第一部分：Multiple choice（单选题） （共5道题）
1.Network core does not include ( ) .
A、routers
B、packet switches
C、links
D、smart phones
答案：D
2.End system does not include （ ）.
A、 router
B、 smart phone
C、 PCs
D、 Web servers
答案：A
3.The original reason for the Internet to use packet switching is ( ).
A、Statistical multiplexing enables efficient sharing of expensive long-haul links.
B、Packet switching enables data to be routed faster than circuit switching.
C、It is less work for a router to process a packet than to process the data in a circuit.
D、 Packet switching requires simpler hardware on the routers than circuit switching.
答案：A
4 Which of the following is not a performance measure in computer networks.
A、delay
B、 packet loss
C、packet size
D、 end-to-end throughput
答案：C
题目解析：
In addition to delay and packet loss, another critical performance measure in computer networks is end-to-end throughput.
5 By which of the following access technology, bandwidth is not shared among the users？
A、 WiFi
B、 HFC
C、 ADSL
D、 None of the above
答案：C
第二部分：blank filling（填空题，填入英文或者数字，不能填中文） （共4道题）
1 The full text in English of FTTH is Fiber To The Home.
The full text in English of ISP is ( ).
The full text in English of TDM is ( ).
参考答案：
Internet Service Provider（或Internet service provider）、
Time-Division Multiplexing（或time-division multiplexing或time division multiplexing或Time Division Multiplexing）
题目解析：
FTTH, ISP,TDM,FDM,ADSL,HFC, TCP, IP, etc
2 There are two fundamental approaches to move data through a network of links and switches: circuit switching and ( ). And in a circuit-switched network, a circuit in a link is implemented with either ( ) or TDM.
参考答案：
packet switching、
frequency-division multiplexing（或FDM或Frequency-Division Multiplexing）
3 Describe the layers in the Internet protocol stack from top to down: application layer, ( ) layer, ( ) layer, ( ) layer and physical layer.
参考答案：
transport 或 Transport、
network 或 Network、
link 或 Link
题目解析：
应用层application、传输层transport、网络层network、链路层link、物理层physical
4 OSI reference model have ( ) layers.
参考答案：
7 或 seven 或 Seven
题目解析：
application layer, presentation layer, session layer, transport layer, network layer,data link layer, and physical layer.
第三部分：True or False（判断题） （共6道题）
1 Packet switching allows more users to use network than circuit switching.
A、true
B、 false
答案：A
2 There is no difference between a host and an end system.
A、 True
B、 False
答案：B
题目解析：
network edge: millions of connected computing devices:
hosts = end systems
running network apps
3 A host doesn't process network-layer in the Internet protocol stack.
A、 True
B、 False
答案：B
题目解析：
Router：network、link、physical[3]；
Link-layer switch： link、physical[2]；
Host：application、transport、network、link、physical[5]。
4 The full text in English of RFC is request for comments.
A、 True
B、False
答案：A
题目解析：
Internet standards:
RFC: Request for comments
5 All communication activity in Internet governed by protocols.
A、 True
B、False
答案：A
题目解析：
network protocols:
machines rather than humans
all communication activity in Internet governed by protocols
6 Packet switching is commonly used in traditional telephone networks
A、True
B、False
答案：B
题目解析：
It's circuit switching that is commonly used in traditional telephone networks!
第四部分：comprehensive questions（综合题） （共3道题）
1 [ 组合题 ]
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.
1 [ 填空题 ]
a. When circuit switching is used, how many users can be supported? ( ) users.
参考答案：
2 或 two 或 Two
2 [ 判断题 ]
b. For the remainder of this problem, suppose packet switching is used. There be a queuing delay if three users transmit at the same time.
A、 True
B、 False
答案：A
3 [ 填空题 ]
c. Find the probability that a given user is transmitting. ( )
参考答案：
0.2 或 20%
4 [ 填空题 ]
d. Suppose now there are three users. Find the probability that at any giventime, all three users are transmitting simultaneously (at the same time). ( ).
