Circuit Switching Source first establishes a connection (circuit) to the destination )) Each router or switch along the way may reserve some bandwidth for the data flow Source sends the data over the circuit ) No need to include the destination address with the data since the routers know the path The connection is torn down Example: telephone network. 6
6 Circuit Switching Source first establishes a connection (circuit) to the destination. » Each router or switch along the way may reserve some bandwidth for the data flow Source sends the data over the circuit. » No need to include the destination address with the data since the routers know the path The connection is torn down. Example: telephone network
Circuit Switching Discussion Traditional circuits: on each hop, the circuit has a dedicated wire or slice of bandwidth Physical connection clearly no need to include addresses with the data Advantages, relative to packet switching: >)Implies guaranteed bandwidth, predictable performance >)Simple switch design: only remembers connection formation, no longest-prefix destination address look up Disadvantages >)Inefficient for bursty traffic(wastes bandwidth) >)Delay associated with establishing a circuit Can we get the advantages without (all)the disadvantages?
7 Circuit Switching Discussion Traditional circuits: on each hop, the circuit has a dedicated wire or slice of bandwidth. » Physical connection - clearly no need to include addresses with the data Advantages, relative to packet switching: » Implies guaranteed bandwidth, predictable performance » Simple switch design: only remembers connection information, no longest-prefix destination address look up Disadvantages: » Inefficient for bursty traffic (wastes bandwidth) » Delay associated with establishing a circuit Can we get the advantages without (all) the disadvantages?
Virtual circuits Each wire carries many" virtual'circuits >)Forwarding based on virtual circuit (vc)identifier iP header: src, dst, etc Virtual circuit header: just Vc >)A path through the network is determined for each vc when the VC is established >Use statistical multiplexing for efficiency Can support wide range of quality of service ))No guarantees: best effort serv ice 》 Weak guarantees: delay<300msec,… Strong guarantees: e.g. equivalent of physical circuit 8
8 Virtual Circuits Each wire carries many “virtual” circuits. » Forwarding based on virtual circuit (VC) identifier – IP header: src, dst, etc. – Virtual circuit header: just “VC” » A path through the network is determined for each VC when the VC is established » Use statistical multiplexing for efficiency Can support wide range of quality of service. » No guarantees: best effort service » Weak guarantees: delay < 300 msec, … » Strong guarantees: e.g. equivalent of physical circuit
Packet Switching and Virtual circuits: similarities < Store and forward'communication based on an address >) Address is either the destination address or a vc identifier Must have buffer space to tem porarily store packets >)E.g. multiple packets for some destination arrive simultaneously Multiplexing on a link is similar to time sharing > No reservations: multiplexing is statistical, i.e. packets are interleaved without a fixed pattern Reservations: some flows are guaranteed to get a certain number of“ slots” DBCIBAA 9
9 Packet Switching and Virtual Circuits: Similarities “Store and forward” communication based on an address. » Address is either the destination address or a VC identifier Must have buffer space to temporarily store packets. » E.g. multiple packets for some destination arrive simultaneously Multiplexing on a link is similar to time sharing. » No reservations: multiplexing is statistical, i.e. packets are interleaved without a fixed pattern » Reservations: some flows are guaranteed to get a certain number of “slots” D B C B A A
Virtual circuits versus Packet Switching Circuit switching: > Uses short connection identifiers to forward packets > Switches know about the connections so they can more easily implement features such as quality of service >) Virtual circuits form basis for traffic engineering: Vc identifies long-lived stream of data that can be scheduled Packet switching >)Use full destination addresses for forwarding packets >) Can send data right away no need to establish a connection first Switches are stateless: easier to recover from failures 》 Adding Qos is hard > Traffic engineering is hard: too many packets 10
10 Virtual Circuits Versus Packet Switching Circuit switching: » Uses short connection identifiers to forward packets » Switches know about the connections so they can more easily implement features such as quality of service » Virtual circuits form basis for traffic engineering: VC identifies long-lived stream of data that can be scheduled Packet switching: » Use full destination addresses for forwarding packets » Can send data right away: no need to establish a connection first » Switches are stateless: easier to recover from failures » Adding QoS is hard » Traffic engineering is hard: too many packets!