ISDN: The Cutting Edge

Old

it might be, but even today few technologies match Integrated

Services Digital Network (ISDN) in terms of the combination of

simplicity and flexibility that it provides. In fact, ISDN as a

technology was ahead of its time when it was conceived

initially. Its perceived failure during its earlier years

happened because the rest of the network (ISDN is just an access

solution) was not ready for services. Nor was the user.

The popularity of the

Internet has revived the interest in ISDN recently. But before

we delve into the details, it would be apt to touch the

fundamentals once again.

What Is It?

In

simple terms, ISDN is a replacement for Plain Old Telephone

Services (POTS), which was never designed to meet the needs of

the information age. "Integrated Services" refers to

its ability to sustain numerous applications, and "Digital

Network" relates to its end-to-end digital connections.

Generally, ISDN networks extend from the local telephone

exchange to the remote user and include all of the telecom and

switching equipment in between. ISDN is based on technology

developed during the 1970s, designed to address the problem of

how to transport digital services across a telephony

infrastructure based on copper wiring originally intended to

carry analog signals only.

ISDN uses the same wiring

that currently serves homes and businesses. You get ISDN service

from the same companies who provide telephone service, and you

use it to connect telephones, computers, and fax machines. The

difference is that you get much faster, more dependable

connections for voice, data, fax, and even video–all through a

single line. There is no other technology that comes close to

delivering such communications benefits today.

ISDN enables faster, more

reliable digital connections over the existing telephony

infrastructure. It offers high quality transmissions with very

low error rates besides providing extreme flexibility–connections

can be established at any time between two locations where ISDN

is available. Tariffed just like a telephone call, ISDN is an

economical proposition as well.

The Different

Configurations

BRI,

the most common ISDN service, provides two 64 Kbps channels per

line. When the two channels are bonded in a single connection,

you get a speed of 128 Kbps, which is about four times the

actual top speed of the fastest analog modems.

BRI service is by far the

most common and is typically found in homes and businesses

alike. The higher capacity PRI service is a central site

solution for extending applications to large numbers of remote

users, who communicate through their BRI connections.

ISDN PRI includes 23

B-channels in North America and Japan (30 B-channels in most of

the rest of the world), plus one 64 Kbps D-channel. The number

of B-channels is limited by the size of the standard trunk line,

which is T1 in North America and Japan and E1 elsewhere. A single PRI connection is

usually much less expensive than obtaining the equivalent number

of B-channels through multiple BRI connections. The primary

benefit of PRI is that its bandwidth can be dynamically

allocated among applications. This is usually accomplished via a

PBX or a server capable of distributing the T1/E1 bandwidth on a

PRI link.

ISDN lines are available

in a number of configurations with Basic Rate Interfaces (BRI)

supporting two 64 Kbps connections, and Primary Rate Interfaces

(PRI) supporting up to 30 such connections (23 in North America

& Japan).

Understanding

the Technology: OSI Model

The

Open Systems Interconnection (OSI) Model provides a framework

that can be used to understand ISDN technology. Figure 1 defines

seven layers of protocols, each of which is used in some way to

facilitate communications across a network. Lower layers relate

to physical and bit-level networking protocols (e.g. physical

connector and bit-level error detection definitions), while

higher layers are usually associated with protocols that allow

applications to communicate with each other across a network

(e.g. the "language" used between an e-mail server and

client).

Specific functions for

each layer are as follows:

(7) Defines the

protocols used for application connectivity

(6) Defines a set of

non-application specific protocols

(5) Specifies

process-to-process communications

(4) Defines a means to

communicate across a network (more than a single physical

connection), error free

(3) Specifies the

protocol used  to link devices that span multiple

connections

(2) Defines the

protocol used in error-free data transfers across a single

physical connection

(1) Defines electrical

and mechanical characteristics of the protocol used to

transfer bits between devices

Although ISDN technology

is defined at the three lowest layers of the model, it is also

considered as merely a physical connection, a high-speed

"pipe" through which protocols at Layer 2 or higher

may be transferred. The reason for these two views is related to

the fact that the protocols used on different types of ISDN

channels are different, with D-channels defined at Layers 2 and

3, and B-channels able to carry anything at Layer 2 or higher.

Always

On/Dynamic ISDN

The

process of moving data through a network is called switching.

ISDN takes advantage of two types of switching:

circuit-switching and packet-switching.

Circuit-switching moves

data between two points by setting up a physical link, or

circuit, between them. You may compare this to a regular

telephone call. Data flows in a stream along a circuit that

lasts as long as necessary, at which time the switches and lines

used to build the circuit are freed for another connection.

The chief advantage of

circuit-switching is that the flow of data is not subject to

delays in the network. The disadvantage is that much of the

connection’s available bandwidth may be wasted due to the

bursty nature of data traffic, which rarely saturates the link’s

capacity. This is especially true when a user keeps the ISDN

B-channel "nailed up" as in continuous remote LAN

access. To address this problem, a new, emerging standard known

as Always On/Dynamic ISDN takes advantage of ISDN’s inherent

packet-switching capability, and joins it with

circuit-switching.