参考答案：
0.008
5 [ 填空题 ]
e. Suppose now there are four users. Find the probability that at any given time, three users are transmitting simultaneously(at the same time).
( ).
参考答案：
0.0256
题目解析：
a. 2Mbps/1Mbps = 2
b. (True)
c. 0.2
d. C(3,3)(0.2*0.2*0.2)=0.2*0.2*0.2 = 0.008
e. C(4,3)(0.2*0.2*0.2)(1-0.2) = 4*0.2*0.2*0.2*0.8 = 0.0256
Notice:
c(m,n)=p(m,n)/n!=m!/((m-n)!*n!)
2 [ 组合题 ]
Consider a packet of length L which begins at end system A and travels over three links to a destination end system.
These three links are connected by two packet switches. Assuming no queuing delays, and suppose now the packet is 1,500 bytes,
the propagation speed on all three links is 2.5 *10^8 m/s, the transmission rates of all three links are 2 Mbps, the packet switch
processing delay is 3 msec, the length of the first link is 5,000 km, the length of the second link is 4,000 km, and the length of the last link is 1,000 km.
1 [ 填空题 ]a. For these values, what is the propagation delay? ( )ms
参考答案：
40
题目解析：
d(proc) = 3ms * 2 = 6ms;
d(trans)=L/R = 1500B/2Mbps * 3 = 1500*8 b * 3 /(2000000b/s) = 18 ms;
d(prop)=(d1+d2+d3)/s = (5000km + 4000km+1000km)/(2.5*10^8m/s) = 40 ms;
d(end-to-end) = d(proc) + d(prop) +ｄ(trans) = 64 ms.
2 [ 填空题 ]For these values, what is the transmission delay? ( )ms
参考答案：
18
3 [ 填空题 ]For these values, what is the processing delay? ( )ms
参考答案：
6
4 [ 填空题 ] For these values, what is the end-to-end delay? ( )ms
参考答案：
64
3 [ 组合题 ] Suppose two hosts, A and B, are separated by 10,000 kilometers and are connected by a direct link of R = 2 Mbps. Suppose the propagation speed
over the link is 2.5*10^8 meters/sec.
1 [ 填空题 ]
a. The value of the bandwidth-delay product is ( ) bit. (Notice: the bandwidth-delay product = R*d(prop), and d(prop) is the propagation rate.)
参考答案：
80000
题目解析：
带宽-时延积R·dprop =R * d/s = 2Mbps * 10000km/(2.5*10^8m/s) = 2*10^6bps * 10^4*10^3 m/(2.5*10^8 m/s) = 8 * 10^4 b = 8*10^4 bit
2 [ 填空题 ]
b. Consider sending a file of 800,000 bits from Host A to Host B. Suppose the file is sent continuously as one large message. What is the maximum
number of bits that will be in the link at any given time? ( )
参考答案：
80000
题目解析：
800,000 bits > R*d(prop), so all the link is full of bit, and some bits are not on the way.
3 [ 填空题 ]
c. Consider sending a file of 8000 bits from Host A to Host B. Suppose the file is sent continuously as one large message. What is the maximum
number of bits that will be in the link at any given time? ( )
参考答案：
8000
题目解析：
8000 bits < R*d(prop),
4 [ 填空题 ]
d. What is the width (in meters) of a bit in the link? ( )
参考答案：
125
题目解析：
该链路上一个bit的宽度 : d/(R*d(prop)) = 10000km/( 8*10^4 bit) = 125 m/b
5 [ 填空题 ]
e. Derive a general expression for the width of a bit in terms of the propagation speed s, the transmission rate R, and the length of the link m.
参考答案：
s/R
题目解析：
一个bit宽度 = d/(R*d(prop)) = d/( R * d/s) = s/R