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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`B-:-6;:<.8<4;,./:,F;F</678:</.-:88-E /D-8:5,;;<,8:-</D-8:..-:;:7/8,66,7a-<5,7-:-.5.87/4<F. -<D,5<-:/D-8:.-8:.>/<@,4;6B-:-6;:<..49//5,/-: 6<9,6,55//B,.5,/.,6./7/-4?8,665bbcA//50<-:/-;,.5-,5<D. ;:,// OPIMKQIUXdMMQR;:7/8,66,7IUMQRMJdSIMIVUdWMRWRZQVUR URMPVJ\//MQRKIJKLIMN]PIMKQIUXdZZJVdKQe feg hiTijkil [mTnokpO q.5,-,8<44.8,-<./BD.5-</.54//,/0<4<..5/7/-4-<,.<-:rs -:4//,/<.-<;,//-:<:,;,8C-E/D-8:5.-D<CB-.5/-<95F55 .-<;,8C-/<00@5<F,,96/t3:/t<0-:;,8C-/5-4.597-:.-E D<C,.5-:<F..;<-<8<6 q.;,8C-/D-8:.B-:/.</<8,66<8,-<.0<,;,8C-3:/4,./-:,- -:/.</F59,.5D5-:<.-:6.C/B,.5-:/.</8:565;<8//.-4