA COMMUNICATIONS
Hayes Microcomputer Products, Inc.
705 Westech Drive
Norcross, Georgia
404/449-8791
INTRODUCTION
Without X.25 in the PC's modem, X.25 error-control and packet switching
functions are not available until the transmitted data is received by
the Packet Switch Network (PSN).
Dial-up X.25 applications are becoming more widely used due to
the expanding availability of service. In addition to more widespread
availability, the Value Added Networks (VANs) are making this service
more attractive by implementing lower price structures. The trend of
increased X.25 usage in business, government and education underlines
the importance this capability has for enhancing a communications
environment. This is of particular interest as users begin to develop
new, diverse communications applications. Through dial-up X.25, users
can transfer data point-to-point in a single session through a direct
dial connection over the Public Switch Telephone Network (PSTN) or a
Packet Switch Network (PSN). Point-to-point, multisession
communications where one workstation accesses one host to execute
multiple applications is available through dial-up X.25 through the
PSTN or a PSN. One workstation can call multiple hosts for data
exchange in a point-to-multipoint session through dial-up X.25 over a
PSN. Multisession and multipoint communications with a V-series X.25
product are supported by using the Hayes AutoStream family of protocols
in Smartcom III or other communications software designed for multiple
session support.
After more than three years of analysis and deliberation, CCITT
Recommendation V.42 has put to rest the modem industry's struggle to
resolve a complex issue. Dial-up modem-to-modem error-control and
other enhanced communications functions can be used with an increasing
degree of confidence due to the work of the CCITT.
Dial-up X.25 and V.42 enhance the evolution of data communications and
provide users with more capabilities for use in meeting their
workstation communications applications needs.
Now, depending upon the user's needs, solutions are available
from Hayes to address the communications requirements for a variety of
applications. Packet switching network and multisession or multipoint
communications are met through CCITT X.25. In addition to addressing
modem error-control, V.42 provides a path for both standard and
vendor-specific enhancements for point-to-point communications. And,
because the solutions use the established performance of V-series
system products as their foundation, PC communications enjoy a new
level of acceptance and reliability.
The V-series products with dial-up X.25 and V.42 are supported
by Hayes software. Smartcom II Version 3.1 for the IBM PC and
compatibles and Smartcom II Version 3.1 for the Macintosh support
single session X.25 and V.42 functions. Smartcom III Version 1.1 for
the IBM PC and compatibles supports V.42 and also has the capability to
support multisession and point-to-multipoint use through dial-up X.25
by supporting the Hayes AutoStream family of protocols. As always,
Hayes is supporting other software developers with technical
information and specifications for use in designing application
programs around these capabilities.
Implementing CCITT's X.25 and V.42 international standards
underlines the company's longstanding commitment to support and
incorporate approved standards in its products. Hayes will only make
extensions to the standards in a logical manner based on technical
merit and systems requirements. This helps ensure that Hayes products
can be easily integrated into the evolving global data communications
network and will not easily or quickly become obsolete for the design
use of the network.
The benefits provided by both X.25 and V.42 address different
applications and environments. An examination of X.25 and Packet
Switching and an analysis of V.42 and its promise and potential will
enable users to evaluate their applications and select the
communications solution best suited to meet the needs of each
application in their data communications network strategy.
INTRODUCTION TO X.25
An X.25 PAD (Packet Assembler/Disassembler) in a dial-up, modem
creates a novel element in the dial-up X.25 environment. This single
port X.25 device may be called a mono-port quad PAD since its single
port can support up to four sessions using Hayes AutoStream.
Incorporating X.25 capability into a V-series system product
extends the usefulness and benefits of an X.25 Packet Switching Network
(PSN) to individual workstations and terminals in widely dispersed
locations.
Growing demands for PC data communications reliability and
flexibility make an X.25 PAD an ideal solution. Since access to an
X.25 network can be accomplished through the Public Switched Telephone
Network (PSTN), use of a mono-port quad PAD opens a variety of
communications opportunities not readily or affordably available using
previous communications equipment.
BACKGROUND - X.25 AND ISO
The diversity of the countries, regulatory agencies and
equipment used worldwide would create a complex jumble with little
chance of interworking if it were not for data communications standards
established by the International Telegraph and Telephone Consultative
Committee (CCITT). To provide stability and conformity for
communications, the CCITT, a United Nations agency, makes
recommendations on a broad range of subjects. As these recommendations
are implemented, standards are established which greatly benefit the
compatibility of data communications equipment and procedures.
CCITT Recommendation X.25 defines the interface between data
terminal equipment (DTE) and data circuit terminating equipment (DCE)
for terminals operating over leased lines in the packet mode while
connected to public data networks. First published in 1976 and updated
in 1980, 1984 and 1988, X.25 enjoys widespread implementation, a fact
that makes certification of X.25 products readily available. The 1984
update, known as X.32, extended X.25's capabilities to include dial-up
operation. Future use of dial-up X.25 will greatly benefit from the
large body of knowledge residing with users and service providers which
has resulted from years of X.25 applications.
