计算机网络课后习翘指导 陕西师范大学 塞)，所有的应用分别从一条或名条特定的网络链路传输。而由于每条链路的带宽足够处理 所有的应用数据，因此不会发生拥塞现象（只会有非常小的队列）。 P4.Consider the ciruit-switched network in Figure 1.13.Recall that there are 4 circuits on each link Label the four switches A.B.C and D,going in the clockwise direction. a.What is the maximum number of simultaneous connections that can be in progress at any one time in this network? b.Suppose that all between switchesAand C.What is the maximum number of simultaneous connections that can be in progress? e.Suppose we want to make four connections between switches A and C,and another four connections between switches B and D.Can we route these calls through the four links to accommodate all eight connections? 答：a因为这4对相邻交换机，每对之间可以建立4条连接，因此最多可以建立16条连 接。 b.我们可以通过右上角交换机有四条连接，通过左下角交换机有四条连接。 一共有8个连接 c是的。对于A和C之间的链接，我们可以选择两条路径通过B两条路径通过D。对于B 和D之间的链接，我们可以选择两条路径通过A两条路径通过C。在这种情况下，任意一条 链路至少有四个连接。 P5.Review the car-caravan analogy in Section 1.4.Assume a propagation speed km/hour. a.Suppose the caravan travels 150 km,beginning in front of one tollbooth,passing through a second tollbooth.and finishing iust after a third tollbooth.What is the end-to-end delav? b.Repeat (a),now assuming that there are eight cars in the caravan instead of ten. 答：由于收费站间隔150km,车速100kmh,收费站以每12s通过一辆汽车的速度提供服 务。 a10辆车，第一个收费站要花费1205，即2分钟来处理。每一辆车要达到第 个收费站都 会有75100-0.75小时，即45分钟的传播延时，因此所有的车在通过第一个收费站时花费47 分钟，在通过第二个收费站和第二条链路时同样花费47分钟，通过第三个收费站花费2分 钟。因此，（端到端）总延时为96分钟。 要站之间的延时为8x2秒+5分=6分36称，端到强总延时 倍再加上第 (46min+36sec)×2+8×12sec=94min+38seg P6.This elementary problem begins to explore propagation delay and transmission delay.two central concepts in data networking Consider two hosts, Aand B.conected by a single link of rate Rbps Suppose that the wo hostsare separated bymmeters,and suppose the propagation speed long th link is s meters/sec.Host A is to send a packet of size L bits to Host B a.Express the propasation delav.dpop in terms of m and s. b.Determine the transmission time of the packet,dirans,in terms of L and R aeanmdqueuinegdelhysobtainaneypresionforteendoenddely d.Suppose ost A begins to transmit the packet at time0.At time where is the last bit of the packet e.Suppose dorop is greater than dirars.At time /drans.where is the first bit of the packet? f.Supposedpp is less than At time where is the first bit of the packet? 3 计算机网络课后习题指导 陕西师范大学 3 塞），所有的应用分别从一条或多条特定的网络链路传输。而由于每条链路的带宽足够处理 所有的应用数据，因此不会发生拥塞现象（只会有非常小的队列）。 P4. Consider the circuit-switched network in Figure 1.13. Recall that there are 4 circuits on each link. Label the four switches A, B, C and D, going in the clockwise direction. a. What is the maximum number of simultaneous connections that can be in progress at any one time in this network? b. Suppose that all connections are between switches A and C. What is the maximum number of simultaneous connections that can be in progress? c. Suppose we want to make four connections between switches A and C, and another four connections between switches B and D. Can we route these calls through the four links to accommodate all eight connections? 答：a. 因为这 4 对相邻交换机，每对之间可以建立 4 条连接，因此最多可以建立 16 条连 接。 b. 我们可以通过右上角交换机有四条连接，通过左下角交换机有四条连接，一共有 8 个连接。 c. 是的。对于 A 和 C 之间的链接，我们可以选择两条路径通过 B 两条路径通过 D。对于 B 和 D 之间的链接，我们可以选择两条路径通过 A 两条路径通过 C。在这种情况下，任意一条 链路至少有四个连接。 P5. Review the car-caravan analogy in Section 1.4. Assume a propagation speed of 100 km/hour. a. Suppose the caravan travels 150 km, beginning in front of one tollbooth, passing through a second tollbooth, and finishing just after a third tollbooth. What is the end-to-end delay? b. Repeat (a), now assuming that there are eight cars in the caravan instead of ten. 答：由于收费站间隔 150km，车速 100km/h，收费站以每 12s 通过一辆汽车的速度提供服 务。 a. 10 辆车，第一个收费站要花费 120s，即 2 分钟来处理。每一辆车要达到第二个收费站都 会有 75/100=0.75 小时，即 45 分钟的传播延时，因此所有的车在通过第一个收费站时花费 47 分钟，在通过第二个收费站和第二条链路时同样花费 47 分钟，通过第三个收费站花费 2 分 钟。因此，（端到端）总延时为 96 分钟。 b. 每两个收费站之间的延时为 8×12 秒＋45 分＝46 分 36 秒， 端到端总延时是该时延两 倍再加上第三个收费站传输时延，即： (46￾￾￾ + 36￾￾￾) × 2+8 × 12￾￾￾ = 94￾￾￾ + 38￾￾￾ P6. This elementary problem begins to explore propagation delay and transmission delay, two central concepts in data networking. Consider two hosts, A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters, and suppose the propagation speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B. a. Express the propagation delay, d prop, in terms of m and s. b. Determine the transmission time of the packet, dtrans, in terms of L and R. c. Ignoring processing and queuing delays, obtain an expression for the endto-end delay. d. Suppose Host A begins to transmit the packet at time t = 0. At time t = dtrans, where is the last bit of the packet? e. Suppose dprop is greater than dtrans. At time t = dtrans, where is the first bit of the packet? f. Suppose dprop is less than dtrans. At time t = dtrans, where is the first bit of the packet?