الطبولوجيات االخرى مثل الباص قد ابعدت ايضا وذلك الن زيادة عدد االجهزة المرتبطة فيها الشبكة سوف يضعف قابلياتها ويقلل من كفائتها.

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Bruce Pearson
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1 Almustansorya University College of Education Computer Science Department Communication and Computer Networks Class 4 (A/B) Lesson 8... Switching The Switching Computer network is a set of connected devices. Whenever we have multiple devices, we have the problem of how to connect them to make one-to-one communication possible. One solution is to make a point-to-point connection between each pair of devices (a mesh topology) or between a central device and every other device (a star topology). These methods, however, are impractical and wasteful when applied to very large networks. هذه الطرق تكون عادة غير عملية عند تطبيقها في شبكات كبيرة الحجم. - The number and length of the links require too much infrastructure to be cost-efficient, and the majority of those links would be idle most of the time. Other topologies employing multipoint connections, such as a bus, are ruled out because the distances between devices and the total number of devices increase beyond the capacities of the media and equipment. الطبولوجيات االخرى مثل الباص قد ابعدت ايضا وذلك الن زيادة عدد االجهزة المرتبطة فيها الشبكة سوف يضعف قابلياتها ويقلل من كفائتها. - A better solution is 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. In a switched network, some of these nodes are connected to the end systems (computers or telephones). المحوالت )switches( هي عبارة عن اجهزة قادرة على خلق ارتباط مؤقت بين االجهزة المرتبطة بها. - Traditionally, three methods of switching have been important: circuit switching, packet switching, and message switching. The first two are commonly used today. The third has been phased out in general communications but still has networking applications. - We can then divide today's networks into three broad categories: circuitswitched networks, packet-switched networks, and message-switched. Packet-switched networks can further be divided into two subcategoriesvirtual-circuit networks and datagram networks as shown in Figure 1. 1

2 Figure 1 Taxonomy of switched networks 1- CIRCUIT-SWITCHED NETWORKS A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM. Figure 2 shows a trivial circuitswitched network with four switches and four links. Each link is divided into n (n is 3 in the figure) channels by using FDM or TDM. Figure 2 A trivial circuit-switched network The end systems, such as computers or telephones, are directly connected to a switch. When end system A needs to communicate with end system M, system A needs to request a connection to M that must be accepted by all switches as well as by M itself. This is called the setup phase; a circuit 2

3 (channel) is reserved on each link, and the combination of circuits or channels defines the dedicated path. After the dedicated path made of connected circuits (channels) is established, data transfer can take place. After all data have been transferred, the circuits are turn down. Note : 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. The actual communication in a circuit-switched network requires three phases: connection setup, data transfer, and connection teardown. Setup Phase Before the two parties (or multiple parties in a conference call) can communicate, a dedicated circuit (combination of channels in links) needs to be established. The end systems are normally connected through dedicated lines to the switches, so connection setup means creating dedicated channels between the switches. Data Transfer Phase After the establishment of the dedicated circuit (channels), the two parties can transfer data. Teardown Phase When one of the parties needs to disconnect, a signal is sent to each switch to release the resources. 2- Packet-Switched Networks The alternative to circuit switching is packet switching, shown in Fig. 3. With this technology, packets are sent as soon as they are available. There is no need to set up a dedicated path in advance, unlike the circuit switching. 3

4 If the message is going to pass through a packet-switched network, it needs to be divided into packets of fixed or variable size. The size of the packet is determined by the network and the governing protocol. When a switch receives a packet, no matter what is the source or destination, the packet must wait if there are other packets being processed. It is up to routers to use store-and-forward transmission to send each packet on its way to the destination on its own. This procedure is unlike circuit switching, in which the result of the connection setup is the reservation of bandwidth all the way from the sender to the receiver. Figure 3 Packet switching. With packet switching there is no fixed path, so different packets can follow different paths, depending on network conditions at the time they are sent, and they may arrive out of order. - Datagram networks In a datagram network, each packet is treated independently of all others. Even if a packet is part of a multi-packet transmission, the network treats it as though it existed alone. Packets in this approach are referred to as datagrams. Datagram switching is normally done at the network layer. Figure 4 shows how the datagram approach is used to deliver four packets from station A to station X. The switches in a datagram network are traditionally referred to as routers. In this example, all four packets (or datagrams) belong to the same message, but may travel different paths to reach their destination. This is so because the 4

5 links may be involved in carrying packets from other sources and do not have the necessary bandwidth available to carry all the packets from A to X. Figure 4 A datagram network with four switches (routers) Packets may also be lost or dropped because of a lack of resources. In most protocols, it is the responsibility of an upper-layer protocol to reorder the datagrams or ask for lost datagrams before passing them on to the application. The datagram networks are sometimes referred to as connectionless networks. The term connectionless here means that the switch (packet switch) does not keep information about the connection state. There are no setup or teardown phases. - Virtual-Circuit Network A virtual-circuit network is a cross between a circuit-switched network and a datagram network. It has some characteristics of both. As in a circuit-switched network, there are setup and teardown phases in addition to the data transfer phase. Resources can be allocated during the setup phase, as in a circuitswitched network, or on demand, as in a datagram network. Also, as in a datagram network, data are packetized and each packet carries an address in the header. A virtual-circuit network is normally implemented in the data link layer, while a circuit-switched network is implemented in the physical layer and a datagram network in the network layer. 5

6 Note that 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. Figure 5 is an example of a virtual-circuit network. The network has switches that allow traffic from sources to destinations. Figure 5 Virtual-circuit network As in the case of the circuit-switched network, a source and destination need to go through three phases in a virtual-circuit network: setup, data transfer, and teardown. In the setup phase, the source and destination use their global addresses to help switches make table entries for the connection. In the teardown phase, the source and destination inform the switches to delete the corresponding entry. Data transfer occurs between these two phases. 6

UNIT- 2 Physical Layer and Overview of PL Switching

UNIT- 2 Physical Layer and Overview of PL Switching 2.1 MULTIPLEXING Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. Figure