课堂小测验及答疑汇总
2020年 5 月 12 日
Q1. Which is the network address after aggregation of following 3 networks: 200.2.50.0/23，
200.2.52.0/22 and 200.2.56.0/22?
A. 200.2.48.0/20
B. 200.2.50.0/20
C. 200.2.48.0/21
D. 200.2.50.0/21
Answer: A
Q2 An IP Datagram with a 20-byte header and total length of 1500 bytes is to be forwarded
into a network with a MTU of 800 bytes, thus fragmentation is necessary. What is the
offset value in the 2nd fragment?
A. 800
B. 780
C. 100
D. 97
Answer: D.
课堂练习
A company (with network 114.23.5.0/24) has 3 departments A, B and C, with 64,
10 and 30 hosts(routers) respectively. How to allocate addresses to them in the
order of A, B and then C? Write out the network address of each department. Is
there a more efficient way for address allocation?
2020年 5 月 7 日
Q1. Network 191.168.0.0 is divided into 8 subnets with same size，which is the correct
subnet mask of each subnet?
A. 255.255.0.0
B. 255.255.224.0
C. 255.255.240.0
D. 255.255.255.0
Answer: B
Q2. For a network with 100 hosts (including routers), which subnet mask shall be adopted as
a best choice? （按照最少够用原则）
A. /23
B. /24
C. /25
D. /26
Answer: C
2020 年 4 月 28 日
Q1. When the load in the network reaches the network capacity, the packet delay _____
and the network throughput reaches ______.
A. increases sharply; its minimum
B. increases sharply; its maximum
C. decreases sharply; its minimum
D. decreases sharply; its maximum
Answer: B
2020 年 4 月 24 日
Q1. Which statement is NOT true for Link State Routing?
A. Compared with Distance Vector Routing, bandwidth consumed is more
due to flooding.
B. It is based on local knowledge since routers only know part of network topology.
C. It makes use of Dijkstra algorithm to build routing table.
D. Compared with Distance Vector Routing, it converges faster and does not
have the problem of count-to-infinity.
Answer: B
Q2. Which congestion control method is reactive and adopts explicit congestion signaling
with the help of destination host?
A. Choke packet
B. Back pressure
C. Traffic aware routing
D. ECN
Answer: D
2020 年 4 月 21 日
Q1. About virtual circuit and datagram, which statement is NOT true?
A. Both use packet switching technology
B. Virtual circuit networks provide connection-oriented service, while datagram
networks provide connectionless service.
C. In datagram networks, packets from same source to same destination may
traverse over different paths, while in virtual circuit networks, belonging to
same VC, they will flow over the same path.
D. Datagram routers always use full address, while virtual circuit routers only deal
with VCI.
Answer: D
2020 年 4 月 17 日
Q1. About VLAN, which statement is NOT TRUE?
A. VLAN decouples the logical topology from the physical topology.
B. For computers connected to a HUB，some can be VLAN-aware while others
are not VLAN-aware.
C. VLAN-aware bridges can limit the scope of broadcasting frames while normal bridges cannot.
D. Legacy Ethernet frame does not support VLAN.
Answer: B
Q2. 一台有 1 个 WAN 口、4 个 LAN 口的路由器，可以连接几个网络？
A. 1
B. 2
C. 4
D. 5
Answer: B
Q3. About virtual circuit, which statement is NOT TRUE?
A. A physical connection is set up before the source sends out data packets.
B. A route from source to destination is chosen at connection setup, so all data
packets will arrive in same sequence as they are sent out.
C. Over an outgoing port of a router, 2 virtual connections cannot use the
same VCI.
D. As VCI is local-significant, it may change along the path from source to
destination.
Answer: A
2020年 4 月 14 日
Q1. In an IEEE802.11 network, at some instant, 3 stations sense the channel and find it is idle,
then multiple frames are to be transmitted, including a RTS, a re-transmitted data frame
and an ACK, which ONE will probably be sent?
A. RTS
B. re-transmitted data frame
C. ACK
D. None of them
Answer: C
Q2. In which case(s), a LAN switch will flood an incoming frame?
A. the destination address is unknown
B. the destination address is known, the outgoing port is same with incoming port
C. the destination address is known, the outgoing port is different from incoming
port
D. the destination address is 0xFF-FF-FF-FF-FF-FF
Answer: A,D
Q2. About Ethernet, consider Ture(T) or False(F) for the statements below:
- In Gigabit Ethernet, if computers connect to a Hub, they work in half-duplex,
which means that data could be transmitted in only one direction at a time,
either from computer to hub, or from hub to computer. - CSMA/CD can be used sometimes in a fully switched Ethernet.
