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￾￾￾     !! "#$ $%" &"&$% '(&$%"!%) !%   *$+"    ,-.$    /$   $   !%0" ##,1 &2% $ &$ %$ $  !%. .2!% 1 $$$ $+#. / 3(#$ $%"!%.$ &!     4(+#$%&2&!& %2"$  #$ $%&!& ! 22&# $% $%25 &2  . 6789/&!.+#$%1 $% $   $...& :&!" !%$ ""2 +  $% 6789 ;<=>?@ABC<=C?<>DE>FG=H ( I+#$%"$2!.++J -+#.0 AGK@ALL=EMM<DN ( +# O  !. $%2. $%!. $%*$+"$$ +#$% 2+#."/ ;<=>?@AB<=C?<>LED><P<E= 1 . 2.. +#. QRSTUV(WSTX%"&&  2$ !Y