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4.4 Switching

4.4 Switching. main problem in a network with connected multiple devices is how to connect them to make one-to-one communication possible. 1 st solution – to make a point-to-point connection between each pair of devices (mesh topology) Problem : impractical for large networks

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4.4 Switching

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  1. 4.4 Switching • main problem in a network with connected multiple devices is how to connect them to make one-to-one communication possible. • 1st solution – to make a point-to-point connection between each pair of devices (mesh topology) • Problem : impractical for large networks • 2nd solution – employing multipoint connection such as in bus topology • problem : distance between devices and the total number of devices increased beyond the capacities of the media and equipment • better solution – switching BENG 4522 Data Communications & Computer Networks

  2. 4.4 Switching • a switched network consists of a series of interlinked nodes, called switches. • switches are devices capable of creating temporary connections between two or more devices linked to the switch. • the end systems (communicating devices) are labeled A, B, C, D and so on, and the switches are labeled I, II, III and IV. • each switch is connected to multiple links. BENG 4522 Data Communications & Computer Networks

  3. 4.4 Switching • methods of switching : • circuit switching and packet switching are commonly used today. • massage-switching has been phased out in general communication. BENG 4522 Data Communications & Computer Networks

  4. 4.4.1 Circuit-switched Networks • A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels. • each link is normally divided into n channels by using FDM or TDM. • In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase. BENG 4522 Data Communications & Computer Networks

  5. 4.4.1 Circuit-switched Networks • circuit switching takes place at the physical layer. • before starting communication, the stations must make a reservation for the resources to be used during the communication. The resources, such as channels (bandwidth in FDM and time slots in TDM), switch buffers, switch processing time, and switch input/output ports, must remain dedicated during the entire duration of data transfer. • data transferred between the two stations are not packetized. The data are continuous flow sent by the source station and received by the destination station. • there is no addressing involved during data transfer. The switch route the data based on their occupied band (FDM) or time slot (TDM). BENG 4522 Data Communications & Computer Networks

  6. 4.4.1 Circuit-switched Networks • Example 1 : application of circuit-switched networks • circuit-switched network used to connect eight telephones in a small area. • communication is through 4 kHz voice channels. • each links uses FDM to connect a maximum of two voice channels • bandwidth of each link is the 8 kHz. BENG 4522 Data Communications & Computer Networks

  7. 4.4.1 Circuit-switched Networks • Example 2 : application of circuit-switched networks • circuit-switched network that connects computers in two remote offices. • the offices are connected using a T-1 line leased from a comm service provider. • two 4 x 8 (4 inputs & 8 outputs) switches with four output ports are folded into the input ports to allow communication between computers in the same office and four other output ports to allow communication between the two offices. BENG 4522 Data Communications & Computer Networks

  8. 4.4.1 Circuit-switched Networks • Three phases • Setup phase, data transfer phase, teardown phase • Delay in a circuit-switched network BENG 4522 Data Communications & Computer Networks

  9. 4.4.2 Packet-switched Networks • in a packet-switched network, there is no resource reservation; resources are allocated on demand. • in a datagram network, each packet is treated independently of all others. Sometimes referred to as connectionless networks. • datagram switching is normally done at the network layer. BENG 4522 Data Communications & Computer Networks

  10. 4.4.2 Packet-switched Networks • in above figure, all four packets (datagrams) belong to the same message, but may travel different paths to reach their destination – based on link availability • this cause a different delay and out f order among the packets at the arrival • referred to as connectionless networks – means that the switch does not keep information about the connection state BENG 4522 Data Communications & Computer Networks

  11. 4.4.2 Packet-switched Networks : Routing table • a switch in a datagram networks uses a routing table that is based on the destination address • the destination address in the header of a packet in a datagram network remains the same during the entire journey of the packet BENG 4522 Data Communications & Computer Networks

  12. 4.4.3 Comparison : Circuit switched vs Packet switch • Efficiency • circuit-switched network is not efficient as the packet-switched network as resources are allocated during the entire duration of the connection. Allowing resources to be dedicated cause other connections to be deprived. • in packet-switched network, resources are allocated only when there are packets to be transferred. • Delay • delay in circuit-switched network is minimal. During data transfer, the data are not delayed at each switch; the resources are allocated for the duration of the connection. • in packet-switch network, datagrams may experience a wait at a switch before it is forwarded BENG 4522 Data Communications & Computer Networks

  13. 4.4.3 Comparison : Circuit switched vs Packet switch • Delay in packet-switched network • delay is not uniform between all packets as each packet may travel through a different route BENG 4522 Data Communications & Computer Networks

  14. 4.4.4 Virtual-circuit Networks • virtual-circuit network is a cross between a circuit-switch network and a datagram network – has some characteristics of both. • Setup, data transfer, and teardown phases as in a circuit-switched network (CSN) • Resource allocated during setup phase, as in a CSN, or on demand as in a datagram network (DN) • As in DN, data are packetized and each packet carries an address in the header. The address has local jurisdiction, not end-to-end jurisdiction (refer to textbook for details) • As in CSN, all packets follow the same path established during the connection • VCN is normally implemented in the data link layer, while CSN is in physical layer and DN in the network layer BENG 4522 Data Communications & Computer Networks

  15. 4.4.4 Virtual-circuit Networks BENG 4522 Data Communications & Computer Networks

  16. 4.4.4 Delay in Virtual-circuit Networks • In virtual-circuit switching, all packets belonging to the same source and destination travel the same path; but the packets may arrive at the destination with different delays if resource allocation is on demand. BENG 4522 Data Communications & Computer Networks

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