The International Standards Organization (ISO) has played an
integral role in defining the reference model of the structure of data
communications networks through Open Systems Interconnection (OSI).
The OSI model is divided into seven functional layers to
clarify the interfaces between services provided by a network and to
avoid confusion in the development of related standards. The goal of
the network is to be as transparent as possible, to the extent that two
users on separate networks will communicate, unaware that the networks
are even involved in the process.
The "open" nature of OSI is directly related to the fact that
the model is versatile enough to enable any host system, regardless of
who made or designed it, to connect with any other host or network.
The OSI model provides a means for thinking about how connections ought
to be structured so that devices wishing to be compatible can implement
the same protocols at every layer.
PACKET SWITCHING
The technology for packet switching has been in use since
ARPANET was developed in 1968. Many technologies are first implemented
in military applications and packet switching was initially used by the
Advanced Research Projects Agency (ARPA) to enable widely dispersed
governmental facilities to share information even though they were
using a variety of computer systems, software and data resources.
Packet switching's basic concept is to route data from a source
to its destination assuring its accurate and in-sequence arrival. In
packet switching, the data is divided into packets (blocks of data) of
a specific length and tagged with a header which contains addressing
and sequencing information.
The path that the packets take through the network and the
routing information remembered by the switches through which they pass
is often called a "virtual circuit." A number of virtual circuits can
share the same physical connection because each packet includes
information identifying its destination in the network.
A sender's Packet Assembler/Disassembler (PAD) assembles
individual characters so they can be sent as packets. Once the packets
have been transmitted, the receiving PAD then disassembles them into
individual asynchronous characters or performs protocol conversion for
other types of interfaces such as SNA.
The addressing and sequencing information gives each packet
switch that the data passes through the necessary information to route
the packet and instructs the receiving PAD concerning the order to use
in reassembling the data into its original form. The receiving PAD
also receives information which enables it to determine if any packets
have been lost in transmission or contained transmission errors and to
request retransmission of missing or broken packets.
Using packet switching, the potential exists to divide a single
document into packets which are routed such that each can take a
different path to the packets' ultimate destination. Usually, however,
most packets moving from one location to another travel over the same
path. Data traveling over a packet switching network uses the most
efficient route available in going from switch to switch. So, the data
may not travel the shortest route to its destination, but it will flow
along the most direct route that has the lowest traffic. The network
uses dynamic routing to handle the packets and keep a balanced data
flow across the switches, and can even react to a broken switch by
re-routing packets across the network.
X.25 packet switching has been available on public and private
networks for a number of years. Unfortunately, X.25 benefits were only
enjoyed by users after the network began handling their data. That is,
the communications link from a users PC to the network was not an
error-control link so data integrity could not be guaranteed.
Additionally, the link to the network did not benefit from the cost
savings available through X.25 and packetizing information for its
entire "journey".
Public X.25 networks, such as Telenet, Tymnet, CompuServe,
Western Union, MCI and GE Information Services in the U.S. and Datapac
in Canada, sell X.25 service to the public. Users are charged based
on the number of packets transmitted, and, in most cases, users do not
own any part of the network in this configuration. Packet switching
will normally be inherently lower in cost than circuit switching when
used for a wide range of data communications applications since network
resources are shared by many users.
Private networks are owned or controlled completely by a
corporation or organization for its exclusive use. These custom
designed networks meet specific corporate applications needs and entail
the corporation owning or having control over all the connecting lines,
hardware, PADs and other equipment required to set up the network.
This type of network is operated exclusively for the benefit of the
organization which established it.
In some cases a company may elect to utilize a hybrid network
which uses both public and private network equipment or features.
X.25 STANDARDS FOR PACKET NETWORK INTERFACE
The combination of X.25 and a PAD in a V-series system product
provides considerably more functionality than the packet
assembly/disassembly associated with a PAD. This results from the
benefits inherent in the implementation of multiple CCITT standards
available through X.25, X.3, X.28 and X.29.
A mono-port quad PAD uses a single RS-232 connection between
the PC and the modem to support up to four simultaneous sessions
through four PADs which can have different PAD parameter settings.
Each of the four PADs supports Triple X-PAD since they conform to three
CCITT standards, X.3, X.28 and X.29.
X.3 defines a set of PAD parameters which controls the behavior
of the PAD it assembles or disassembles packets, and which also
provides operating parameters for activities such as flow control and
data forwarding conditions.
The interface between the DTE and the PAD is defined by X.28.
