sw basicsct

7/27/2019 SW BasicsCT

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We represent a router as a black box that acceptsincoming packets from one of the input ports(interfaces), uses a routing table to find thedeparting output port, and sends the packet

from this output port.

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The crossbar switch is the simplest kind ofswitch.

In case of n input lines and n out put lines ithas n X n cross points.

where input and out put lines can beconnected by semiconductor switches.


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The problem with a cross bar switch is that thenumber of crossbars grows as the square ofthe number of lines into the switch. If weassume that all lines are full duplex and that

there are no self connect ions, only thecrosspoint above the diagonal are needed. Still,n( n - 1) / 2 crosspoints are needed.

F or n= 1000, we need 499500 crosspoint s. Itpossible to build a VLSI chip with this numberof transistor switches, but not with 1000 pinson t he chip.

Thus a single crossbar switch is only useful forrelatively small end offices.


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By splitting the cross bar switch into smallerones and interconnecting them, it is possible tobuild multistage switches with fewer crosspoints.

These are called space division switches


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The number of crosspoints needed f or a three-stage switch is 2kN + k( N/ n)2

For N = 1000, n = 50, and k = 10, we need only24000 crosspoint s instead of t he 499500 requiredby a single-stage crossbar .


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Its a completely different kind of switch. With time division switch

the n input lines are scanned in sequence to buildup an input frame with n slots .

Each slot has k bits. For T1 switches, the slots are 8 bits, with 8000

frames processed per second.


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The heart of the time division switch is the time slot interchanger,

which accepts input frames and produces out put frames , in whichthe time slots have been reordered according to mapping table inthe memory of the switch.

Finally, the out put frame is de-multiplexed with out put slot 0

going to line 0, and so on.

In essence, the switch moves data from input lines to out put linesaccording to the mapping table even though there are no physicalconnections between these lines.

The problem that limits the number of input lines to a time divisionswitch is the time necessary to transform an input frame into thecorresponding out put frame.

It is necessary to store n slots in the buffer RAM and then to readthem out again within one frame period of 125 ( sec. Wit h memoryaccess time T, we need a time interval 2nT, so wit h T = 100 nsec wecan support at most n = 125/ 2T = 625 lines.


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TCP/IP ProtocolSuite


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