Packet-switching differs

from circuit switching in that data is segmented into discrete

units, or packets. Each packet contains a piece of the original

data, plus information about the sender, the recipient, and

where that packet fits in with the others. Packet-switching

networks simply forward these packets from one switch to another

until they are delivered at their destination. No dedicated

connection is ever formed.

The advantage of

packet-switching is that short messages can be transferred with

little latency since no end-to-end link needs to be set up.

Also, carrier bandwidth can be shared by a large number of

users, resulting in lower costs.

The new AO/DI standard

combines the best of circuit-switching and packet-switching to

provide telephone companies, ISPs, and end users with what they

truly need–bandwidth on demand, with minimal waste. It

addresses the problem of nailed-up connections by allowing users

to keep them, by making better use of the lower-bandwidth ISDN

D-channel and freeing up B-channels until they are actually

needed.

AO/DI initiates each

connection using X.25 on the D-channel, where it maintains an

open link. This eliminates a major burden on the

circuit-switched network; X.31 traffic is carried on the telco

packet network, whose virtual circuits are tailor-made for

low-cost, less data-intensive activity. When more bandwidth is

required for such data operations as file transfers, the

Bandwidth Allocation Control Protocol automatically switches to

one B-channel (64 Kbps) or both B-channels (128 Kbps). When

activity settles down, the B-channels are automatically

"un-nailed," with communication transparently

reverting to the D-channel.

That lets ISDN users

remain always "on" a remote LAN or the Internet

without wasting capacity, and provides immediate access to all

the bandwidth they truly need when data activity steps up. The

D-channel’s X.31 link provides ample capacity for e-mail

notification (or even e-mail itself), pointcast data, schedule

updates, and channel-subscription delivery to Web browsers, and

keeps both B-channels open for voice calls and faxing until

needed for data. Throughout the network, B-channels are

dynamically and transparently allocated only to those who are

actually making use of them. Telcos and ISPs are able to better

handle traffic loads, even in the heaviest peaks, with efficient

B-channel sharing.

If you have investigated

getting ISDN in the past, you may have heard that it is more

difficult to get than regular telephone service. That is

especially true in India, where the "novelty" of the

technology has just recently begun to give way to business as

usual.

The telephone companies

have already made the investment that delivers ISDN to the world’s

doorsteps. The existing telephone wiring in your home or

business should be compatible with ISDN, and your ISDN provider

will outfit you with an appropriate wall jack. Still, how easy

it is to order ISDN depends on the experience of your carrier.

You can contact the telephone company or other ISDN provider for

your region. True Cost of

ISDN

There

are three basic cost components to an ISDN line: set-up fee,

monthly fee, and line fee. All vary widely from area to area,

meaning that the real cost of ISDN depends on mainly where you

are.

In general, set-up fees

are minimal, and of course non-recurring. Monthly fees tend to

be competitive with POTS and, considering that each ISDN BRI

line is actually two lines, are usually quite reasonable. The

line fee–a per-minute charge for actual line usage (where

applicable)–is typically the fee to consider most when looking

into ISDN.

In some areas, the real

cost of ISDN lines is lower than the cost of business phone

lines, making the decision to use ISDN an easy one. In areas

with high line fees, real usage must be scrutinized to justify

the rate. How much time will the telecommuter spend accessing

the LAN? How many file transfers are involved in a medical

imaging application? How many hours a day does a particular

worker need to spend on the Web? If those events are time

intensive with modems and it is best that they happen quickly,

over a reliable connection, the cost of ISDN is typically highly

justified.

Real World

ISDN Applications

If

you plan to connect to the Internet via ISDN, you will need

Internet service from a provider with ISDN support. Premier ISPs

typically supply ISDN service to both individuals and

organizations at a reasonable cost. Simply contact an ISDN ISP

with coverage in your area.

Internet access is one of

the most obvious, and popular, applications for ISDN. Internet

access, however, is really just the beginning of the advantages

that come with ISDN’s higher speeds, clear connections, and

broad communications capabilities. But there are some more

innovative ways in which people are using ISDN today.

• Telecommuting
  
  
  
  "Telecommuting"–which can mean a complete,
  remote office or occasional work at home–has been a hot
  
  topic for some time. ISDN, more than any other innovation,
  
  is making it a practical reality. With a single ISDN line
  
  and an ISDN adapter, telephone, and fax machine, remote
  
  professionals can enjoy the same communications capabilities
  
  as office-bound workers.
  
