What is ISDN?

ISDN, which stands for integrated services digital network, is a system of
digitizing phone networks which has been in the works for over a decade. This
system allows audio, video, and text data to be transmitted simultaneously
across the world using end-to-end digital connectivity.

The original telephone system used analog signals to transmit a signal across
telephone wires. The voice was carried by modulating an electric current with a
waveform from a microphone. The receiving end would then vibrate a speaker coil
for the sound to travel back to the ear through the air. Most telephones today
still use this method. Computers, however, are digital machines. All information
stored on them is represented by a bit, representing a zero or a one. Multiple
bits are used to represent characters, which then can represent words, numbers,
programs, etc. The analog signals are just varying voltages sent across the
wires over time. Digital signals are represented and transmitted by pulses with
a limited number of discrete voltage levels. [Hopkins]

The modem was certainly a big breakthrough in computer technology. It allowed
computers to communicate with each other by converting their digital
communications into an analog format to travel through the public phone network.
However, there is a limit to the amount of information that a common analog
telephone line can hold. Currently, it is about 28.8 kbit/s. [Hopkins] ISDN
allows multiple digital channels to be operated simultaneously through the same
regular phone jack in a home or office. The change comes about when the
telephone company\'s switches are upgraded to handle digital calls. Therefore,
the same wiring can be used, but a different signal is transmitted across the
line. [Hopkins] Previously, it was necessary to have a phone line for each
device you wished to use simultaneously. For example, one line each for the
phone, fax, computer, and live video conference. Transferring a file to someone
while talking on the phone, and seeing their live picture on a video screen
would require several expensive phone lines. [Griffiths] Using multiplexing (a
method of combining separate data signals together on one channel such that they
may be decoded again at the destination), it is possible to combine many
different digital data sources and have the information routed to the proper
destination. Since the line is digital, it is easier to keep the noise and
interference out while combining these signals. [Griffiths] ISDN technically
refers to a specific set of services provided through a limited and standardized
set of interfaces. This architecture provides a number of integrated services
currently provided by separate networks.

ISDN adds capabilities not found in standard phone service. The main feature is
that instead of the phone company sending a ring voltage signal to ring the bell
in your phone, it sends a digital package that tells who is calling (if
available), what type of call it is (data/voice), and what number was dialed (if
multiple numbers are used for a single line). ISDN phone equipment is then
capable of making intelligent decisions on how to answer the call. In the case
of a data call, baud rate and protocol information is also sent, making the
connection instantaneous. [Griffiths] ISDN Concepts:

With ISDN, voice and data are carried by bearer channels (B channels) occupying
a bandwidth of 64 kbit/s each. A delta channel (D channel) handles signalling at
16 kbit/s or 64 kbit/s. H channels are provided for user information at higher
bit rates. [Stallings] There are two types of ISDN service: Basic Rate ISDN
(BRI) and Primary Rate ISDN (PRI).

BRI: consists of two 64 kbit/s B channels and one 16 kbit/s D channel for a
total of 144 kbit/s. The basic service is intended to meet the needs of most
individual users. PRI: intended for users with greater capacity requirements.
Typically the channel structure is 23 B channels plus one 64 kbit/s D channel
for a total of 1.544 Mbit/s. H channels can also be implemented: H0=384 kbit/s,
H11=1536 kbit/s, H12=1920 kbit/s. [Stallings]

In this paper, I will concentrate on defining the specifics of Basic Rate ISDN
for local loop transmission. I will provide an in depth view of ISDN as it
relates to layer 1 to 3 of the seven layer OSI model. I will also provide the
specification for communication at the S/T customer interface.

Basic Rate ISDN:

Basic Rate Interface (BRI) - The BRI is the fundamental building block of an
ISDN network. It is composed of a single 16 kbit/s "D-channel" which is used for
call setup and control and two 64 kbit/s "B-channels". The B-channels can be
used to carry voice and both circuit mode and packet mode