What is it all about? Types of switching elements How do we move traffic from one part of the network to another? How do we move traffic from one part of the network to another? Telephone switches Telephone switches Connect end-systems to switches, and switches to each other Connect end-systems to switches, and switches to each other switch samples switch samples Datagram routers Datagram routers Switching Data arriving to an input port of a switch have to be moved to Data arriving to an input port of a switch have to be moved to one or more of the output ports one or more of the output ports switch datagrams switch datagrams ATM switches ATM switches switch ATM cells switch ATM cells An Engineering Approach to Computer Networking An Engineering Approach to Computer Networking Classification Other switching element functions Requirements Packet vs. circuit switches Packet vs. circuit switches Participate in routing algorithms Participate in routing algorithms Capacity of switch is the maximum rate at which it can move Capacity of switch is the maximum rate at which it can move information, assuming all data paths are simultaneously active information, assuming all data paths are simultaneously active packets have headers and samples don packets have headers and samples don ʼ t to build routing tables to build routing tables Connectionless vs. connection oriented Connectionless vs. connection oriented Resolve contention for output trunks Resolve contention for output trunks Primary goal: Primary goal: maximize capacity maximize capacity subject to cost and reliability constraints subject to cost and reliability constraints connection oriented switches need a call setup connection oriented switches need a call setup scheduling scheduling setup is handled in setup is handled in control plane control plane by switch controller Admission control Admission control Circuit switch must reject call if can Circuit switch must reject call if can ʼ t find a path for samples t find a path for samples from input to output from input to output connectionless switches deal with self-contained datagrams to guarantee resources to certain streams to guarantee resources to certain streams goal: goal: minimize call blocking minimize call blocking We We ʼ ll discuss these later ll discuss these later Packet switch must reject a packet if it can Packet switch must reject a packet if it can ʼ t find a buffer to store t find a buffer to store Here we focus on pure data movement Here we focus on pure data movement Connectionless Connection-oriented it awaiting access to output trunk it awaiting access to output trunk (router) (switching system) Packet Internet router ATM switching system goal: goal: minimize packet loss minimize packet loss switch Don Don ʼ t reorder t reorder packets packets Circuit Telephone switching switch system
A generic switch Outline Circuit switching Circuit switching Circuit switching Moving 8-bit samples from an input port to an output port Moving 8-bit samples from an input port to an output port Packet switching Packet switching Recall that samples have no headers Recall that samples have no headers Switch generations Switch generations Destination of sample depends on Destination of sample depends on time time at which it arrives at the at which it arrives at the Switch fabrics Switch fabrics switch switch Buffer placement Buffer placement actually, relative order within a actually, relative order within a frame frame Multicast switches Multicast switches We We ʼ ll first study something simpler than a switch: a multiplexor ll first study something simpler than a switch: a multiplexor Multiplexors and demultiplexors More on multiplexing Inverse multiplexing Most trunks time division multiplex voice samples Most trunks time division multiplex voice samples Demultiplexor Demultiplexor Takes a high bit-rate stream and scatters it across multiple Takes a high bit-rate stream and scatters it across multiple trunks trunks At a central office, trunk is demultiplexed and distributed to At a central office, trunk is demultiplexed and distributed to one input line and N outputs that run N times slower one input line and N outputs that run N times slower active circuits active circuits samples are placed in output buffer in round robin order samples are placed in output buffer in round robin order At the other end, combines multiple streams At the other end, combines multiple streams Neither multiplexor nor demultiplexor needs addressing Neither multiplexor nor demultiplexor needs addressing resequencing resequencing to accommodate variation in delays to accommodate variation in delays Synchronous multiplexor Synchronous multiplexor information (why?) information (why?) Allows high-speed virtual links using existing technology Allows high-speed virtual links using existing technology N input lines N input lines Can cascade multiplexors Can cascade multiplexors Output runs N times as fast as input Output runs N times as fast as input need a standard need a standard example: DS hierarchy in the US and Japan example: DS hierarchy in the US and Japan
A circuit switch Call blocking Time division switching A switch that can handle N calls has N logical inputs and N A switch that can handle N calls has N logical inputs and N Can Can ʼ t find a path from input to output t find a path from input to output Key idea: when Key idea: when demultiplexing demultiplexing, position in frame determines , position in frame determines logical outputs logical outputs output trunk output trunk Internal blocking Internal blocking N up to 200,000 N up to 200,000 Time division switching interchanges sample position within a Time division switching interchanges sample position within a slot in output frame exists, but no path slot in output frame exists, but no path In practice, input trunks are multiplexed In practice, input trunks are multiplexed frame: time slot interchange (TSI) frame: time slot interchange (TSI) Output blocking Output blocking example: DS3 trunk carries 672 simultaneous calls example: DS3 trunk carries 672 simultaneous calls no slot in output frame is available no slot in output frame is available Multiplexed trunks carry Multiplexed trunks carry frames frames = set of samples = set of samples Output blocking is reduced in Output blocking is reduced in transit transit switches switches Goal: extract samples from frame, and depending on position in Goal: extract samples from frame, and depending on position in need to put a sample in one of need to put a sample in one of several several slots going to the desired frame, switch to output frame, switch to output next hop each incoming sample has to get to the right output line and the each incoming sample has to get to the right output line and the right slot in the output frame right slot in the output frame demultiplex demultiplex, switch, multiplex , switch, multiplex How large a TSI can we build? Space division switching Crossbar Limit is time taken to read and write to memory Limit is time taken to read and write to memory Each sample takes a different path through the switch, Each sample takes a different path through the switch, Simplest possible space-division switch Simplest possible space-division switch depending on its destination depending on its destination For 120,000 circuits For 120,000 circuits Crosspoints Crosspoints can be turned on or off need to read and write memory once every 125 microseconds need to read and write memory once every 125 microseconds For multiplexed inputs, need a switching schedule (why?) each operation takes around 0.5 each operation takes around 0.5 ns ns => impossible with current => impossible with current Internally nonblocking technology technology but need N 2 crosspoints Need to look to other techniques Need to look to other techniques time taken to set each crosspoint grows quadratically vulnerable to single faults (why?)
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