Integrated Services Digital Network
Integrated Services Digital Network (ISDN) is a
circuit-switching technology that enables the local loop of a PSTN to carry
digital signals, resulting in higher capacity switched connections. ISDN
changes the internal connections of the PSTN from carrying analog signals to
time-division multiplexed (TDM) digital signals.
ISDN turns the local loop into a TDM digital connection.
This change enables the local loop to carry digital signals that result in
higher capacity switched connections. The connection uses 64 kb/s bearer
channels (B) for carrying voice or data and a signaling, delta channel (D) for
call setup and other purposes.
There are two types of ISDN interfaces:
Basic Rate Interface (BRI)-ISDN is intended for the home and
small enterprise and provides two 64 kb/s B channels and a 16 kb/s D channel.
The BRI D channel is designed for control and often underused, because it has
only two B channels to control. Therefore, some providers allow the D channel
to carry data at low bit rates, such as X.25 connections at 9.6 kb/s.
Primary Rate Interface (PRI)-ISDN is also available for
larger installations. PRI delivers 23 B channels with 64 kb/s and one D channel
with 64 kb/s in North America, for a total bit rate of up to 1.544 Mb/s. This
includes some additional overhead for synchronization. In Europe, Australia,
and other parts of the world, ISDN PRI provides 30 B channels and one D
channel, for a total bit rate of up to 2.048 Mb/s, including synchronization
overhead.
For small WANs, the BRI ISDN can provide an ideal connection
mechanism. BRI has a call setup time that is less than a second, and the 64
kb/s B channel provides greater capacity than an analog modem link. If greater
capacity is required, a second B channel can be activated to provide a total of
128 kb/s. Although inadequate for video, this permits several simultaneous
voice conversations in addition to data traffic.
Another common application of ISDN is to provide additional
capacity as needed on a leased line connection. The leased line is sized to
carry average traffic loads while ISDN is added during peak demand periods.
ISDN is also used as a backup if the leased line fails. ISDN tariffs are based
on a per-B channel basis and are similar to those of analog voice connections.
With PRI ISDN, multiple B channels can be connected between
two endpoints. This allows for videoconferencing and high-bandwidth data
connections with no latency or jitter. However, multiple connections can be
very expensive over long distances.
Note: Although ISDN is still an important technology for
telephone service provider networks, it is declining in popularity as an
Internet connection option with the introduction of high-speed DSL and other
broadband services.
NB;
PSTN –Public Switched Telephone Network -This’ a general term referring to the
variety of telephone networks and services in place worldwide.
TDM Time Division Multiplexing – Technique in which info.
From multiple channels can be allocated bandwidth on a single wire based on
preassigned time slots.
Cavin Ouma
BMCS, CCNA, CISA(ON GOING)
