Electronic Switching Systems.
• A network is a set ofSWITCHING:
connected devices.
• These devices are connected to each other through switches.
• Switches are capable of creating temporary connections between
two or more devices linked to the switch.
• In a switched network, some of these nodes are connected to the
end systems (Computer or telephones).
• Others are used only for routing. Figure 6.1 shows a switched
network.
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Three methods of switching:
Circuit switching
Packet switching
Message switching
Switches are used in circuit switching and packet switching
A circuit-switched network |
Packet Switching
Circuit switching
• Circuit switching uses two technologies
• Space-division switch
• Time-division switch
Limitation of crossbar switch:
Multistage switch:
• The problem in crossbar switches can be solved by using
multistage switches in which we combine crossbar switches in
several stages.
• In this case devices are linked to switches that in turn are linked to
a hierarchy of other switches.
• The design of a multistage switch depends on the number of
stages and the number of switches required in each stage.
• Normally the middle stages have fewer stages than do the first and
the last stages.
Multiple Paths:
Blocking:
•
Multistage switches will reduce the cost but reduction in cross
points will result in blocking during periods of heavy traffic.
• Blocking refers to times when one input cannot be connected to
output because there is no path available.
• In case of crossbar switch blocking does not occur because every
combination of input and output has its own cross points.
• In case of multistage switch only two of five inputs can use the
switch at a time which will cause the problem of blocking during
period of heavy traffic.
Time Division Switches:
• It uses time division multiplexing to achieve
switching. There are two methods to implement:
• Time slot Interchange (TSI)
• Time division multiplexing (TDM) bus
Time slot interchange (TSI):
• Figure 6.4 shows a system connecting four input lines to four
output lines. Imagine each input line wants to send data to an
output line according to following pattern. In the figure 6.4
we can see that the desired task cannot be accomplished.
• The data in the output will be in the same order of input. Thus data can
be moved only from 1 to 1,2 to 2, 3 to 3 and 4 to 4.
• In figure 6.5 a device called Time slot interchange (TSI) is inserted into
the link.
• A TSI changes the ordering of the slots based on the desired
connections. It changes the order of data from A, B, C and D to C, D,
A and B and the de-multiplexer separates it to the proper outputs.
• TSI consists of random access memory (RAM) with several memory
locations.
• The size of each location is the same as the size of single time slot.
• The RAM fills up the incoming data from time slots in the order
received.
• Slots are then sent out in an order based on the decisions of a control
unit
TSI
Time
Division Multiplexing (TDM) Bus:
• In the figure 6.6, the input and output lines are connected to a high speed
bus through input and output gates.
• Each input gate is closed during one of the four time slots.
• During the same time slot, only one output gate is also closed.
• This pair of gates allows a burst of data to be transferred
from one specific input line to one specific output line using
the bus.
• The control unit opens and closes the gates according to
switching need.
• For example, at the first time slot the input gate 1 and output
gate 3 will be closed; during the second time slot input gate 2
and output gate 4 will be closed and so on.
Space and Time Division
Switching combinations:
• The advantage of Space division switching is that it is
instantaneous and the disadvantage is the number of cross
points.
• The advantage of TDM is that it needs no cross points but
the disadvantage is the delay introduced due to processing
each connection.
• Thus by combining both of them, we will take the
advantage of both and remove the disadvantage of both.
• Thus multistage switches can be designed with combination
of time- space- space-time (TSST), space-time-time-space
(STTS).
• Figure 6.7 shows a simple time-space-time (TST) switch that consists of
two time stages and one space stage and has 12 inputs and 12 outputs.
• In this example instead of one TDM switch, it divides input into three
groups of four inputs each and directs them to three time slot interchanges.
• Thus the delay is minimized one- third of that which would result from
using one time slot interchange to handle all 12 inputs
• The last stage is same like first stage and the middle is space division switch
(Crossbar) that connects TSI groups together to allow connectivity between
all possible input and output pairs.