Included in this definition is how data delivery to and from the DTE is
controlled as well as a command language to control the virtual circuit.
The packet-level control mechanism for the PAD is defined by
X.29. It provides the guidelines for remote and local PADs to use when
communicating with each other and allows remote host computers to
control how data is presented by the PAD to the attached terminal.
BENEFITS OF X.25 AND PACKET SWITCHING
Using an X.25 packet switching network for data transmission
provides users with affordable, reliable, flexible service that can be
economical for applications needing short, randomly occurring data
transfers. Activities such as a credit card verification in a retail
store, cash advances from an Automated Teller Machine (ATM) or access
to medical or other records are ideally suited for an X.25 packet
switching implementation.
The cost benefits of X.25 packet switching result from
combining messages from many sources for transmission over the same
circuits. With Direct Distance Dialing (DDD) or a leased line, the
user pays the same for the circuit whether it is used often or
infrequently. By contrast, networks base their charges on the amount
of data sent. The economy of scale in the network methodology versus
the DDD or leased line approach can be like comparing the U.S. Postal
Service with a private courier.
With a private courier you pay for the cost of transporting a
letter in a vehicle which may only have your letter in it. As a
result, you pay a premium for this "customized" delivery of your
letter. Using the post office, your message is combined with those of
many others, placed in a shared vehicle, and delivered. The cost
savings are passed along to you by this more efficient delivery system
and the end result, the delivery of your message, is the same as with
the custom solution.
An X.25 packet switching network maximizes the accuracy of data
transferred using special internal error-control protocols, redundant
hardware and redundant paths. This means that if the Atlanta network
node is down, data going from Miami to San Francisco can be rerouted
through Baltimore and St. Louis to speed it on its way. And, this will
be accomplished transparent to the user who is only aware that the data
has been transferred to San Francisco, not what route the data may have
taken. Dynamic routing helps distribute and balance the data flow on a
network and provides enhanced network availability by routing data
through back-up equipment when necessary.
Flexibility over the network lies in several areas, including
the ease of connectivity X.25 provides since it is an agreed upon
connection standard for the DTE/DCE interface.
An additional benefit to the PC user who communicates using a
V-series X.25 PAD product is multisession and multipoint capability.
This feature, for example, enables a product manager in Chicago to
check the corporate mainframe for inventory levels on particular
products while maintaining a connection to the engineering library in
Dallas to check specifications in a design document, and to conduct an
e-mail exchange of advertising copy with the marketing department in
San Francisco. Multiple access sessions such as this will enable a
user to switch between information sources and destinations to retrieve
and provide the most up-to-date information available. This feature
will be especially useful in coordinating the various activities
necessary for bringing new products to market or for companies whose
main asset is information. Having the proper information at the proper
place at the proper time is a critical success factor for virtually any
company. For example, from his weekend home in the mountains, a
product manager could simultaneously view three sessions of different
program models t olution required for a research analysis report.
X.25 AND DIAL UP MODEMS
The ubiquitous nature of the PSTN results in great convenience
for users worldwide since it provides both intra- and inter- country,
company, government, agency and industry communications. The
establishment of various CCITT standards has permitted communications
equipment manufactured and used in a variety of countries to transfer
data over the PSTN with relative ease using X.25 as the common protocol.
While the PSTN was, and is, designed for voice transmission, it
has become increasingly utilized for data transmission through the
implementation of dial-up modems.
Dial-up modems, such as Hayes V-series Smartmodem 9600,
incorporating X.25 for communications are taking PC communications to a
new level of sophistication. These modems combine the cost
effectiveness, flexibility and reliability of packet switching with the
wide availability of dial-up service to bring X.25 benefits to the
desktop.
Without an X.25 PAD in the modem, a user dials an X.25 network
asynchronously, data passes into a receiving modem at the network, and
a network PAD packetizes the data and sends it out on the network to
its destination. Using an X.25 PAD-equipped dial-up V-series system
product means that the data leaves the user's modem in packetized form
and can go directly onto the network for transmission.
The X.25 mono-port quad PAD modem extends X.25's error-control
capability from the network to the PC, thus making the entire
transmission "protected" by error-control. Without the PAD in the
modem, the data is not subject to error-control techniques until it s
received by the network for processing and transmission unless the PC
and the network are equipped with CCITT V.42 error-control modems. (The
V.42 function in the V-series system products is covered in detail
later in this document.) By providing error-control capability across
the complete scope of the data transmission, the X.25 mono-port quad
PAD modem offers extremely high data integrity, reduces data transfer
delays, and helps to lower transmission overhead by reducing the steps
data must go through after it leaves the PC. Furthermore, this PAD
provides the evolving multitasking PC environment with access to
multiple sessions where each session will be used by one of the PC
tasks.
