Circuit Switching

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Switches: hardware and/or software devices allowing temporary connections between two or more devices...

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Nội dung Text: Circuit Switching

Circuit Switching by Dr. Nguyen Minh Hoang hoang.dhbk@gmail.com March 2007
References  Reference books:  Data Communications and Networking, 4/e, by Behrouz A. Forouzan, DeAnza College (http://highered.mcgraw hill.com/sites/0072967757/information_center_view0/)  Data & Computer Communications by William Stallings (http://williamstallings.com/DCC/DCC7e.html)
Introduction  How can we connect multiple devices?  Pointtopoint vs.  Multipoint  Better solution  switching  Switches: hardware and/or software devices allowing temporary connections between two or more devices  Three methods: circuit, packet, and message
Example of a Switched Network
Circuit Switching  Creates a direct physical connection between two devices  Originally developed for voice traffic  Does not use pointtopoint connections between every device to every other device  Instead, devices link to a switch which may in turn link to other switches  Allows for a dedicated connection for duration of transmission
Circuit Switching  Spacedivision switching Circuit switching: create a direct  Timedivision switching physical connection between 2  TDM Bus devices such as phones or computers  Combinations
Circuit Switch vs. Folded Switch n inputs, m outputs n lines in full duplex Folded Switch: An nbyn folded switch allows every device to connect to every other device in fullduplex mode
SpaceDivision Switch  The paths in the circuit are separated from each other spatially  Originally for analog networks, but now for both analog and digital  Crossbar switch  Multistage switch
Crossbar Switch Inputs and outputs connected in a grid using electronic microswitches (transistors) at crosspoints Disadvantage??? Too many crosspoints, only a few used at any given time Inefficient; fewer than 25% of switches are in used at a given time
Multistage Switches  Combine crossbar switches in several stages  Devices are linked to switches that are linked to a hierarchy of other switches  Middle stages usually have fewer switches than first and last stages  Fewer crosspoints but still allowing multiple paths through the network  increased reliability
Multistage Switch (1) A set of crossbar switches in several stages Use a 15x15 crossbar => 225 crosspoints Use the above multistage =>78 crosspoints
Multistage Switch Multiple paths available for each pair of devices => reliable Disadvantage??? Blocking problem when heavy traffic, experienced in public phone systems
Blocking  Reduced number of crosspoints may mean that at times of heavy traffic, an input may not be able to connect to an output if there is no path available (i.e. all switches are occupied)  Does not occur in singlestage switch; a nonblocking path is always available
TimeDivision Switches  Uses timedivision multiplexing to achieve switching  Used by most modern switches  TD switching involves partitioning low speed bit stream into pieces that share higher speed stream  Two methods:  Timeslot interchange (TSI)  TDM bus  Based on synchronous time division multiplexing (TDM)  Each station connects through controlled gates to high speed bus  Time slot allows small amount of data onto bus  Another line’s gate is enabled for output at the same time
TDM with a Time Slot Interchange Without a TSI, order is unchanged 1->3, 2->4, 3->1, 4->2
Time Slot Interchange (TSI)  Consists of RAM with several memory locations  Fills up with incoming data from time slots in order received  Slots are sent out in order based on control unit logic #memory locations = # inputs Cannot handle bursty size of mem. loc. = size of a time slot traffic
TDM Bus  Input and output lines are connected to a highspeed bus through input and output gates  Control unit opens and closes gates as needed
Time vs. Space Division Switch  Time division  No crosspoints  Delay  Space division  Many crosspoints  Zero delay
Space and TimeDivision Switching Combinations  Advantage of spacedivision switching is it is instantaneous; however, requires too many crosspoints  Advantage of timedivision switching is it needs no crosspoints; however, processing connections creates delays  Combining them optimizes the number of crosspoints and reduces the amount of delay  TSS: TimeSpaceSpace  TSST: TimeSpaceSpaceTime  STTS: SpaceTimeTimeSpace
TST Switch