GUYDOSRM@snyplava.bitnet (Ray Guydosh) (08/19/89)
{{{ Here is another special single-issue digest. Replies have }}}
{{{ been directed to Ray. -chip }}}
TELECOM Digest Sat, 26 Aug 89 02:00:00 CDT Introduction to ISDN
Today's Topics: Moderator: Patrick Townson
Introduction to ISDN (Dory Leifer via Ray Guydosh)
[Moderator's Note: I am pleased to present this special essay by Dory
Liefer, and I thank Ray Guydosh, a long-time Digest reader and
contributor for sending it along. PT]
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Date: Sat, 19 Aug 89 09:13 EDT
From: Ray Guydosh <GUYDOSRM@snyplava.bitnet>
Subject: Article on ISDN
Patrick,
I stumbled upon the article below and thought that you might be interested
in seeing it if you hadn't seen it already.
Regards,
Ray
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An Introduction to ISDN
Author: Dory Leifer
Filename Filetype: ISDN LEIFER_D
(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. Permission to use this article is granted
provided the original source is cited and a hardcopy of the article is sent
to the editor at Merit, 5115 I.S.T. Bldg, 2200 Bonisteel Blvd, Ann Arbor,
MI 48109-2099. Further information about Merit, the Merit Network News, or
this article may be obtained by sending electronic mail to Info@merit.edu.)
===========================================================================
- Introduction to ISDN -
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.
ISDN will spur technological development of new and innovative products and
services for both research and business. 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. Analog means that signals are transmitted by varying the
frequency and intensity of the electric current in response, in this case,
to changes in the speaker's voice. Digital signals, in contrast, consist of
only two discrete voltage levels corresponding to binary 0 and 1. 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.
Digital signals can be carried easily over long distance lines because they
can be combined or multiplexed for transmission on high capacity links.
Digital signals also are not very susceptible to noise during
amplification. When the destination switch receives the digital signal, it
converts the digital signal back into analog and sends it out over the
local loop at that end.
This conversion between digital and analog 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 digital signal can only be a sampling of the "noise" coming
out of the modem, not a regeneration of the original bit stream from the
computer. 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 a
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.
On a phone line, you cannot start dialing unless you hear a dial tone. A
dial tone means that your phone is connected to a device at the telephone
company ready to accept call initiation. If a call is in progress and you
try to dial, the person at the other end hears an unpleasant tone. Sharing
a single circuit to convey both transmission and signalling information
imposes serious limitations.
ISDN relieves the limitations of both in-band signalling and analog
transmission. The next section describes a standard ISDN interface which
provides end-to-end digital transmission and separates the signalling
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 signalling 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 signalling or packet data (16,000 bps)
These channels provide both signalling 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. (For information
on OSI protocols, refer to the Dec. 1988--Jan. 1989 "Merit Network News",
which may be obtained by sending a message to INFO@merit.edu )
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 signalling
but later was re-specified to include both signalling 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 signalling protocol on the D-channel is based on the set of signalling
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 networks
* Required data rate
* IEC (long distance carrier) selection if applicable
* Notification if line forwarded to another address
* User information text
Signalling 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 allocation of resources, and accurate
billing and accounting between various service providers. A protocol called
Common Channel Signalling Number Seven (CCS7) performs these functions.
CCS7 was designed by AT&T and is based on the international standard CCITT
Signalling System Seven (SS7). CCS7 is already used on a wide scale for
signalling 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 signalling 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?
The concept of an integrated service is an abstraction rather than a set of
particular CCITT recommendations. 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
Co-existence.
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. A switch that can
handle thousands of such lines requires technology far beyond conventional
VLSI design. The power to be exploited by future designs of this type of
switch will be mind-boggling.
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.
References
Armbruester, H. "Universal Broadband ISDN: Greater
Bandwidth, Intelligence and Flexibility." Proceedings from the
IEEE ICC, June 1988.
Dorros, I., "A Systems Approach for National
Telecommunications Policy." IEEE Communications, Jan 1989, Vol 27,
Num 1.
Habara, K. "ISDN: A Look at the Future Through the
Past." IEEE Communications, Nov 1988, Vol 26, Numb 11. pp 25-32.
Roehr, W., "Signalling System Number 7." Open
Systems Data Transfer, February 1985. pp 1-16.
Smith, E.A., et al "Impact of Non-Voice Services on
Network Evolution." Electrical Communications, number 1, 1981. pp
17-30.
Stallings, William. Tutorial - Integrated
Services Digital Networks (ISDN). IEEE Computer Society, 1985.
"Standard Makers Cementing ISDN Subnetwork Layers."
Data Communications, Oct 1987, Vol 16, Num 11.
Williams, R., and R. Gillman "ISDN Access Protocols -
Status and Applications." National Communications Forum, 1984. pp
181-190.
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[Moderator's Note: Again, my thanks to Ray Guydosh for sending this along
to share with you. In Sunday's Digest (327) more messages from John Covert
explaining how AT&T sets the rates foreign PTT's must charge for calls to
the United States when the call is placed on an AT&T card. PT]
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End of TELECOM Digest : Introduction to ISDN
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