Information Technology Reference
In-Depth Information
G
ISDN
G.1 Introduction
A major problem in data communications and networks is the integration of real-time sam-
pled data with non-real-time (normal) computer data. Sampled data tends to create a constant
traffic flow whereas computer-type data has bursts of traffic. In addition, sampled data nor-
mally needs to be delivered at a given time but computer-type data needs a reliable path
where delays are relatively unimportant.
The basic rate for real-time data is speech. It is normally sampled at a rate of 8 kHz and
each sample is coded with eight bits. This leads to a transmission bit rate of 64 kbps. ISDN
uses this transmission rate for its base transmission rate. Computer-type data can then be
transmitted using this rate or can be split to transmit over several 64 kbps channels. The basic
rate ISDN service uses two 64 kbps data lines and a 16 kbps control line, as illustrated in
Figure G.1. Table G.1 summarizes the I series CCITT standards.
Typically, modems are used in the home for the transmission of computer-type data. Un-
fortunately, modems have a maximum bit rate of 56 kbps. With ISDN, this is automatically
increased, on a single channel, to 64 kbps. The connections made by a modem and by ISDN
are circuit switched.
64 kbps
(B channel)
ISDN
connection
16 kbps
(D channel
for control)
Circuit-
switched
connection
64 kbps
(B channel)
ISDN
connection
16 kbps
(D channel
for control)
Figure G.1
Basic rate ISDN services
