计算机网络原理（双语）C答案.docx

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1、C卷答案一、AACCDCBCCB二、TFF T T T T F T F 三、1、Ethernet is a popular CSMA/CD protocol. Explain how it works.Solution:A station first senses the wire to see whether it is free. If there is currently no other activity, the station will start transmitting its data while it continues to listen. If a collision

2、is detected, transmission stops immediately and a next attempt is made some random period later determined by an exponential backoff algorithm.2、What is the difference between a switch and a router? What effect does this have on the number of times they must forward a message.Solution:A switch is la

3、yer 2, a router is layer 3. A switch builds its switch table by eavesdropping on packets. Thus, the switch plugs and plays but will need to forward a messages to all output links if it has not heard from the destination node. A router builds its routing table through routing algorithms (eg. distance

4、 vector or link state). Thus, a router can forward a message to almost any host by sending only a single message.3、What is the purpose of ARP? Describe how ARP works.Solution:The purpose of ARP is to resolve IP addresses to MAC addresses within a subnet. ARP uses MAC-level broadcast where only the h

5、ost with the matching IP address responds to the host sending the ARP. From this response, the sending host can learn the MAC address of the responding host.4、 What is performance? Give at least three performance measures of interest for computer networks.Solution:A quantitative measurement of a sys

6、tem.Delay, throughput, loss (also resource consumption, utilization, reliability, availability).5、What is the difference between a network service and a network protocol? Is it possible to have different implementations of the same protocol?Solution:A service describes what the network offers to its

7、 clients by means of an interface. A protocol is a set of rules describing exactly how that service is to be realized through the exchange of messages between computers. These rules may be implemented differently (e.g., using different programming languages), as long as those implementations coincid

8、e with the rules.四、1、Calculate how long it takes to send a 1Mb file over a 1,5Mbps link(1) a circuit switched network that uses 24 dijferent frequency slots and a 500msec connectionestablishment time(2) a lOhop packet switched network where each link is a 1.5Mbps link. Assume no congestion and no pa

9、cket segmentation.(3) Redo your calculations for the packet switched network above assuming your maximum segment size (MSS) is 1KB.Solution:(l)latency = 1/ (1.5/24) + .5 = 16.5 seconds(2) latency = 1/ (1.5) * 10 = 6.66 seconds(3) latency = time to send pipeline + forwarding delay = 1 / 1.5 + .001/1.

10、5*10 = .673 seconds2、Suppose that root name server has the two following records :(xust.edu, NS)(, 128.119.40.Ill, A)Also, suppose that machine dns .umass.edu has the following resource record: (, 128.119.40.200, A)Suppose that you are working at XJTU and your host machine needs to resolve the name

11、. Describe the sequence of queries between your host and the servers involved in the query process. What is the number of communications needed to obtain the IP address of ? You should assume that the resolved IP address is not cached at the local host and that the processing of all queries is recur

12、sive.Solution:1. Local host contacts the local name server.2. Local name server contacts the root server.3. Root server contacts the authoritative server.4. Authoritative server sends the resolved IP address.5. Root server forwards the answer to the local name server.6. Local name server forwards the answer to the host.