CHAPTER 8 Switching A 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. 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. A better solution is switching.A switched network consists of a series ofinterlinked 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,for example).Others are used only for routing.Figure 8.1 shows a switched network. Figure 8.1 Switched network B A The end systems (communicating devices)are labeled A,B,C,D,and so on,and the switches are labeled I,II,III,IV,and V.Each switch is connected to multiple links. 213
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214 CHAPTER 8 SWITCHING 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:circuit-switched networks, packet-switched networks,and message-switched.Packet-switched networks can funher be divided into two subcategories-virtual-circuit networks and datagram networks- as shown in Figure 8.2. Figure 8.2 Taxonomy ofswitched networks Switched networks Circuit-switched Packet-switched Message-switched networks networks networks Datagram Virtual-circuit networks networks We can say that the virtual-circuit networks have some common characteristics with circuit-switched and datagram networks.Thus,we first discuss circuit-switched networks,then datagram networks,and finally virtual-circuit networks. Today the tendency in packet switching is to combine datagram networks and virtual- circuit networks.Networks route the first packet based on the datagram addressing idea, but then create a virtual-circuit network for the rest of the packets coming from the same source and going to the same destination.We will see some of these networks in future chapters. In message switching,each switch stores the whole message and forwards it to the next switch.Although,we don't see message switching at lower layers,it is still used in some applications like electronic mail (e-mail).We will not discuss this topic in this book. 8.1 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 ofone or more links.How- ever,each connection uses only one dedicated channel on each link.Each link is nor- mally divided into n channels by using FDM or TDM as discussed in Chapter 6. A circuit-switched network is made ofa set of switches connected by physical links, in which each link is divided into n channels. Figure 8.3 shows a trivial circuit-switched 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
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SECTION 8.1 CIRCUIT-SWITCHED NETWORKS 215 Figure 8.3 A trivial circuit-switched network One link,n channels Path →M We have explicitly shown the multiplexing symbols to emphasize the division of the link into channels even though multiplexing can be implicitly included in the switch fabric. The end systems,such as computers or telephones,are directly connected to a switch.We have shown only two end systems for simplicity.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 (channel)is reserved on each link,and the combination of circuits or channels defines the dedicated path.After the dedicated path made ofconnected circuits(channels) is established,data transfer can take place.After all data have been transferred,the circuits are tom down. We need to emphasize several points here: D Circuit switching takes place at the physical layer. D Before starting communication,the stations must make a reservation for the resources to be used during the communication.These 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 until the teardown phase. D Data transferred between the two stations are not packetized(physical layer transfer of the signal).The data are a continuous flow sent by the source station and received by the destination station,although there may be periods of silence. D There is no addressing involved during data transfer.The switches route the data based on their occupied band(FDM)or time slot(TDM).Of course,there is end-to- end addressing used during the setup phase,as we will see shortly. 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
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216 CHAPTER 8 SWITCHING Example 8.1 As a trivial example,let us use a circuit-switched network to connect eight telephones in a small area.Communication is through 4-kHz voice channels.We assume that each link uses FDM to connect a maximum of two voice channels.The bandwidth ofeach link is then 8 kHz.Figure 8.4 shows the situation.Telephone 1 is connected to telephone 7;2 to 5;3 to 8;and 4 to 6.Of course the situation may change when new connections are made.The switch controls the connections. Figure 8.4 Circuit-switched network used in Example 8.1 Circuit-switched network ® 04 6 2® kHz kHz 3③ 4③ 魔金 8 kHz kHz 0 4 kHz kHz 0 -G8 kHz kHz Example 8.2 As another example.consider a circuit-switched network that connects computers in two remote offices of a private company.The offices are connected using a T-l line leased from a communi- cation service provider.There are two 4 X 8(4 inputs and 8 outputs)switches in this network.For each switch,four output ports are folded into the input ports to allow communication between computers in the same office.Four other output ports allow communication between the two offices.Figure 8.5 shows the situation. Figure 8.5 Circuit-switched network used in Example 8.2 Circuit-switched network 4x8 4x8 swltch T-I line with switch 1.544 Mbps
