Data and Computer Communications (Eighth Edition)

1 CHAPTER297 CIRCUITSWITCHING Communications Reading and Web Terms, Review Questions, and Problems10He got into a District Line train at Wimbledon Park, changed on to the Victoria Line atVictoria and on to the Jubilee Line at Green Park for West Hampstead. It was a longand awkward journey but he enjoyed it. King Solomon s Carpet, Barbara Vine (Ruth Rendell)KEY POINTS Circuit switching is used in public telephone networks and is the basisfor private networks built on leased lines and using on-site switching was developed to handle voice traffic butcan also handle digital data , although this latter use is often inefficient. With circuit switching, a dedicated path is established between twostations for communication .

2 Switching and transmission resourceswithin the network are reserved for the exclusive use of the circuit forthe duration of the connection is transparent: Once it isestablished, it appears to attached devices as if there were a direct con-nection. Packet switching was designed to provide a more efficient facilitythan circuit switching for bursty data packet switching, astation transmits data in small blocks, called packets. Each packet con-tains some portion of the user data plus control information neededfor proper functioning of the network. A key distinguishing element of packet-switching networks is whetherthe internal operation is datagram or virtual circuit.

3 With internal vir-tual circuits, a route is defined between two endpoints and all packetsfor that virtual circuit follow the same route. With internal datagrams,each packet is treated independently, and packets intended for thesame destination may follow different routes. is the standard protocol for the interface between an end systemand a packet-switching network. Frame relay is a form of packet switching that provides a streamlinedinterface compared to , with improved Two describes how information can be encoded and transmitted over a com-munications link. We now turn to the broader discussion of networks, which canbe used to interconnect many devices.

4 The chapter begins with a general discus-sion of switched Communications networks. The remainder of the chapter focuseson wide area networks and, in particular, on traditional approaches to wide areanetwork design: circuit switching and packet / SWITCHED Communications NETWORKS2991We use this term here in a very general sense, to include voice, image, and video, as well as ordinary data ( , numerical, text).Since the invention of the telephone, circuit switching has been the dominanttechnology for voice Communications , and it has remained so well into the digitalera. This chapter looks at the key characteristics of a circuit-switching 1970, research began on a new form of architecture for long-dis-tance digital data Communications : packet switching.

5 Although the technology ofpacket switching has evolved substantially since that time, it is remarkable that(1) the basic technology of packet switching is fundamentally the same today as itwas in the early 1970s networks, and (2) packet switching remains one of the feweffective technologies for long-distance data chapter provides an overview of packet-switching technology. We willsee, in this chapter and later in this part, that many of the advantages of packetswitching (flexibility, resource sharing, robustness, responsiveness) come with acost. The packet-switching network is a distributed collection of packet-switchingnodes. Ideally, all packet-switching nodes would always know the state of theentire network.

6 Unfortunately, because the nodes are distributed, there is a timedelay between a change in status in one portion of the network and knowledge ofthat change elsewhere. Furthermore, there is overhead involved in communicat-ing status information. As a result, a packet-switching network can never perform perfectly, and elaborate algorithms are used to cope with the time delay andoverhead penalties of network operation. These same issues will appear againwhen we discuss internetworking in Part , this chapter provides an overview of a popular form of packetswitching known as frame SWITCHED Communications NETWORKSFor transmission of data1beyond a local area, communication is typically achievedby transmitting data from source to destination through a network of intermediateswitching nodes; this switched network design is typically used to implement LANsas well.

7 The switching nodes are not concerned with the content of the data ; rather,their purpose is to provide a switching facility that will move the data from node tonode until they reach their destination. Figure illustrates a simple network. Thedevices attached to the network may be referred to as stations maybe computers, terminals, telephones, or other communicating devices. We refer tothe switching devices whose purpose is to provide communication as connected to one another in some topology by transmission links. Each stationattaches to a node, and the collection of nodes is referred to as a a switched communication network, data entering the network from astation are routed to the destination by being switched from node to node.

8 For300 CHAPTER 10 / CIRCUIT SWITCHING AND PACKET SWITCHINGF igure Switching NetworkBAEFCD1452367 PersonalcomputerPersonalcomputerMainfram eSwitchingnodePersonalcomputerPersonalco mputerServerexample, in Figure , data from station A intended for station F are sent tonode 4. They may then be routed via nodes 5 and 6 or nodes 7 and 6 to the desti-nation. Several observations are in nodes connect only to other nodes ( , 5 and 7). Their sole task is theinternal (to the network) switching of data . Other nodes have one or more sta-tions attached as well; in addition to their switching functions, such nodesaccept data from and deliver data to the attached links are generally dedicated point-to-point links.

9 Node-node linksare usually multiplexed, using either frequency division multiplexing (FDM) ortime division multiplexing (TDM). , the network is not fully connected; that is, there is not a direct linkbetween every possible pair of nodes. However, it is always desirable to havemore than one possible path through the network for each pair of enhances the reliability of the different technologies are used in wide area switched networks: circuitswitching and packet switching. These two technologies differ in the way thenodes switch information from one link to another on the way from source / CIRCUIT-SWITCHING CIRCUIT-SWITCHING NETWORKSC ommunication via circuit switching implies that there is a dedicated communica-tion path between two stations.

10 That path is a connected sequence of links betweennetwork nodes. On each physical link, a logical channel is dedicated to the connec-tion. communication via circuit switching involves three phases, which can beexplained with reference to Figure any signals can be transmitted, an end-to-end(station-to-station) circuit must be established. For example, station A sends arequest to node 4 requesting a connection to station E. Typically, the link from Ato 4 is a dedicated line, so that part of the connection already exists. Node 4 mustfind the next leg in a route leading to E. Based on routing information and mea-sures of availability and perhaps cost, node 4 selects the link to node 5, allocates afree channel (using FDM or TDM) on that link, and sends a message requestingconnection to E.