Homework Network and Concept
Final Exam of Computer Networks (ICE 1230)
2008.5. 27.
Answer in English and Total 15 points
1. Why is it said that packet switching employs statistical multiplexing? Contrast statistical multiplexing with the multiplexing that takes place in TDM. (1 point)
Answer: In a packet switched network, the packets from different sources flowing on a link do not follow any fixed, pre-defined pattern. In TDM circuit switching, each host gets the same slot in a revolving TDM frame.
2. In BitTorrent, suppose Alice provides chunks to Bob throughout a 30-second interval. Will Bob necessarily return the favor and provide chunks to Alice in the same interval? Why or why not? (1 point)
Answer: It is not necessary that Bob will also provide chunks to Alice. Alice has to be in the top 4 neighbors of Bob for Bob to send out chunks to her; this might not occur even if Alice provides chunks to Bob throughout a 30 second interval.
3. What is a way using Last-Modified: header line in HTTP? (1 point)
Answer: The header line is used by Conditional GET to check whether the object asked has been modified.
4. We saw that there is no network-layer protocol that can be used to identify the hosts participating in a multicast group. Given this, how can multicast applications learn the identities of the hosts that are participating in a multicast group? (1 point)
Answer: The protocol must be built at the application layer. For example, an application may periodically multicast its identity to all other group members in an application-layer message.
5 In these days, the Internet needs to include the wireless communications network in addition to the wired networks. What is a way to extend the congestion control of TCP in this case? Why? (2 point)
Answer: Packet losses occur from network congestion and wireless communication. Since the wired Internet does not consider the packet loss due to errors in wireless communication, the congestion control of TCP should be revised. A way
2008.5. 27.
Answer in English and Total 15 points
1. Why is it said that packet switching employs statistical multiplexing? Contrast statistical multiplexing with the multiplexing that takes place in TDM. (1 point)
Answer: In a packet switched network, the packets from different sources flowing on a link do not follow any fixed, pre-defined pattern. In TDM circuit switching, each host gets the same slot in a revolving TDM frame.
2. In BitTorrent, suppose Alice provides chunks to Bob throughout a 30-second interval. Will Bob necessarily return the favor and provide chunks to Alice in the same interval? Why or why not? (1 point)
Answer: It is not necessary that Bob will also provide chunks to Alice. Alice has to be in the top 4 neighbors of Bob for Bob to send out chunks to her; this might not occur even if Alice provides chunks to Bob throughout a 30 second interval.
3. What is a way using Last-Modified: header line in HTTP? (1 point)
Answer: The header line is used by Conditional GET to check whether the object asked has been modified.
4. We saw that there is no network-layer protocol that can be used to identify the hosts participating in a multicast group. Given this, how can multicast applications learn the identities of the hosts that are participating in a multicast group? (1 point)
Answer: The protocol must be built at the application layer. For example, an application may periodically multicast its identity to all other group members in an application-layer message.
5 In these days, the Internet needs to include the wireless communications network in addition to the wired networks. What is a way to extend the congestion control of TCP in this case? Why? (2 point)
Answer: Packet losses occur from network congestion and wireless communication. Since the wired Internet does not consider the packet loss due to errors in wireless communication, the congestion control of TCP should be revised. A way