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SECTION 8.1 CIRCUIT-SWITCHED NEIWORKS 217 Three Phases The actual communication in a circuit-switched network requires three phases:connec- tion 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 sys- tems are normally connected through dedicated lines to the switches,so connection setup means creating dedicated channels between the switches.For example,in Figure 8.3, when system A needs to connect to system M,it sends a setup request that includes the address of system M,to switch I.Switch I finds a channel between itself and switch IV that can be dedicated for this purpose.Switch I then sends the request to switch IV, which finds a dedicated channel between itself and switch III.Switch III informs sys- tem M of system A's intention at this time. In the next step to making a connection,an acknowledgment from system M needs to be sent in the opposite direction to system A.Only after system A receives this acknowledgment is the connection established. Note that end-to-end addressing is required for creating a connection between the two end systems.These can be,for example,the addresses of the computers assigned by the administrator in a TDM network,or telephone numbers in an FDM network. Data Transfer Phase After the establishment ofthe 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. Efficiency It can be argued that circuit-switched networks are not as efficient as the other two types of networks because resources are allocated during the entire duration ofthe con- nection.These resources are unavailable to other connections.In a telephone network, people normally terminate the communication when they have finished their conversation. However,in computer networks,a computer can be connected to another computer even ifthere is no activity for a long time.In this case,allowing resources to be dedicated means that other connections are deprived. Delay Although a circuit-switched network normally has low efficiency,the delay in this type ofnetwork is minimal.During data transfer the data are not delayed at each switch;the resources are allocated for the duration of the connection.Figure 8.6 shows the idea of delay in a circuit-switched network when only two switches are involved. As Figure 8.6 shows,there is no waiting time at each switch.The total delay is due to the time needed to create the connection,transfer data,and disconnect the circuit.The
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218 CHAPTER 8 SWITCHING Figure 8.6 Delay in a circuit-switched network B Data transfer Time Time Time Time delay caused by the setup is the sum of four parts:the propagation time of the source computer request(slope of the first gray box),the request signal transfer time (height of the first gray box),the propagation time of the acknowledgment from the destination computer(slope ofthe second gray box),and the signal transfer time of the acknowledg- ment (height of the second gray box).The delay due to data transfer is the sum of two parts:the propagation time(slope of the colored box)and data transfer time (height of the colored box),which can be very long.The third box shows the time needed to tear down the circuit.We have shown the case in which the receiver requests disconnection, which creates the maximum delay. Circuit-Switched Technology in Telephone Networks As we will see in Chapter 9,the telephone companies have previously chosen the circuit- switched approach to switching in the physical layer;today the tendency is moving toward other switching techniques.For example,the telephone number is used as the global address,and a signaling system (called SS7)is used for the setup and teardown phases. Switching at the physical layer in the traditional telephone network uses the circuit-switching approach. 8.2 DATAGRAM NETWORKS In data communications,we need to send messages from one end system to another.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 net- work and the governing protocol. In packet switching,there is no resource allocation for a packet.This means that there is no reserved bandwidth on the links,and there is no scheduled processing time
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SECTION8.2 DATAGRAMNETWORKS 219 for each packet.Resources are allocated on demand.The allocation is done on a first- come,first-served basis.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.As with other systems in our daily life,this lack ofreservation may create delay.For example,if we do not have a reservation at a restaurant,we might have to wait. 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.Even if a packet is part of a multipacket 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.We briefly discuss datagram networks here as a comparison with circuit-switched and virtual-circuit- switched networks.In Part 4 of this text,we go into greater detail. Figure 8.7 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.That is why we use a different symbol for the switches in the figure. Figure 8.7 A datagram network with four switches (routers) Datagram network 32团 3 2 2341 2 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 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.This approach can cause the datagrams of a transmission to arrive at their destination out oforder with different delays between the packets.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.Each packet is treated the same by a switch regardless ofits source or destination
