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課程名稱︰計算機網路
課程性質︰系必修
課程教師︰逄愛君
開課學院：電資學院
開課系所︰資工系
考試日期（年月日）︰2010.4.21
考試時限（分鐘）：180
是否需發放獎勵金：是
（如未明確表示，則不予發放）
試題 :
1.(6%)Why is it said that pachet switching emplots statistical multiplexing?
Constrast statistical multiplexing with the multiplexing that takes places
in TDM.
Ans:
(a)In a packet switched network, the packets from different sources
flowing on a link do not follow any fixed, pre-defined pattern.
(b)In TDM circuit seitching, each host gets the same slot in a
revolving TDM frame. Statistical multiplexing ensures that slots
will not be wasted (whereas TDM can waste slots).
2.(8%)Consider a 1Gbps link. Users are generationg data at a rate of 100kbps
when busy, but are busy generation data only with probability p=0.1.
a.What is N, the maximum number of users that can be supported simultaneously
under circuit switching?
b.Now consider packet switching and a user population of M users. Give a
formula (in terms of p, M, N) for the probability that more than N users
are sending data.
3.(8%)Consider the car-caravan analogy shown in Figure 1. Assume a propagation
speed of 50 km/hour, and a toll booth takes 12 seconds to service a car.
a.Suppose the caravan travels 200km, beginning in front of one tollbooth,
passing through a second tollbooth, and finishing just before a third
tollbooth.
What is the end-to end delay?
b.Repeat (a), now assuming that there are five cars in the caravan instead of
ten
4.(12%)In modern packet-switched networks, the source host segments long,
application-layer messages (for example, an image or a music file) into
smaller packets and sends the packets into the network. The receiver then
reassembles the packets back into the original message. We refer to this
process as message segmentation. Figure 2 illustrates the end-to-end
transport of a message with and without message segmentation. Consider a
message that is 8*10^6 bits long that is to be sent form source to
destination. Suppose each link in the figure is 2Mbps. Ignore propagation,
queuing, and processing delays.
a.Consider sending the message from source to destination without message
segmentation. How long does it take to move the message from the source
host to destination host?
b.Now suppose that the message is segmented into 4,000 packets, with each
packet being 2,000 bits long. Compare this result with your answer in part
(a).
c.Discuss the drawbacks of message segmentation.
i.Pachets have to be put in sequence at the destination.
ii.Message segmentation results in many smaller packets.
Since header size is usually the same for all packets regardless of
their size, with message segmentation the total amount of header
bytes is more.
5.(6%)Layering of Internet
a.Why do we need the layering concept for Internet?
b.Please claborate on the pros and cons of the Layering.
6.(6%)Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP?
Ans:
The applications associated with those protocols require that all
application data be received in the correct order and without gaps.
TCP provides this service whereas UDP does not.
7.(10%)Suppose Alice, with a Web-based e-mail account (such as Hotmail or
gmail), sends a message to Bob, who accesses his mail from his mail server
using POP3. Discuss how the nessage gets from Alice's host to Bob's host. Be
sure to list the series of application-layer protocols that are used to move
the message between the two hosts.
8.(16%)HTTP and DNS
a.What is web caching?
b.Why HTTP/1.1 uses persistent TCP connection for web accessing?
c.What are Top-level domain(TLD) servers and Authoritative DNS servers?
d.Bob access the Internet at home via a dynamic IP maintained by HiNet. If
Bob would like to install a web server at home with the demain name
"babhome.idv.tw". Please elaborate on how Bob register and activates the
domain name. Note that the provider of "idv.tw" is "TWNIC", and the
authoritative DNS server is "ns1.twnic.net.tw"
9.(8%)Consider a new peer Alice who joins BitTorrent without possessing any
chunks. Without any chunks, she cannot become a top-four uploader for any of
the other peers, since she has nothing to upload. How then will Alice get her
first chunk?
10.(20%)Realiable Data Transfer
a.Protocol rdt 2.1 provides reliable data transfer by using ACK/NAK and
putting a sequence number into the transmitted data. Instead of using rdt
2.1, some of the students in the class proposed a new protocol to let the
sequence numver "1" represent "New Packet" and the wequence number "0"
represent "Retransmitted Packet". Please re-draw the state diagrams of
senders and receivers.
b.Is there any problem for the new protocol? Please elaborate on ti with a
message flow. If so, does the situation also occur for rdy 2.1? Why of Why
not?
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