东北大学软件学院计算机网络平时测验1

东北大学软件学院计算机网络的平时测验，授课老师是lyx，考试形式是开卷考试。题目是计算机网络（自顶向下）教材的课后习题。

课程名称：计算机网络 (Computer Networks )

Q1. The word protocol is often used to describe diplomatic relations. How does Wikipedia describe diplomatic protocol?

Q2. List six access technologies. Classify each one as home access, enterprise access, or widearea wireless access.

Q3. Dial-up modems, HFC, DSL and FTTH are all used for residential access. For each of these access technologies, provide a range of transmission rates and comment on whether the transmission rate is shared or dedicated.

Q4. Describe the most popular wireless Internet access technologies today. Compare and contrast them.

Q5. Suppose there is exactly one packet switch between a sending host and a receiving host.

The transmission rates between the sending host and the switch and between the switch and the receiving host are R1 and R2, respectively. Assuming that the switch uses store-and-forward packet switching, what is the total end-to-end delay to send a packet of length I? (Ignore queuing, propagation delay, and processing delay.)

Q6. What advantage does a circuit-switched network have over a packet-switched network?

What advantages does TDM have over FDM in a circuit-switched network?

Q7. Suppose users share a 2 Mbps link. Also suppose each user transmits continuously at 1 Mbps when transmitting, but each user transmits only 20 percent of the time.

When circuit switching is used, how many users can be supported?
For the remainder of this problem, suppose packet switching is used. Why will there be essentially no queuing delay before the link if two or fewer users transmit at the same time? Why will there be a queuing delay if three users transmit at the same time?
Find the probability that a given user is transmitting.
Suppose now there are three users. Find the probability that at any given time, all three users are transmitting simultaneously. Find the fraction of time during which the queue grows.

Q8. Why will two ISPs at the same level of the hierarchy often peer with each other? How does an IXP earn money?

Q9. Some content providers have created their own networks. Describe Google’s network. What motivates content providers to create these networks?

Q10. Consider sending a packet from a source host to a destination host over a fixed route. List the delay components in the end-to-end delay. Which of these delays are constant and which are variable?

Q11. How long does it take a packet of length 1,000 bytes to propagate over a link of distance 2,500 km, propagation speed 2.5*10^8 m/s, and transmission rate 2 Mbps? More generally, how long does it take a packet of length I to propagate over a link of distance d, propagation speed s, and transmission rate R bps? Does this delay depend on packet length? Does this delay depend on transmission rate?

Q12. Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has three links, of rates R4=500 kbps, R2=2 Mbps, and R=1 Mbps.

Assuming no other traffic in the network, what is the throughput for the file transfer?
Suppose the file is 4 million bytes. Dividing the file size by the throughput, roughly how long will it take to transfer the file to Host B?
Repeat (a) and (b), but now with R reduced to 100 kbps.

Q13. Suppose end system A wants to send a large file to end system B. At a very high level, describe how end system A creates packets from the file. When one of these packets arrives to a router, what information in the packet does the router use to determine the link onto which the packet is forwarded? Why is packet switching in the Internet analogous to driving from one city to another and asking directions along the way?

Q14. What are the five layers in the Internet protocol stack? What are the principal responsibilities of each of these layers?

Q15. Suppose Alice and Bob are sending packets to each other over a computer network.

Suppose Trudy positions herself in the network so that she can capture all the packets sent by

Alice and send whatever she wants to Bob; she can also capture all the packets sent by Bob and send whatever she wants to Alice. List some of the malicious things Trudy can do from this position.

Q16. What are some of the possible services that a link-layer protocol can offer to the network layer? Which of these link-layer services have corresponding services in IP? In ТСР?

Q17. How big is the MAC address space? The IPv4 address space? The IPv6 address space? What is the maximum number of VLANs that can be configured on a switch supporting the 802.1Q protocol? Why?

Q18. Suppose nodes A, B, and C each attach to the same broadcast LAN (through their adapters). If A sends thousands of IP datagrams to B with each encapsulating frame addressed to the MAC address of B, will C’s adapter process these frames? If so, will C’s adapter pass the IP datagrams in these frames to the network layer C? How would your answers change if A sends frames with the MAC broadcast address?

Q19. Why is an ARP query sent within a broadcast frame? Why is an ARP response sent within a frame with a specific destination MAC address?

Q20. Suppose that N switches supporting K VLAN groups are to be connected via a trunking protocol. How many ports are needed to connect the switches? Justify your answer.

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