4.4 Switching

Chapter 4 : Network Layer Logical Addressing

BENG 4522 Data Communications & Computer Networks 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



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.



4.4 Switching methods of switching :

circuit switching and packet switching are commonly used today. massage-switching has been phased out in general communication.



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.



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).



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.



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.



4.4.1 Circuit-switched Networks Three phases

Setup phase, data transfer phase, teardown phase Delay in a circuit-switched network



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.



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



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



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



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



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



4.4.4 Virtual-circuit Networks



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.

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4.4 Switching