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Circuit Switching Dr. Indranil Sen Gupta Circuit Switching Slide 1 Introduction • For transmission of data beyond a LAN, communication is typically achieved by transmitting data from source to destination through a number of intermediate switching nodes. – The switching nodes are not concerned with the contents of data. • Their purpose is to provide a switching facility that will move the data from node to node until they reach the destination. – The end devices that wish to communicate are called stations. – The switching devices are called nodes. Dr. Indranil Sen Gupta Circuit Switching Slide 2 Simple Switching Network Dr. Indranil Sen Gupta Circuit Switching Slide 3 Some Observations • Some nodes connect only to other nodes. – Their sole task is the internal switching of data. • Inter-node links are usually multiplexed, either using FDM or TDM. • Multiple paths between a source-destination pair enhances the network reliability. • Two different techniques used in wide-area switching networks. – Circuit switching and packet switching. – Differ in the way the nodes switch information from one link to another. Dr. Indranil Sen Gupta Circuit Switching Slide 4 Circuit Switching Networks • Communication via circuit switching implies that there is a dedicated communication path between two stations. – The path is a connected sequence of links between network nodes. – On each physical link, a logical channel is dedicated to the connection. • Communication via circuit switching involves three phases: – Circuit establishment – Data transfer – Circuit disconnect Dr. Indranil Sen Gupta Circuit Switching Slide 5 Contd. • Circuit establishment – An end-to-end circuit must be established before any signal can be transmitted. – Some segments of the circuit may be a dedicated link, while some other segments may be shared. • Data transfer – Information is transmitted from the source to the destination. – The data may be analog or digital, depending on the nature of the network. – The connection is generally full-duplex. • Circuit disconnect – The connection is terminated at the end of the data transfer. – Signals must be propagated to deallocate the dedicated resources. Dr. Indranil Sen Gupta Circuit Switching Slide 6 Contd. • Circuit switching can be inefficient. – Channel capacity is dedicated for the duration of the connection, even if no data is being transferred. • For voice connection, utilization is typically high. • For terminal-computer connection, the capacity may be idle during most of the time of the connection. – Performance-wise, there is a delay prior to signal transfer. • Call establishment phase is there. • Once the circuit is established, the network is transparent to the users. • Information is transmitted at a fixed rate with no delay other than that required for propagation through the communication medium. • Originally developed for handling voice traffic, but is now also used for data traffic. – Best known example is the Public Telephone Network. Dr. Indranil Sen Gupta Circuit Switching Slide 7 Public Telephone Network Dr. Indranil Sen Gupta Circuit Switching Slide 8 Switching Concepts • We first examine the operation of a single circuit switched node. – Collection of stations attached to a central switching unit. – The central switch establishes a dedicated path between any two devices that wish to communicate. • Major elements of a single-node network. – Digital switch: provides a transparent (full-duplex) signal path between any pair of attached devices. – Network interface: represents the functions and hardware needed to connect digital devices to the network (like telephones). – Control unit: establishes, maintains, and tears down a connection. Dr. Indranil Sen Gupta Circuit Switching Slide 9 Elements of a circuit-switch node Dr. Indranil Sen Gupta Circuit Switching Slide 10 Blocking and Non-blocking Networks • An important characteristic of a circuit-switch node is whether it is blocking or non-blocking. – A blocking network is one which may be unable to connect two stations because all possible paths between them are already in use. – A non-blocking network permits all stations to be connected (in pairs) at once and grants all possible connection requests as long as the called party is free. • For a network which supports only voice traffic, a blocking configuration may be acceptable, since most phone calls are of short duration. • For data applications, where a connection may remain active for hours, non-blocking configuration is desirable. Dr. Indranil Sen Gupta Circuit Switching Slide 11 Space Division Switching • Originally developed for the analog environment, and has been carried over to the digital domain. • In a space division switch, the signal paths are physically separate from one another (divided in space). • The basic building block of the switch is a metallic crosspoint or semiconductor gate that can be enabled or disabled by a control unit. – Essentially a crossbar matrix. Dr. Indranil Sen Gupta Circuit Switching Slide 12 Space-division Switch: block diagram Dr. Indranil Sen Gupta Circuit Switching Slide 13 Limitations of crossbar switch • The number of crosspoints grows with the square of the number of attached stations. – Costly for a large switch. • The failure of a crosspoint prevents connection between the two devices whose lines intersect at that crosspoint. • The crosspoints are inefficiently utilized. – Only a small fraction are engaged even if all of the attached devices are active. • Solution is to build multistage space division switches. Dr. Indranil Sen Gupta Circuit Switching Slide 14 3-stage Space Division Switch Dr. Indranil Sen Gupta Circuit Switching Slide 15 Multistage Switches • By splitting the crossbar switch into small chunks and interconnecting them, it is possible to build multistage switches with many fewer crosspoints. • In the previous example: – The number of crosspoints needed goes down from 100 to 48. – There is more than one path through the network to connect two endpoints, thereby increasing reliability • Multistage switches may lead to blocking. – The problem may be tackled by increasing the number or size of the intermediate switches, which also increases the cost. Dr. Indranil Sen Gupta Circuit Switching Slide 16 Time Division Switching • Both voice and data can be transmitted via digital signals. • All modern circuit switches use digital time-division techniques for establishing and maintaining circuits. – Synchronous TDM allows multiple low-speed bit streams to share a highspeed line. – A set of inputs is sampled in turn. The samples are organized serially into slots (channels) to form a recurring frame of slots. – During successive time slots, different I/O pairings are enabled, allowing a number of connections to be carried over the shared bus. • To keep up with the input lines, the data rate on the bus must be high enough so that the slots recur sufficiently frequently. • For 100 full-duplex lines at 19200 bps, the data rate on the bus must be greater than 1.92 Mbps. – The source-destination pairs corresponding to all active connections are stored in the control memory. • Thus the slots need not specify the source & destination addresses. Dr. Indranil Sen Gupta Circuit Switching Slide 17 Routing in Circuit Switched Networks • In large circuit-switched networks, connections often require a path through more than one switch. • Routing function in public telecommunication networks has been traditionally quite simple. – Switches are organized as a tree structure. – To add some resilience to the network, additional high-usage trunks are added that cut across the tree structure to connect exchanges with high volumes of traffic between them. – Usually a static approach. • To cope with growing demands, all providers today use a dynamic approach. – Routing decisions are influenced by current traffic conditions. – Switching nodes have a peer relationship with each other rather than a hierarchical one. – Routing is more complex and more flexible. Dr. Indranil Sen Gupta Circuit Switching Slide 18 Method 1: Alternate Routing • The possible routes to be used between two end offices are predetermined. – It is the responsibility of the originating switch to select the appropriate route for each call. • In practice, usually a different set of pre-planned routes is used for different time periods. – Takes advantage of different traffic patterns in different time zones and different times of day. Dr. Indranil Sen Gupta Circuit Switching Slide 19 Alternate Routing Example Dr. Indranil Sen Gupta Circuit Switching Slide 20 Method 2: Adaptive Routing • This is designed to enable switches to react to changing traffic patterns on the network. – Greater management overhead (switches must exchange information). – Has the potential for more effectively optimizing the use of network resources. Dr. Indranil Sen Gupta Circuit Switching Slide 21