13-03-2014, 02:20 PM
Switching Techniques
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INTRODUCTION
For transmission of data, communication is typically achieved
by transmitting data from source to destination through a network of
intermediate switching nodes. The switching nodes are not
concerned with the content of the data; rather, their purpose is to
provide a switching facility that will move the data from node to
node until they reach their destination.
Circuit Switching
Circuit switching is the most familiar technique used to
build a communications network. It is used for ordinary
telephone calls. It allows communications equipment and
circuits to be shared among users. Each user has sole access to a
circuit (functionally equivalent to a pair of copper wires) during
network use.
Switching Concept
Time-division switching is based on the use of
synchronous time-division multiplexing (TDM).
For a switch that supports 100 devices, there must be 100
repetitively occuring time slots, each one assigned to an input
and an output line. One iteration for all time slots is referred to
as a frame.
Routing in Circuit-Switched Networks
Two broad classes of routing algorithms have been implemented:
static routing and dynamic routing. The static routing algorithm is
not able to adapt to changing conditions. Thus, the network must be
designed to meet some typical heavy demand. A dynamic routing
approach is one in which routing decisions are influenced by current
traffic conditions
Alternate Routing
The essence of alternate-routing schemes is that the possible
routes to be used between two end offices are predefined. Each
switch is given a set of preplanned routes for each destination.
Source switch selects appropriate route. The preferred choice is a
direct trunk connection between two switches. If this trunk is
occupied, then select alternate route from remaining routes.
A simple example is shown in the figure. The originating
switch X has four possible routes to the destination switch Y. The
direct route (a) will always be tried first. If this trunk is unavailable
(busy, out of service), the other routes will be tried in a particular
order, depending on the time period. For example, during weekday
mornings, route (b) is tried next.