计算机网络课后习题指导 陕西师范大学 g.Suppose s-2.5 10s,L-120 bits,and R=56 kbps.Find the distance m so that dpopequals 答：a传播时延：drop=m/s秒 b.传输时延dtrans=L/R秒： c.端到端时延demd-ta-end=m/s+L/R秒： d.该分组的最后一个bit刚刚离开主机A: c第一个比特距离主机A为dras×s米，还没有到达B: 第一个比特已经到达B: g由g=得，m=片×s=0×2.5×10m≈536km。 P7.In this problem.we consider sending real-time voice from Host a to Host b over a packet-switched network (VolP)Host analog voice toa digital 6 kbps bit stre on the fly.Hostthen groups the bits into 56-byte packets.There is one link between Hosts A and B:its transmission rate is 2 Mbps and its propagation delay is 10 msec.As soon as Host A gathers a packet.it sends it to Host B.As soon as Host B receives an entire packet,it converts the packet's bits to an analog signal How much time elapses from the time a bit is created (from the original analog signal at Host A) until the bit is decoded (as part of the analog signal at Host B)? 答：考虑分组第一个比特bi。在该b传输之前，需要将分组所有的b都产生，这个分组产 生时延为： 64x10s=7×10-3s=7ms 然后是分组传输时延为： =6s=0.224ms 最后由题意可得分组在链路上的传播时延为：10ms 综上，节点总时延为上面三个时延之和： =7+0.224+10=17.224ms P8.Suppose users share a Mbps link.Also suppose each user requires 150 kbps when transmitting. but each user transmits only 10 percent of the time.(See the discussion of packet switching versus circuit switching in Section 1 3 a When circuit switching is used,how many users can be su orted? b.For f this packet switching is used.Find the given user is transmitting. c.Suppose there are 120 users.Find the probability that at any given time.exactly n users are transmitting simultaneously.(Hint:Use the binomial distribution.) d.Find the probability that there are 21 or more users transmitting simultaneously 答：a当使用电路交换时，由于每一个用户共享带宽，所以用户数量为： 0=20个： b.由题知每个用户仅有10%的时间在传输，所以任意时刻某用户传输概率为10%： c.当有120个用户，其中恰好有n个用户同时传输的概率为：计算机网络课后习题指导 陕西师范大学 4 g. Suppose s = 2.5 · 108, L = 120 bits, and R = 56 kbps. Find the distance m so that dprop equals dtrans. 答：a.传播时延: ￾￾￾￾￾ = ￾/￾秒； b.传输时延 ￾￾￾￾￾￾ = ￾/￾秒； c.端到端时延 ￾￾￾￾￾￾￾￾￾￾￾ = m/s +L/R秒； d.该分组的最后一个 bit 刚刚离开主机 A； e.第一个比特距离主机 A 为d￾￾￾￾￾ × ￾米，还没有到达 B； f.第一个比特已经到达 B； g.由 ￾ ￾ = ￾ ￾ 得，m = ￾ ￾ × ￾ = ￾￾￾ ￾￾×￾￾￾ × 2.5 × 10￾￾ ≈ 536￾￾。 P7. In this problem, we consider sending real-time voice from Host A to Host B over a packet-switched network (VoIP). Host A converts analog voice to a digital 64 kbps bit stream on the fly. Host A then groups the bits into 56-byte packets. There is one link between Hosts A and B; its transmission rate is 2 Mbps and its propagation delay is 10 msec. As soon as Host A gathers a packet, it sends it to Host B. As soon as Host B receives an entire packet, it converts the packet’s bits to an analog signal. How much time elapses from the time a bit is created (from the original analog signal at Host A) until the bit is decoded (as part of the analog signal at Host B)? 答：考虑分组第一个比特 bit。在该 bit 传输之前，需要将分组所有的 bit 都产生，这个分组产 生时延为： ￾￾×￾ ￾￾×￾￾￾ ￾ = 7 × 10￾￾￾ = 7￾￾ 然后是分组传输时延为： ￾ ￾ = ￾￾×￾ ￾×￾￾￾ ￾ = 0.224￾￾ 最后由题意可得分组在链路上的传播时延为：10ms 综上，节点总时延为上面三个时延之和： d￾￾￾￾￾ = 7 + 0.224 + 10 = 17.224￾￾ P8. Suppose users share a 3 Mbps link. Also suppose each user requires 150 kbps when transmitting, but each user transmits only 10 percent of the time. (See the discussion of packet switching versus circuit switching in Section 1.3.) a. When circuit switching is used, how many users can be supported? b. For the remainder of this problem, suppose packet switching is used. Find the probability that a given user is transmitting. c. Suppose there are 120 users. Find the probability that at any given time, exactly n users are transmitting simultaneously. (Hint: Use the binomial distribution.) d. Find the probability that there are 21 or more users transmitting simultaneously. 答：a. 当使用电路交换时，由于每一个用户共享带宽，所以用户数量为： ￾￾￾￾￾ ￾￾￾￾￾￾￾ = 20个； b. 由题知每个用户仅有 10%的时间在传输，所以任意时刻某用户传输概率为 10%； c. 当有 120 个用户，其中恰好有 n 个用户同时传输的概率为：