ISDN Tutorial
- Integrated Services Digital Network (ISDN) digital
telecommunications system
ISDN or Integrated Services Digital Network is an
international standard for end to end digital transmission of voice, data and
signaling. It can operate over copper based systems and allows the transmission
of digital data over the telecommunications networks, typically ordinary copper
based systems and providing higher data speeds and better quality than analogue
transmission. The ISDN specifications provide a set of protocols that enable the
set up, maintenance and completion of calls.
ISDN, Integrated Services Digital Network, provides a number
of significant advantages over analogue systems.
In is basic form it enables two simultaneous telephone calls to be made over the
same line simultaneously Faster call connection. It typically takes a second to
make connections rather than the much longer delays experienced using purely
analogue based systems. Data can be sent more reliably and faster than with the
analogue systems Noise, distortion, echoes and crosstalk are virtually
eliminated The digital stream can carry any form of data from voice to faxes and
internet web pages to data files - this gives the name 'integrated services'
Usage
ISDN is in use around the world, but with the introduction of
ADSL it is facing strong competition. The technology never gained much market
share in the USA, although it used in other countries. In Japan it became
reasonably popular in the late 1990s although it is now in decline with the
advent of ADSL. The system was also introduced in Europe where providers such as
BT, France Telecom and Deutsche Telekom introduced services.
Configurations
There are two types of channel that are found within ISDN.
These are the 'B' and 'D' channels. The B or 'bearer' channels are used to carry
the payload data which may be voice and / or data, and the d or 'Delta' channel
is intended for signalling and control, although it may also be used for data
under some circumstances.
Additionally there are two levels of ISDN access that may be
provided. These are known as BRI and PRI.
BRI (Basic Rate Interface) - This
consists of two B channels, eac of which provides a bandwidth of 64 kbps under
most circumstances. One D channel with a bandwidth of 16 kbps is also provided.
Together this configuration is often referred to as 2B+D.
The basic rate lines connect to the network using a standard
twisted pair of copper wires. The data can then be transmitted simultaneously in
both directions to provide full duplex operation. The data stream is carried as
two B channesl as mentioned above, each of which carry 64 kbps (8 k bytes per
second). This data is interleaved with the D channel data and this is used for
call management: setting up, clearing down of calls, and some additional data to
maintain synchronisation and monitoring of the line.
The network end of the line is referred to as the 'Line
Termination' (LT) while the user end acts as a termination for the network and
is referred to as the 'Network Termination' (NT). Within Europe and Australia,
the NT physically exists as a small connection box usually attached to a wall
etc, and it converts the two wire line (U interface) coming in from the network
to four wires (S/T interface or S bus). The S/T interface allows up to eight
items or 'terminal equipments' to be connected, although only two may be used at
any time. The terminal equipments may be telephones, computers, etc, and they
are connected in what is termed a point to point configuration. In Europe the
ISDN line provides up to about 1 watt of power that enables the NT to be run,
and also enables a basic ISDN phone to be used for emergency calls. In North
America a slightly different approach may be adopted in that the terminal
equipment may be directly connected to the network in a point to point
configuration as this saves the cost of a network termination unit, but it
restricts the flexibility. Additionally power is not normally provided.
PRI (Primary Rate Interface) - This
configuration carries a greater number of channels than the Basic Rate Interface
and has a D channel with a bandwidth of 64 kbps. The number of B channels varies
according to the location. Within Europe and Australia a configuration of 30B+D
has been adopted providing an aggregate data rate of 2.048 Mbps (E1). For North
America and Japan, a configuration of 23B+1D has been adopted. This provides an
aggregate data rate of 1.544 Mbps (T1).
The primary rate connections utilise four wires - a pair for
each direction. They are normally 120 ohm balanced lines using twisted pair
cable. Primary rate connections always use a point to point configuration.
Primary rate lines are widely used to conenct to Private
Branch eXchanges (PBX) in an office etc. Typically this may be used to provide a
number of POTS (Plain Old Telephone System) or basic rate ISDN lines to the
users.
Summary
Although ISDN is has been overtaken by technologies such as
ADSL it is nevertheless still widely used in many areas, particularly where
existing services need to be maintained, or where compatibility needs to be
guaranteed. As such it is still an important technology that will be encountered
for many years to come.
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