网络课后作业
Chap.2
3.Television channels are 6 MHz wide. How many bits/sec can be sent if four-level digital signals are used? Assume a noiseless channel.
电视频道的带宽是6MHz。如果使用4级数字信号，则每秒钟可以发送多少位？假设电视频道为无噪声信道。
4. If a binary signal is sent over a 3-kHz channel whose signal-to-noise ratio
is 20dB, what is the maximum achievable data rate?
如果在一条3kHz信道上发送一个二进制信号，该信道的信噪比为20dB，则最大可达到的数据传输率为多少？
28.Ten signals, each requiring 4000 Hz, are multiplexed on to a single channel
using FDM. How much minimum bandwidth is required for the multiplexed
channel? Assume that the guard bands are 400 Hz wide.
有10个信号，每个都要求4000Hz，现在用FDM将它们复用在一条信道上。对于被复用的信道，最小要求多少带宽？假设防护频段为400Hz宽。
43.Suppose that x bits of user data are to be transmitted over a k-hop path in a
packet-switched network as a series of packets, each containing p data bits and h
header bits, with x >>p + h. The bit rate of the lines is b bps and the propagation
delay is negligible. What value of p minimizes the total delay?
假定x位用户数据将以一系列分组的形式，在一个分组交换网络中沿着一条共有
k跳到路径向前传输，每个分组包含p位数据和h位的头，这里x>>p+h。线路的传输率为b bps，传播延迟忽略不计。请问什么样的p值使总延迟最小？
Chap.3
3.The following data fragment occurs in the middle of a data stream for whichthe byte-stuffing algorithm described in the text is used: A B ESC C ESC FLAGFLAG D. What is the output after stuffing?
下面的数据片段发生数据流用于文本中所述的梯段字节填充算法 ︰ B ESC C ESC FLAG FLAG D.在填料后的输出是什么？
5. A bit string, 0111101111101111110, needs to be transmitted at the data lin
k layer. What is the string actually transmitted after bit stuffing?
位串0111101111101111110需要在数据链路层上被发送，请问，经过位填充之后实际被发送出去的是什么？
8.To provide more reliability than a single parity bit can give,an error-detecting coding schema uses one parity bit for checking all the odd-number bits and a second parity bit for all the even-numbered bits.What is the Hamming distance of this code?
提供比单奇偶校验位可以给更高的可靠性，错误检测的编码架构为所有偶数位使用一个奇偶校验位为检查所有奇数位和第二的奇偶校验位。此代码的汉明距离是什么？
17.A channel has a bit rate of 4 kbps and a propagation delay of 20 msec. For what range of frame sizes does stop-and-wait give an efficiency of at least 50 percent?
一个信道的位速率为4kbps，传输延迟为20ms。请问帧的大小在什么范围内，停
-等协议才可以获得至少50％的效率？
29. Frames of 1000 bits are sent over a 1-Mbps channel using a geostationary satellite whose propagation time from the earth is 270 msec. Acknowledgements are always piggybacked onto data frames. The headers are very short. Three-bit sequence numbers are used. What is the maximum achievable channel utilization
for (a) Stop-and-wait. (b) Protocol 5. (c) Protocol 6.
利用地球同步卫星在一个1Mbps的信道上发送1000位的帧，该信道离开地球的传输延迟为270ms。确认信息总是被捎带在数据帧傻姑娘。头部非常短，并且使用3位序列号。在下面的协议中，最大可获得的信道利用率是多少？(a)停-等协议(b)协议5(c)协议6
37.What is the minimum overhead to send an IP packet using PPP?Count only the over-head introduced by PPP itself ,not the IP header overhead.
什么是开销最少要发送 IP 数据包使用 PPP 的？数只介绍了 PPP 本身，不 IP 标头开销过度头。
Chap.4
2.A group of N stations share a 56-kbps pure ALOHA channel.Each station outputs a 1000-bit frame on an average of once every 100 sec,even if the previous one has not yet been sent (e.g.,the stations can buffer outgoing frames). What is the maximum value of N?