Using X.25 protocols in the modem eliminates concerns over
dial-up error-control and allows a user to communicate confidently with
a network. Without X.25, users must determine if a modem on the network
implements a standard (V.42) or non-standard error-control protocol.
Thus, in addition to the performance and cost saving benefits existing
with X.25, there is also a high degree of user comfort with this
internationally approved, widely used protocol.
HAYES IMPLEMENTATION OF X.25 PAD CAPABILITY FOR DIAL-UP MODEMS
X.25 technology helps bring multisession and multipoint
communications to a personal computer workstation through a Public Data
Network (PDN) or the PSTN and provides multiple session connectivity to
workstations on an Integrated Services Digital Network (ISDN).
The X.25 V-series system products provide the user with
error-control from the local DTE, through the X.25 network, to the
remote computer without any protocol conversion overhead. Having a
local PAD allows the user to set up and establish multiple virtual
calls over the physical link and provides greater control over data
flow.
Functions included in the X.25 communications solution are:
% Link Layer Support - The PAD uses CCITT recommended
Link Access Procedure Balanced (LAPB). The link layer is responsible
for error-control from the local DTE to the network node. Access to
link layer protocol parameters allows the user to adjust the protocol
to fit specific applications. Parameters which can be adjusted include
the mode of operation (X.25,V-series system product or standard
asynchronous), and negotiation steps (detection, XID exchange).
% Packet Layer Support - After a link has been brought
up, the packet layer of X.25 is responsible for establishing virtual
circuit calls. The V-series X.25 products implement four PADs to
support up to four virtual calls over a single link (X.25 allows for a
maximum of up to 4096 virtual calls on a physical connection). CCITT
Recommendation X.25 describes several user facilities such as Reverse
Charge, Network User ID and Flow Control Parameter Negotiation. These
and others which may be required when connecting to X.25 networks
supported in the Hayes PAD.
% Triple X PAD - The Hayes PAD adheres to CCITT
Recommendations X.3, X.28 and X.29 to ensure compatibility with
existing and future X.25 equipment. The PAD design is based upon the
1984 specifications for X.25 and all of the mandatory specifications of
the 1988 recommendation.
% X.32 Support - X.32 defines the functional and
procedural aspects for the DTE to dial into an X.25 packet switch
network. In order to dial into the network, the Reverse Charging and
Network User ID packet layer user facilities supported in the PAD are
used to identify the calling DTE and allow the network to accrue
charges against it.
% Hayes AutoStream - This family of Hayes-developed
protocols is included in the AT Command processor to provide enhanced
functionality to a single communications link. Smartcom III
communications software and X.25 V-series system products support Hayes
AutoStream to provide for multiple simultaneous virtual circuits, per
channel flow control and device flow control as well as control and
setting of PAD parameters over the asynchronous link between the PC and
the modem.
A mono-port quad PAD's asynchronous dial-up capability
addresses a different range of applications compared to a leased line
implementation. These new modems make X.25 links affordable for single
sites, an important feature for companies which operate a variety of
remote offices, stores or branches.
Prior to the availability of dial-up X.25, many applications
had to be handled through costly leased lines. The availability of
those leased circuits was guaranteed, but users paid a high price for
that constant access. In the case of low volume or sporadic use, leased
lines often become prohibitively expensive and difficult to cost
justify.
Now, using dial-up X.25, businesses can improve the
profitability of existing branches. For instance, it may be
advantageous for a car rental company to open small, minimally staffed
branch offices in small airports, hotels, travel agencies, etc. These
remote sites can use dial-up X.25 for access to the corporate
communications network to process orders, handle billing, etc.
Affordable, remote sites for a variety of businesses are possible using
dial-up X.25.
X.25 NETWORK SUPPORT FOR DIAL-UP ACCESS
Several industry observers have described the issue of dial-up
X.25 capability as a "chicken and egg" situation. Users have been
reluctant to use dial-up X.25 until the price is lowered, while service
providers did not want to cut prices until the volume of use increases.
Both sides recognize the potential held by dial-up X.25 and the
users, modem manufacturers and service providers have taken the
initiative to help dial-up X.25 realize its potential.
Many networks have already implemented dial-up X.25 service and
others have indicated they will be implementing the service.
Additionally, indications are that dial-up X.25 service will soon be
extended into more cities around the world and across the U.S. and
trends toward lower costs are being implemented by many packet switch
carriers as tariffs are adjusted.
THE ROLE OF X.25 AND PACKET SWITCHING IN THE FUTURE
As the Integrated Services Digital Network (ISDN) grows in its
implementation, X.25 packet switching usage will grow as a result of
its use for ISDN.