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220 CHAPTER 8 SWITCHING Routing Table If there are no setup or teardown phases,how are the packets routed to their destinations in a datagram network?In this type of network,each switch(or packet switch)has a rout- ing table which is based on the destination address.The routing tables are dynamic and are updated periodically.The destination addresses and the corresponding forwarding output ports are recorded in the tables.This is different from the table of a circuit- switched network in which each entry is created when the setup phase is completed and deleted when the teardown phase is over.Figure 8.8 shows the routing table for a switch. Figure 8.8 Routing table in a datagram network Destination Output address port 1232 4150 2 9130 21 3 A switch in a datagram network uses a routing table that is based on the destination address. Destination Address Every packet in a datagram network carries a header that contains,among other infor- mation,the destination address of the packet.When the switch receives the packet,this destination address is examined;the routing table is consulted to find the corresponding port through which the packet should be forwarded.This address,unlike the address in a virtual-circuit-switched network,remains the same during the entire journey of the packet. The destination address in the header ofa packet in a datagram network remains the same during the entire journey ofthe packet. Efficiency The efficiency of a datagram network is better than that of a circuit-switched network; resources are allocated only when there are packets to be transferred.If a source sends a packet and there is a delay of a few minutes before another packet can be sent,the resources can be reallocated during these minutes for other packets from other sources
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SECTION8.3 VIRTUAL-CIRCUIT NETWORKS 221 Delay There may be greater delay in a datagram network than in a virtual-circuit network. Although there are no setup and teardown phases,each packet may experience a wait at a switch before it is forwarded.In addition,since not all packets in a message necessarily travel through the same switches,the delay is not uniform for the packets of a message. Figure 8.9 gives an example of delay in a datagram network for one single packet. Figure 8.9 Delay in a datagram network A B Transmission[ time Waiting time Waiting time Time Time Time Time The packet travels through two switches.There are three transmission times (37), three propagation delays (slopes 3t of the lines),and two waiting times (wI +w2)'We ignore the processing time in each switch.The total delay is Total delay =3T+3t+WI +W2 Datagram Networks in the Internet As we will see in future chapters,the Internet has chosen the datagram approach to switching at the network layer.It uses the universal addresses defined in the network layer to route packets from the source to the destination. Switching in the Internet is done by using the datagram approach to packet switching at the network layer. 8.3 VIRTUAL-CIRCUIT NETWORKS A virtual-circuit network is a cross between a circuit-switched network and a datagram network.It has some characteristics ofboth. 1.As in a circuit-switched network,there are setup and teardown phases in addition to the data transfer phase
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222 CHAPTER 8 SWITCHING 2.Resources can be allocated during the setup phase,as in a circuit-switched network, or on demand,as in a datagram network. 3.As in a datagram network,data are packetized and each packet carries an address in the header.However,the address in the header has local jurisdiction (it defines what should be the next switch and the channel on which the packet is being canied),not end-to-end jurisdiction.The reader may ask how the intermediate switches know where to send the packet if there is no final destination address carried by a packet. The answer will be clear when we discuss virtual-circuit identifiers in the next section. 4.As in a circuit-switched network,all packets follow the same path established during the connection. 5.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 net- work in the network layer.But this may change in the future. Figure 8.10 is an example of a virtual-circuit network.The network has switches that allow traffic from sources to destinations.A source or destination can be a computer, packet switch,bridge,or any other device that connects other networks. Figure 8.10 Virtual-circuit network End system End system End system A Switches BT】 D End system Addressing In a virtual-circuit network,two types of addressing are involved:global and local (virtual-circuit identifier). Global Addressing A source or a destination needs to have a global address-an address that can be unique in the scope of the network or internationally if the network is part of an international network.However,we will see that a global address in virtual-circuit networks is used only to create a virtual-circuit identifier,as discussed next. Virtual-Circuit Identifier The identifier that is actually used for data transfer is called the virtual-circuit identifier (Vel).A vel,unlike a global address,is a small number that has only switch scope;it
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