N个站共享一个56kbps的纯ALOHA信道？每个站平均每100秒输出一个1000位帧，即使前面的帧还没有被送出，它也这样进行（比如这些站可以将送出的帧缓存起来）。请问N的最大值是多少？
3. Consider the delay of pure ALOHA versus slotted ALOHA at low load. Which one is less? Explain your answer.考虑在底载荷情况下纯的ALOHA和分槽的ALOHA的
延迟。哪一个延迟更小？请说明你的理由。
5.A large population of ALOHA users manages to generate 50 request/sec, including both originals and retransmissions. Time is slotted in units of 40 mesc.
一大群ALOHA用户每秒钟产生50个请求，包括原始的请求和重传的请求。时槽单位为40ms。
(a)What is the chance of success on the first attempt?首次发送成功的几率是多少？
(b)恰好k次冲突之后成功的概率是多少? What is the probability of exactly k collisions and then a success?
(c)所需传送次数的期望值是多少？What is the excepted number of transmission attempts needed?
44. To make VLANs work,configuration tables are needed in the switches and bridges. What if the VLANs of Fig.4-49(a) use hubs rather than multidrop cables?Do the hubs need configurations tables,too?Why or why not?
为了使 Vlan 工作，配置表需要在交换机和桥梁。如果 Fig.4-49(a) Vlan 使用集线器，而不是多点电缆？集线器太需要配置表了吗？为什么或为什么不？
Chap.5
10.If delays are recorded as 8-bit numbers in a 50-router network, and delay vectors are exchanged twice a second, how much bandwidth per (full-duplex) line is chewed up by the distributed routing algorithm? Assume that each router has three lines to other routers.
假设在一个50台路由器的网络中，用8位数值记录延迟信息，并且每秒钟交换延迟矢量两次，请问，分布式路由算法需要在每条（全双工）线路上消耗多少宽带？假设每台路由器有三条线路连接到其他的路由器。
22.A datagram subnet allows routers to drop packets whenever they need to. The probability of a router discarding a packet is p. Consider the case of a source host connected to the source router, which is connected to the destination router, and then to the destination host. If either of the routers discards a packet, the source host eventually times out and tries again. If both host-router and router-router lines are counted as hops, what is the mean number of
(a) hops a packet makes per transmission?
(b) transmissions a packet makes?
(c) hops required per received packet?
一个数据报子网允许路由器在必要的时候丢弃分组。一台路由器丢弃一个分组的概率为p。请考虑这样的情形：源主机连接到路由器，源路由器连接到目标路由器，然后目标路由器连接到目标主机。如果任一台路由器丢掉了一个分组，则源主机最终会超时，然后
再重试发送。如果主机至路由器以及路由器之间的线路都计为一跳，那么：
（a）一个分组每次传输中的平均跳数是多少？
（b）一个分组的平均传输次数是多少？
（c）每个接收到的分组平均要求多少跳？
39.A network on the Internet has a subnet mask of 255.255.240.0. What is the maximum number of hosts it can handle?
一个 网络在互联网上有 255.255.240.0 的子网掩码。它可以处理的主机的最大数目是什么？
53.When the IPv6 protocol is introduced,does the ARP protocol have to be changed?IF so,are the changes conceptual or technical?
介绍IPv6协议时，ARP协议必须改变吗？如果变化，是概念变化还是技术变化？
Chap.6
14.Why does UDP exists? Would it not have been enough to just let user processes send raw IP packets?
UDP为什么有必要存在？难道只让用户进程发送原始的IP分组还不够吗？
16.A client sends a 128-byte request to a server located 100 km away over a 1-gigabit optical fiber.What is the efficiency of the line during the remote procedure call?
一个客户端发送 128 字节的请求到服务器位于 1 千兆位光纤 100 公里。在远程过程调用过程中的效率是线的什么？
20.Datagram fragmentation and reassembly are handled by IP and are invisible to TCP.Does this meant that TCP does not have to worry about data arriving in the wrong order?
数据报的分段和重组机制是由IP来处理的，对于TCP是不可见的。这是否意味着TCP不用担心数据错序到达的问题呢？
23.A process on host 1 has been assigned port p,and a process on host 2 has been assigned port q.Is it possible for there to be two or more TCP connections between these two ports at the same time?
主机上1的一个进程已经被分配了端口p，主机2上的一个进程已经被分配了端口q,请问这两个端口之间有可能同时存在两个或多个TCP连接吗？
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