Since ISDN provides packet switching services on both its
signaling (D-channel) and voice/data (B-channel) channels, packet
switching capability becomes automatic for ISDN users equipped for data
transfer. As ISDN islands appear, X.25 can link these islands through
X.25 gateways. These gateways result from integrating packet switches
into the overall digital switching system. In addition to V-series
system products, equipment such as the Hayes ISDN PC Card which offers
X.25 support will help to expedite and enhance the growing use of
dial-up X.25. ISDN terminal adapters convert a PC to an ISDN terminal
capable of accessing ISDN's 2B+D Basic Rate Interface.
HAYES SUPPORT FOR X.25
X.25 is a mature standard, widely implemented and supported by
a large and growing number of data communications equipment and service
companies. This widespread acceptance of X.25 provides a tremendous
foundation for use in meeting the data communications needs of the
future.
Hayes has been an outspoken advocate for the support of
recognized CCITT standards by data communications products. The
company will continue to be an active participant in helping to
establish new standards.
The company intends to continue its support of CCITT standards and
plans to help develop the data communications promise held by X.25 and
packet chnology through working in the standards setting process at the
CCITT and through participation in this growing market.
INTRODUCTION TO V.42
As modem technology continued to make ever increasing
advancements in data transfer speeds, interactive communications
applications and PC-to-host asynchronous links, the requirement for
modem-to-modem error-control came to the forefront as an issue needing
a standardized procedure.
Hayes announced the implementation of V.42 in V-series system
products in November 1988. By offering V.42 compliant products the
company underscored its support for international standards and helped
establish this new standard by making it available in the market.
Study Group XVII of the International Telegraph and Telephone
Consultative Committee (CCITT) began work on an error-control
recommendation in 1984. The result of the group's efforts is CCITT
Recommendation V.42, Error-Correcting Procedures for DCEs Using
Asynchronous-to-Synchronous Conversion.
The primary V.42 protocol, Link Access Procedure for Modems (LAPM),
is based on High-level Data Link Control (HDLC) procedures specified by
the International Standards Organization (ISO). Basing the new V.42
standard on such a widely tested, accepted and utilized technique will
provide great benefits for LAPM implementation.
This new standard is important to the present, but it is
equally important for the future. V.42 provides an international
error-control standard for point-to-point communications which also
provides the foundation for developing advanced modem capabilities.
Also provided is a means by which manufacturers can provide proprietary
enhancements without interfering with the future development and
evolution of the functions covered by the standard.
BACKGROUND
CCITT is a United Nations agency whose voting members are
countries, i.e. each country has only one vote which is cast by a
designated representative. Most countries select CCITT representatives
from their Postal, Telephone and Telegraph Administration (PTT), but
the United States, lacking such a body, is represented by the U.S.
CCITT National Committee, a Department of State body. The U.S.
committee is comprised of five study groups, of which one, Study Group
D, is in charge of positions related to modems and provides input to
CCITT Study Group XVII.
The International Standards Organization is made up of the
national standards-making bodies of each country, ANSI in the case of
the U.S. ISO, in cooperation with the International Electrotechnical
Commission (IEC), develops information processing standards. Of
particular interest to modem manufacturers is the ISO's activities
concerning the bottom four layers of the Open Systems Interconnection
(OSI) reference model.
CCITT, ISO and a variety of governmental and data
communications industry experts began work in 1984 to sort out the
variety of concerns, issues, features and related matters that needed
to be addressed by a point-to-point modem error-control protocol.
Following more than three years of analysis and discussion,
CCITT Study Group XVII met in Geneva, Switzerland in April 1988 to
finalize Recommendation V.42 for error-control in asynchronous modems.
The final recommendation specifies LAPM as the primary error-control
protocol and includes an alternative protocol in Annex A for backward
compatibility with MNP class 2-4 modems. The CCITT specifies an
alternate procedure to recommendations in either an annex or an
appendix. Procedures in annexes are mandatory for full compliance with
the recommendation while implementation of procedures defined in
appendices is optional. Recommendation V.42 states that "Compliance
with this Recommendation requires implementation of both protocols.
However, unless otherwise specified by user options, two V.42 DCEs will
commicate using LAPM."
As proposed in January 1988, all future enhancements for
error-control in Recommendation V.42 will be directed toward LAPM and
not the annex protocol. At the request of several European PTTs, the
Annex A protocol was made mandatory for full compliance, but this could
be dropped in four years. Study Group XVII completed its work in the
spring of 1988 and passed the V.42 recommendation to the full CCITT for
approval at its Plenary Meeting in Melbourne, Australia in November
1988.
V.42 - ITS FUNCTION AND THE NEEDS IT FULFILLS
V.42 provides the process by which data communications
equipment (DCE) handles error-control during an exchange of data. That
is, V.42 specifies an error-control protocol for V.22, V.22bis, V.26ter
and V.32 modems to implement.
