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Modernz 45
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* Intro to X.25 *
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* Another Modernz Presentation *
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* by *
* Digital-demon *
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* (C)opyright March 21, 1992 *
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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.
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Disclaimer
~~~~~~~~~~
This publication is for informational purposes ONLY.
In no way are the above authors, or organizations, liable for the
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allowed...and downright encouraged.
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