  
  
  
  
  
  In terms of data communications, ISDN succeeds in making
  remote LAN access seem very much like being locally attached
  
  to the LAN. And because a single ISDN line can carry
  
  simultaneous data and analog communications, such as
  
  telephone calls and faxes, one line can turn the home
  
  desktop
  
  
  
  into an efficient, cost-effective communications centre. For
  workers
  
  
  
  whose jobs require a good deal of face-to-face
  communications, ISDN-based videoconferencing can complete
  
  the picture. Large-scale
  
  file transfers
  
  
  
  
  
  
  
  
  As computer applications have become increasingly
  graphically oriented, PC communications have the ability,
  
  more than ever, to include the pictures, sounds, and even
  
  full-motion video that enable people to show rather than
  
  just tell. Before ISDN, it was not practical to communicate
  
  this way over distance–downloading a 1 megabyte file with
  
  even the fastest modem takes several minutes. With ISDN’s
  
  faster speed and compression, it takes seconds.
  
  
  
  
  
  
  If you have ever encountered a web site that features a
  video clip you are supposed to watch on your screen, you
  
  understand the limits of modems in multimedia
  
  communications. With ISDN, however, multimedia is not only
  
  practical, it is welcome.
  
  
  
  
  
  
  And web pages are just the start. Many organizations are
  beginning to turn to imaging systems to store documents,
  
  rather than keeping endless files of paper. Once a document
  
  has been scanned, it is a graphical file that can be viewed
  
  on a computer screen. To transmit it to someone requires the
  
  bandwidth of ISDN, and it is becoming very popular with
  
  publishers, banks, photo agencies, hospitals, police
  
  departments, manufacturers, government agencies, and
  
  countless other industry segments that are driven by
  
  digitally stored documents that need to be sent to others.
  
  
  
  
  
  
  
  
  
  
  
  
  
• Education
  
  
  
  "Distance learning" can be thought of as
  telecommuting for students. And just as ISDN has made
  
  telecommuting a reality for thousands of professional
  
  workers, it is making distance learning a practicality for
  
  ground-breaking public and private grade schools, high
  
  schools, and colleges throughout North America.
  
  Computer-based, interactive voice, data, image, and video
  
  can go a long way toward helping students who are unable to
  
  physically visit a classroom to participate in the learning
  
  experience.
  
  
  
  
  
  
  
  
  
• Videoconferencing
  
  
  
  While videoconferencing has been around for awhile,
  widespread videoconferencing is just beginning to come into
  
  its own. The ability to transmit quality video and voice
  
  across distances used to require very expensive equipment,
  
  and costly leased-lines, that were justified only by the
  
  most rigorous needs in the largest of companies. And because
  
  they depended on leased-lines, those videoconferencing
  
  solutions were point-to-point; a headquarters facility could
  
  be linked to satellite facilities, for example, but
  
  videoconferencing on the scale of teleconferencing was
  
  simply impractical.
  
  
  
  
  
  
  ISDN, along with new, lower-cost hardware, is changing that–rapidly.
  Both desktop videoconferencing (where a participant joins
  
  from a PC equipped with ISDN, a video camera, and a
  
  microphone), and conference room videoconferencing (where
  
  more sophisticated equipment such as remote control cameras
  
  allow group participation) have become as easy to set up as
  
  voice conferencing. And because of ISDN’s versatility,
  
  videoconferences can include the sharing of documents,
  
  images, and other files with all participants, through file
  
  transfers or screen display. This capability is proving
  
  highly popular for workgroup collaboration, telecommuting,
  
  security and surveillance, and dozens of highly innovative
  
  applications.

  Alternatives to ISDN

  ISDN is just one of the technologies available to you for linking LANs. Others are: Leased-line: Usually installed for a minimum of one year, leased-lines are best suited for constant data transmission. You pay the charges even if you are not actually transmitting any data. Highly expensive for ad hoc communication. X.25: The charges are based on line-speed, call duration, and volume of data sent. Good for interactive applications but expensive for file transfer. Dial-up: Limited by speed, dial-up services can be of poor quality. The call set-up time can be very long. Suitable for short, infrequent communication. ISDN offers conspicuous advantages over all the above. It is tariffed like a normal phone call, based on call duration, time of day and distance. It is most cost-effective for file transfer as there is no charge on volume of data transferred. The in-built security makes it a better bet than dial-up.

  
  
  For general business use, ISDN videoconferencing can make
  
  meetings much more practical and productive. There is far
  
  less travel time and expense involved, as participants no
  
  longer need to physically congregate. That also makes
  
  scheduling much easier. And since participants are typically
  
  at their own offices, videoconferencing means everyone has
  
  access to their computers and file cabinets for information
  
  during the meeting. Other people can also be called into the
  
  meeting as needed.
  
  
  
  
  
  
  
  

  
  
  
  
• Telephony
  
  
  
  Much of the excitement regarding ISDN focuses on its data
  and video capabilities. It is important to remember that
  
  ISDN is also a voice solution, providing two clear,
  
  all-digital telephone connections per BRI line. Connections
  
  are established in 2 to 4 seconds, versus the 10 to 30
  
  seconds required for analog telephone connections. And
  
  because of ISDN’s digital nature, ISDN telephone sets
  
  include microprocessors that can drive features unavailable
  
  in regular telephones, such as automated call-back and
  
  direct links to computer-based call centre
  
  applications.