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COMMUNICATION TECHNOLOGIES:-
Circuit Switching Technologies
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Circuit switching is a dedicated communications path established between
two stations or multiple end points through nodes of the WAN
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Transmission path is a connected sequence of physical link between nodes.
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On each link, a logical channel is dedicated to the connection. Data
generated by the source station are transmitted along dedicated path as
rapidly as possible.
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At each node, incoming data are routed or switched to the appropriate
outgoing channel without excessive delay. However, if data processing is
required, some delay is experienced.
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Example of circuit switching above is the telephone networks.
Packet Switching Technologies
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It is not necessasry (as in circuit switching) to dedicate transmission
capacity along a path through the WAN rather data are sent out in a
sequence of small chucks, called packets.
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Each packet, consisting of several bits is passed through the network from
node to node along some path leading from the source to the destination
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At each node along the path, the entire packet is received, stored
briefly, and then transmitted to the next node.
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At destination all individual packets are assembled together to form the
complete text and message from the source. Each packet is identified as to
its place in the overall text for reassembly.
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Packet switching networks are commonly used for terminal-to-computer and
computer-to-computer communications.
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If packet errors occur, the packet is retransmitted.
Frame Relay Techniques
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Packet switching was developed at a time (1960’s) when digital long
distance transmission facilities exhibited a relatively high error rate
compared to today’s facilities. A large amount of overhead was included
for error detection and control. Each packet included additional bits and
each node performed additional processing to insure reliable transmission.
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Frame relay has removed the overhead bits and additional processing. It
has become unnecessary to invoke these overhead checks and thereby enables
higher capacity transmission rates.
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Frame relay takes advantage of these high rates and low error rates.
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Frame relay networks are designed to operate efficiently at user data
rates of 2 Mb/s and higher. (packet switching originally designed with a
64 Kb/s data rate to the end user).
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Frame relay achieves these higher rates by stripping out most of the
overhead involved with error control.
Asynchronous Transfer Mode (ATM)
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ATM also referred to as “Cell Relay”
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Evolution from frame relay and circuit switching.
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Major differences: Frame relay uses variable length packets called
“frames”. ATM uses fixed length packets called “cells”.
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ATM provides little overhead for error control like frame relay, and
depends on inherent reliability of the transmission system and on higher
layers of logic in the end systems to identify and correct errors.
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ATM is designed to operate in range of 10s to 100 Mb/s compared to frame
relay (2 Mb/s)
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ATM allows multiple virtual channels with higher data rates for
transmission paths. Each channel dynamically sets on demand.
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