These modems are full-duplex, two wire, dial-up products used on the
Public Switched Telephone Network (PSTN) that use
asynchronous-to-synchronous conversion.
When two V-series system products implement LAPM to exchange
data, the receiving modem uses a Frame Check Sequence (FCS) to verify
the accuracy of the data it has received in a data frame. Based on the
FCS, the receiving modem acknowledges receiving accurate data or tells
the sending modem to retransmit the data frame if FCS indicates an
error has occurred.
The protocol defines the link establishment process,
error-control procedures and negotiation parameters for establishing,
maintaining and conducting data transfer.
The V.42 standard does not explicitly apply to half-duplex ping
pong modems. Hayes has enhanced the V-series Smartmodem 9600 to use
LAPM over the high-speed link between two of these modems and allows
data compression to operate in this mode.
LAPM features include:
% Benign detection phase - V.42 modems must have the
capability to detect the presence or non-presence of another V.42 modem
at the other end of a connection. This detection phase must not
interfere with a V.42 modem's capability to establish a connection with
a non-V.42 modem. This detection phase may be enabled or disabled with
no impact on the V.42 modem's performance.
% Extension of LAPB and LAPD - LAPM is an extension of
LAPB and LAPD. It uses basically the same connection establishment and
termination procedures, as well as similar data transfer procedures.
Implementors familiar with either of these protocols should have no
difficulty with a LAPM implementation.
% Poll/Final bit procedure - The P/F bit procedures allow one
modem to force the other to transmit a response. This LAPM feature
improves error recovery capabilities by bypassing timer expiration
recovery mechanisms.*
% Separate primitives for ACK, NAK and BUSY - LAPM
provides seperate frame types for these functions which improves
protocol reliability and eliminates the chance of lockups due to
misinterpretation of frame contents.* V.42 includes an enhanced Reject
capability to improve error recovery performance in the presence of
line noise.
% 8-bit address field default - The address field allows
for differentiation of commands and responses, and in the future will
also allow for multiple simultaneous virtual data paths between the
modems for remote configuration, network management or user data such
as multiplexing multiple terminals or devices.*
% Address extension bit used - The address extension bit
may be used to provide for multi-octet addresses.
% Modulo-128 I-frame sequence numbers - The large
numbering base for information frames permits a larger "window size"
(number of outstanding frames) than would be permitted under modulo-8
sequence numbering. This improves performance on connections with long
propagation delays, such as satellite links.
% XID frame exchange for negotiation - LAPM uses the
internationally standardized procedures for negotiation defined by ISO
and CCITT. Using this standard mechanism, V.42 modems can negotiate
standard parameters as well as manufacturer-specific enhancements.
% Private parameter negotiation - Enhancements provided
by individual modem manufacturers may be negotiated through a mechanism
defined in V.42.
% Parameter renegotiation - V.42 permits the
renegotiation of link parameters between the stations any time during
the connection. This may be useful if line or user data flow
conditions change and the modem determines that different data link
parameters would improve performance.
% UI frame exchange for break signalling - Unnumbered
Information (UI) frames are used for break signalling out-of-band with
the user. V.42 supports three types of breaks: in-sequence, expedited
and destructive.
% Break length preserved (10 msec to 2.54 seconds) - In
some environments the length of the break sent is important. LAPM
preserves the break length up to a maximum of 2540 milliseconds in 10
millisecond increments.*
The rich functionality of V.42's LAPM satisfies a variety of
needs for personal computer modem data communications.
First among the needs met is the establishment of the
point-to-point error-control international standard. Much had been
said in recent years about how best to meet the error-control needs for
PC communications, but the rhetoric confused potential users and
delayed the progress needed for continuing the evolution of data
communications. LAPM provides an expandable standard for
error-control. This enables manufacturers to provide products based on
an internationally recognized standard and provides users with both a
reliable protocol and peace of mind because it is an approved
international standard.
Users' comfort level needs are especially addressed by V.42
since the previous environment featured multiple, incompatible
techniques. Knowing that future products implementing V.42's LAPM
protocol will interact with other V.42 modems will be of great benefit
to users. Planning for future point-to-point communications systems
can proceed with the knowledge that error-control concerns have been
fully and adequately addressed by LAPM.
Another future benefit provided by V.42 is a well defined
platform for advanced functions. These advanced functions will be
applicable to LAPM only and not the alternative protocol in Annex A of
V.42. Enhancements to V.42 will furnish the next-generation
communications features required by the high-speed, sophisticated PCs
of the future.
Future V.42 plans include addressing issues as they relate to
LAPM such as:
% Data compression - Improving data throughput by means
of data compression is one of the most widely discussed error-control
topics in the data communications industry. A standardized technique
is likely to be approved through accelerated procedures early in the
next CCITT study period. Contributions relating to existing techniques
have already been made to the CCITT and others are expected in the
future. The resulting data compression standard will likely be based
on the best attributes of the existing methods.
