8.4CIRCUIT-SWITCHED NETWORKSCIRCUIT-SWITCHED NETWORKSA circuit-switched network consists of a set of switchesA circuit-switched network consists of a set of switchesconnected by physical links. A connection between twoconnected by physical links. A connection between twostations is a dedicated path made of one or more links.stations is a dedicated path made of one or more links.However, each connection uses only one dedicatedHowever, each connection uses only one dedicatedchannel on each link. Each link is normally dividedchannel on each link. Each link is normally dividedinto n channels by using FDM or TDM.into n channels by using FDM or TDM.
8.6In circuit switching, the resources needto be reserved during the setup phase;the resources remain dedicated for theentire duration of data transfer until theteardown phase.Note
8.7As a trivial example, let us use a circuit-switched networkto connect eight telephones in a small area.Communication is through 4-kHz voice channels. Weassume that each link uses FDM to connect a maximumof two voice channels. The bandwidth of each link is then8 kHz. Figure shows the situation. Telephone 1 isconnected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Ofcourse the situation may change when new connectionsare made. The switch controls the connections.Example
8.10Switching at the physical layer in thetraditional telephone network usesthe circuit-switching approach.Note
8.11DATAGRAM NETWORKSDATAGRAM NETWORKSIn data communications, we need to send messagesIn data communications, we need to send messagesfrom one end system to another. If the message isfrom one end system to another. If the message isgoing to pass through a packet-switched network, itgoing to pass through a packet-switched network, itneeds to be divided into packets of fixed or variableneeds to be divided into packets of fixed or variablesize. The size of the packet is determined by thesize. The size of the packet is determined by thenetwork and the governing protocol.network and the governing protocol.
8.12In a packet-switched network, thereis no resource reservation;resources are allocated on demand.Note
8.13A datagram network with five switches (routers)Packets may also be lost/dropped due to lack of resources
8.14In a packet-switched (connectionless)network ,there are no setup or teardown phases. Eachpacket is treated the same regardless of itssource or destination.Note
8.19Switching in the Internet is done byusing the datagram approach to packetswitching at the network layer.Note
Comparison of circuit switched andpacket-switched networks
8.21VIRTUAL-CIRCUIT NETWORKSVIRTUAL-CIRCUIT NETWORKSA virtual-circuit network is a cross between a circuit-A virtual-circuit network is a cross between a circuit-switched network and a datagram network. It hasswitched network and a datagram network. It hassome characteristics of both.some characteristics of both.
8.27Switch and switching tables in a virtual-circuit network
8.28Setup request frame from source to destination in a virtual-circuit networkB assigns a VCI ifready to receiveframes from A
8.29A special Setup acknowledgment frame completes the table entries
8.30Source-to-destination data transfer in a virtual-circuit network
8.31• In virtual-circuit switching, all packets belongingto the same source and destination travel thesame path; but the packets may arrive at thedestination with different delays due to:— Varying packet queue lengths in thenetwork nodes— Varying load from other users sharing thesame network resources
8.33Switching at the data link layer in aswitched WAN is normallyimplemented by usingvirtual-circuit techniques.Note
8.34STRUCTURE OF A SWITCHSTRUCTURE OF A SWITCHWe use switches in circuit-switched and packet-We use switches in circuit-switched and packet-switched networks. We will see the structures of theswitched networks. We will see the structures of theswitches used in packet-switched networks.switches used in packet-switched networks.