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）︰2009.4.15
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 :
九十七學年度第二學期(April 15, 2009)計算機網路 期中考
1.(20%) Circuit-switching vs. packet-switching.
a.(4%) Why is it said that packet switching employs statistical
multiplexing?
b.(8%) Suppose users share a 2Mbps link. Also suppose each user
requires 1Mbps when transmitting, but each user transmits only
20 percent of the time.
i. When circuit-switching is used, how many users can be
supported?
ii. Suppose packet switching is used. 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.
c.(8%) Consider an application that transmits data at a steady rate
(for example, the sender generates an N-bit unit of data
every k times units, where k is small and fixed).
Also, when such an application starts, it will continue
running for a relatively long period of time. Answer the
following questions, briefly justifying your answer.
i. Would a packet-switched network or a circuit-switched
network be more appropriate for this application? Why?
ii. Suppose that a packet-switched network is used and the
only traffic in this network comes from such applications
as described aboved.
Furthermore, assume that the sum of the application data
rates is less than the capacities of each and every link.
Is some form of congestion control needed? Why?
2.(10%) Packet Loss and Delay
a.(2%) How do packet loss and delay occur in the Internet?
b.(4%) How long does it take a packet of length 2,000 bytes to
propagete over a link of distance 2,000 km, propagation speed
2x10^8 m/s, and transmission rate 2 Mbps?
c.(4%) 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 R1=250 kbps,
R2=500kbps, R3=1Mbps. Suppose that the file is 2 million bytes.
Roughly, how long will it take to transfer the file to Host B?
3.(10%) Layering of Internet
a.(2%) Why do we need the layering concept for Internet?
b.(4%) Please elaborate on the pros and cons of the Layering.
c.(4%) A system has an n-layer protocol hierarchy. Applications
generate messages of the M-byte length. At each of the layers,
a K-byte header is added. What fraction of the headers is for a
packet being transmitted?
4.(10%) HTTP and DNS
a.(3%) What is web caching?
b.(3%) Why HTTP/1.1 uses persistent TCP connection for web accessing?
c.(4%) What are Top-level domain (TLD) servers and Authoritative DNS
servers?
5.(20%) P2P Applications
a.(10%) Consider a new peer Alice who joins BitTorrent without
possessing any chunks. Without any chunks, she connot 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?
b.(10%) Consider distributing a file of F=5Gbits to N peers. The
server has an upload rate of us=20Mbps, and each peer has
download rate of di=1Mbps and an upload rate of u. For N=10,100
and 1,000 and u=100Kbps, 250 Kbps, and 500Kbps, prepare a chart
giving the minimum distribution time for each of the
combinations of N and u for both client-server distribution
and P2P distribution.
6.(10%) Answer true or false to the following questions and briefly justify
your answer:
a. With the SR protocol, it is possible for the sender to receive an
ACK for a packet that falls outside of its current window.
b. With GBN, it is possible for the sender to receive an ACK for
packet that falls outside of its current window.
c. The alternating-bit protocol is the same as the SR protocol with a
sender and receiver window size of 1.
d. The alternating-bit protocol is the same as the GBN protocol with a
sender and receiver window size or 1.
7.(20%) Consider a scenario in which Host A wants to simultaneously send
packets to Hosts B and C. A is connected to B and C via a broadcast
channel - a packet sent by A is carried by the channel to both B and
C. Suppose that the broadcast channel connecting A, B and C can
independently lose and corrupt packets(and so, for example, a packet
sent from A might be correctly received by B, but not by C).
Design a stop-and-wait-like error-control protocol for reliably
transferring
packets from A to B and C, such that A will not get new data from the upper
layer until it knows that both B and C have correctly received the current
packet.
Give FSM descriptions of A and C. (Hint: the FSM for B should be essentially
the same as for C). Also, give a description of the packet format(s) used.
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