% Network management and remote configuration - In large
networks there is a great need to receive status reporting and
diagnostic information from widely dispersed, often unmanned equipment.
Additionally, the capability to set parameters and run tests remotely
is desirable. Error-control modems are currently among the different
types of equipment being studied in the area of network management by
ISO and CCITT.
% Data encryption - Currently, this topic is still being
approached at the feasibility level. Issues which need resolution
include implementation at the data link layer versus higher layers such
as presentation layer and key management. Significant progress on this
feature will most likely come in the 1990s provided other regulatory
issues such as export controls can be overcome.
% Forward error correction (cellular radio) - Cellular
radio applications present monumental challenges to modem designers.
Not only do drop-outs occur during cell transitions, but even normal
traffic (i.e. a large truck driving by) can interfere with the signal
and produce significant fading and other impairments. Error rates can
be very high, causing any normal error-control protocol to break down
and not be able to transfer even a single frame (the human ear masks
the resulting noise, but a modem cannot). Forward error correction,
such as used in compact discs, could be applied to V.42 modems.
% Transport of interface state information - In addition
to prerving user data, it is sometimes desirable to have end-to-end
carriage of interface state information. For example, this may occur
if the remote device is a printer with a paper-out signal that needs to
be received by the host. V.120 has this capability today and a similar
scheme could be added to V.42.
% Statistical multiplexing (multi-port) - As mentioned in
the address field section of LAPM features, the capability exists in
V.42 for multiple simultaneous virtual circuits between the modems.
High speed modems are currently used to connect multiple terminals or
remote terminals. This capability would also be desirable in an
error-control modem.
% ISDN compatibility (terminal adapter interworking) -
The similarity between the V.42 LAPM protocol and the LAPD-like
protocol used in the V.120 terminal adaption standard will permit the
development of rules for interworking between these devices. This
allows devices on the ISDN to easily interwork with devices on the PSTN
without significant protocol conversion resources.
% Asymmetrical and half-duplex operation - Many existing
error-control modems, such as Hayes V-series Smartmodem 9600, use
half-duplex ping-pong or asymmetrical transmission techniques to
achieve high throughput at reduced cost. Most of these modems use
proprietary techniques (since there was no standard) resulting in one
manufacturer's modems not being able to communicate with another
manufacturer's at 9600 bps. Work is in progress in the CCITT to
develop standard techniques for such lower-cost, high-speed solutions,
and error-control using V.42 will be an essential part. Changes in the
timers and acknowledgment rules may be necessary.
% Modem rate negotiation (multi-speed modems) - Although
significant degradation of circuit quality during a single call is
quite rare, there may be some benefit gained by the ability for modems,
based on error rates or other objective factors, to request a change to
alternative (slower) modulation methods with improved performance (and
to switch back if conditions improve).
% Character format indication and negotiation - Some
confusion currently exists in error-control connections due to the fact
that the character format (parity, stop bits) is independently set on
each DTE-modem interface, with an 8-bit format used between the modems.
Rules are provided in V.42 for encoding 5,6,7 and 8-bit data into
protocol frames, but no method is provided to coordinate this setting
between the two modems. Establishment of a method to coordinate data
frame formats settings between the modems and a method for warning the
user of possible problems need to be addressed.
% Preservation of framing and parity errors - There are
some cases where it is desirable for modems to pass along data with
improper parity rather than adjusting it as currently performed by
existing error-control modems. Tandem modem links where part of the
connection has error-control and part does not would benefit from
preserving irregular parity formats.
% Multi-frame selective reject - This will allow several
individual frames to be requested in one SREJ frame, thereby reducing
substantially the overhead on asymmetrical links.
There is no guarantee that any of these will become part of the
V.42 standard. As work continues in the CCITT study groups, the fact
that all of these are on the agenda gives an idea of the intensity of
work focused on this standard by the international telecommunications
community. While some of these may have little or no market value, the
study groups will continue to evaluate and establish the enhancements
which are practical, based on the technical merit arguments of the
participating members.
V.42 COMPLIANCE
V.42 specifies that a modem claiming full "compliance" to the
standard must implement all parts of the standard, including both the
primary and alternative protocols. The standard is written to be
compatible even with modems having no error-control capabilities. As a
result, modems implementing a portion of the standard or other modems
(such as a non-error-control V.22 or V.32 modem) which can communicate
with a V.42 modem may claim "compatibility". In short, a claim of V.42
compatibility does not necessarily mean a modem provides error control.
While these semantic differences may seem insignificant, users must
understand the product capabilities they will receive with a compliant
versus compatible modem. A V.42 compatible modem may only implement
selected portions of the standard, rendering it less useful in some
applications where it must interact with a V.42 compliant modem which
offers users the full range of V.42 error-control capabilities.
