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Switch is a device in network which is used to route call to desired destination.

First generation systems used to have electromechanical STROWGER switches ( based on relay functioning )…next generation systems use LOGIC GATES for switching which is done with help from number dialed by subscriber.

Switching fundamentals are based on ::

a)     TDM—time division multiplexing

b)     PACKET SWITCHING ( NGN NETWORKS—next generation networks )

Telecom network deploys switches based on number of subscribers and pattern of dialing i.e number of calls within access network ( local calls ) / number of calls on long distance network ( STD calls or intercity calls ) / number of calls on international network ( ISD calls ).

Typical diagram of PSTN ( public switched telephone network ) for analog / digital fixed telephone lines ( fig-7 ) ::

a)     local calls :: subscriber of location A calls to subscriber of location B & vice-versa…this routing is done by access or local switch

b)     national long distance or STD calls :: subscriber of city X calls to subscriber of city Y & vice-versa…this routing is done by both local & long distance switch

c)      international long distance or ISD calls :: subscriber of city X in country X’ calls to subscriber of city Y in country Y’…this routing is done by local, long distance & gateway switches

Traffic pattern in switches is measured in ERLANGS.

ERLANG is defined as = no. of calls generated x duration of calls

                                        --------------------------------------------------------

                                              Busiest hour of the day

0.1 ERLANGS / subscriber --has a traffic of 01 call generated for 6 min during busiest hour of the day

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