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Unformatted text preview: can be combined
into one 2-Mbps channel. The narrow-band ISDN, however, cannot support the
requirements of several types of data services, especially those needed for
multimedia applications. For example, the bandwidth required for full-definition
digital video is in the 100 Mbps range. To handle this sort of traffic, as well as
bursts of data traffic from computers, broad-band ISDN (often referred to as BISDN) was introduced. B-ISDN is based on optical fibers and asynchronous timedivision multiplexing. The advantage of asynchronous time-division multiplexing
over conventional time-division multiplexing is that it allows the total bandwidth
available to be divided between contending activities in a much more flexible way.
ISDN has become so popular that a term to describe the standard analog service
has been coined: Plain Old Telephone Service (POTS).
Value Added Network (VAN). Some companies specialize in providing
value added data transmission service. The value added over and above the
standard services of the common carriers may include electronic mail, data
encryption/decryption, access to commercial databases, and code conversion for
communication between incompatible computers. These companies normally
lease dedicated lines of a common carrier, do value addition to enhance the
communication facilities, and then sell that enhanced service.
This type of
service is popularly known as value-added network (VAN) service.
In a data communications network, the task of network designers is to select and
coordinate the network components so that the necessary data is moved to the
right place, at the right time, with a minimum of errors, and at the lowest possible
cost. A number of communication processors (typical micro- or mini- computers)
are used by network designers to achieve these goals. The functions of some of the
commonly used communication processors are briefly described below.
Multiplexers There are many applications in which several terminals are connected to a
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This document was uploaded on 04/07/2014.
- Spring '14