HAYES COMMITMENT TO V.42
Hayes believes strongly that V.42 is the point-to-point modem
error-control technique of the present and the future, consistent with
existing standardized techniques and independent of proprietary
control. The achievement of the goal of a standardized error-control
technique will eliminate uncertainty in the marketplace, greatly
increasing the demand for modems with LAPM error-control capability.
Additionally, the establishment of an international standard will
promote the evolution of a more homogeneous communications environment.
In keeping with its history of implementing international
standards, Hayes is offering V.42 compliant data communications
products which will also be compatible with the installed base of the
company's current V-series system products. Where applicable, the
company will also offer upgrades to add V.42 capabilities to existing
V-series system products.
The CCITT's recommendation of an HDLC-based procedure for modem
error-control affirms the Hayes decision to base error-control in its
V-series system products on an extension of the LAPB protocol.
Knowledge concerning HDLC-based protocols has enabled Hayes to be an
active, early supporter of V.42's LAPM. The new standard includes a
number of error-control features (XID frames and benign recognition
sequence) which Hayes has already successfully used in its V-series
system products.
Hayes engineers have taken a very active role in the
development of the new error-control standard and will continue to work
within the CCITT and ANSI to develop new standards and enhance the
existing standards.
X.25 AND V.42 - FITTING THE APPLICATION
Selection of a V-series system product with X.25 or V.42 is
relatively straight forward since each standard addresses the specific
interest of the user and provides identifiable migration paths for data
communications.
Since V-series system products implementing either protocol are
accompanied by the V-series feature set (error-control, adaptive data
compression, automatic feature negotiation and automatic speed
buffering), users enjoy a consistent foundation for their current and
future dial-up communications needs. Recognized for their expansion of
communications technology, V-series system products provide the
reliability and quality which has become synonymous with all Hayes
products.
Because both X.25 and V.42 are CCITT approved international
standards, either solution may be selected with confidence throughout
the world. Both are clearly defined and documented by an impartial
"governing" body which makes thsi tet
implementation of these standards more certain. Consistent
implementation allows users to select X.25 or V.42 products with the
knowledge that they will interwork with other products which also
embrace these protocols or will work with non-error-control modems
using compatible modulation standards. Further, since both contain the
original V-series functions, they will properly interwork with each
other or previously installed Hayes V-series products using all
features.
Therefore, no matter which product is chosen for a particular
application, users benefit from the technological advantage of Hayes
engineering expertise, quality of design, and from the implementation
of approved international standards.
Cost of using either X.25 or V.42 for dial-up communications
also enters in the decision. Dial-up X.25 service is becoming more
widespread in its availability. Increased competition for X.25
customers will help drive tthe in increasing availability of dial-up
X.25 as will lower prices for the service. As noted before, lower DDD
charges will help make point-to-point V.42 error-control usage
attractive for other user applications. So, if connect time or usage
occurs in short, erratic bursts (even though they may be frequent),
X.25 usage provides an effective means of transferring data. Also,
access to existing X.25 networks in companies or other organizations
makes the selection of the X.25 V-series system products logical.
Where infrequent dial-up access is needed, the V.42 standard may prove
best.
One important difference in the two standards is the more
direct migration to ISDN provided by X.25. Many corporations which
plan to move to ISDN or who have already made initial implementation of
ISDN will benefit from the X.25 gateway available through ISDN switches.
Selection of the V-series system product and the CCITT standard
best suited for an individual user corporation or other organization is
entirely dependent on the communications applications and the data
communications strategy of the information management plan. By
considering a few applications related questions, the decision can be
made for each application and the proper V-series system product can be
selected.
Those questions are:
% Do I want the benefits of error-control, adaptive compression
and automatic feature negotiation? These are standard in all V-series
system products regardless of whether X.25 or V.42 is ultimately
selected.
% Do I need a product which follows international standard
communications protocols? Since X.25 and V.42 adhere to CCITT
recommendations, users may select any V-series system product with
confidence.
% Does my corporation already use X.25 packet switching service or
would it like to? If so, then obviously the X.25 product is the
correct choice.
% Do I need communications capability centering on exchanging
data with one host at a time or several? For calling from one PC to
another, V.42 may be best suited. For dialing a corporate mainframe
to conduct multiple session of accessing and exchanging data, X.25 is
probably better suited.
The selection of the standard to be used is dependent on the
analysis of each communications application and the data communications
strategy of the organization. Regardless of the solution selected,
Hayes V-series system products can meet the need of a wide range of
uses and provide a compatible base for interworking with the V-series
functions. Many organizations will find that they have applications
such that they will deploy both standards in their systems and can do
so with confidence that all of the Hayes V-series products can
interwork with error-control and compression.
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