394 Chapter 11: Remote Access
versatile technology. ISDN can offer a maximum bit rate of about 2 Mbps in Europe or about
1.5 Mbps in the USA and Japan.
ISDN services are targeted at both businesses and private residential users. Residential users
might occasionally use ISDN for videoconferencing or possibly teleconferencing in a company
meeting. Although even 128 K would be a slight throughput problem for video, 384 K or
bundling six 64 K bearer channels together would provide much better-quality video.
ISDN can allocate transmission bandwidth on a demand basis, which means that it suits the
bandwidth to the particular data being transmitted. A computer network often transmits data in
irregular bursts.
In addition to providing a suitable transmission medium for very different services, ISDN
guarantees a specific quality of service. It guarantees fast connection setup, low bit-error rates,
and short end-to-end delays between messages.
Common services that use ISDN are graphics and voice, which might require up to 128 Kbps
(two B channels), as compared to a text transfer, which might need as little as 4.8 Kbps.
NOTE
The CCITT/ITU-T is the body primarily responsible for producing international ISDN
standards.
ISDN Terminology
It's important to review ISDN terminology because it's often the subject of test material.
ISDN has what are called function and reference points. Terminal Endpoint Identifiers (TEIs)
are used to identify the equipment. Terminal equipment (TE) refers to the end-user devices--
the equipment actually used by the customer. Examples of TE devices are ISDN-capable
workstations, routers, or ISDN telephones.
Functions are basically representations of hardware components:
·
TE1--A device that has an ISDN component built into it, such as an ISDN external
modem.
·
TE2--A device without an ISDN component, such as a laptop.
·
TA--A terminal adapter, which converts computer electrical signals into ISDN signals.
An example is an ISDN internal modem in a PC.
·
NT1--The termination point for the two-wire local loop. This device also converts the
signals from the two-wire local loop to four-wire ISDN.
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