- Manchester encoding is not used in Ethernet with data rate higher than 10Mbps.
Answer: TFT
2020 年 4 月 7 日
Q1.For a CSMA/CD network with bus topology, assume the frame time is T0, and the
propagation delay of the cable is τ, which are the vulnerable period and
contention slot time respectively?
A. T0, 2T0
B. T0, 2τ
C. τ, 2T0
D. τ, 2τ
Answer: D
Q2. About IEEE802 Reference Model, determine True(T) or False(F) for the statements
below:
- Different LAN may have different LLC sub-layer protocols.
- Different LAN may have different MAC sub-layer protocols.
- A LAN protocol may include multiple physical layer protocols.
Answer: FTT
2020 年 4 月 3 日
Q1. What type of service is provided by Ethernet?
A. connection-oriented service
B. connectionless service
C. connectionless service with explicit ACK
D. None of the above
Answer: B
Q2. Which statement is NOT TRUE for stations using pure ALOHA?
A. A station transmits at will without coordination with other stations
B. A station can only find collision when timer expires
C. After timer expires, the station retransmits the frame at once.
D. The vulnerable period is twice the frame time, i.e. 2T0
Answer: C
2020 年 3 月 31 日
Q1. （多选）For protocol 6, in which cases NAK will be sent while frame-expected=6 and no_nak is true?
A. received a damaged frame with seq=6
B. received a frame with seq=4
C. received a duplicated ACK
D. received a frame with seq=7
Answer: A、B、D
Q2. Consider a 50-kbps satellite channel with a 500-msec round-trip propagation delay, 1000-
bit frames are sent with Go-back-N ARQ and short ACK frames as acknowledgement. If
a line utilization of 50% is required, how many bits at least shall be used for sequence
number?
A. 3
B. 4
C. 5
D. 6
Answer: B
. 2020 年 3 月 27 日
Q1. For a Go-back-N ARQ with 3-bit sequence number, if the seq number in
current receiving windows is 0,while the entity receives an un-damaged frame with r.seq=1, what will the entity do for a proper operation?
(Assume the entity always has a packet to send)
A. accept the frame and send a frame with s.ack=1
B. discard the frame
C. discard the frame and send a frame with s.ack=0
D. discard the frame and send a frame with s.ack=7
Answer：D
. 2020 年 3 月 24 日
Q1. Assume 3-bit sequence number is used in Protocol 4. When a node receives a frame with r.seq=7 and r.ack=7, what will s.seq and s.ack be in the frame s it will send as a response?
A. 7 and 7
B. 7 and 0
C. 0 and 7
D. 0 and 0
Answer：C
Q2. Assume 3-bit sequence number is used in a Go-back-N protocol. Now the
sender has sent out 8 frames with sequence number 0,1,... and 7. When
timing out, the sender only received ACK1 and ACK3, how many frames will the sender retransmit?
A. 2
B. 4
C. 6
D. 8
Answer：B
2020 年 3 月 20 日
Q1. A data link layer sending entity has 3 jobs before delivering bit stream to its physical layer,
- bit stuffing
- adding header flag and trailer flag
- adding checksum bits
What is the correct sequence of the 3 jobs?
A. 123
B. 312
C. 132
D. 321
Answer：B
Q2. 计算题
若 G(x)=x5+x2+x+1，接收方收到的数据是：1001 1011 1110 0101 1000，请判断该帧是否传输出错。
Answer：传输出错。
Q3.附加作业题
The following character encoding is used in a data link protocol: A: 01000111; B: 11100011; FLAG: 01111110; ESC: 11100000
Show the bit sequence transmitted (in binary) for the four-character frame: A B ESC FLAG when each of the following framing methods are used:
(a) Character count.
(b) Flag bytes with byte stuffing.
© Starting and ending flag bytes, with bit stuffing.
(d) Calculate the efficiency (as a percentage of real data per bit sent) of your answers to (a), (b)and © .
Answer：
1）字符计数法：
00000101 01000111 11100011 11100000 01111110
增加了一个长度字节（长度值包括其本身），效率为 4/5=80%
2）字节填充法：
01111110 01000111 11100011 11100000 11100000
11100000 0111 1110 011111110
增加了首尾标志和 2 个转义字符，效率为 4/8=50%
3）比特填充法：
01111110 01000111 110100011 111000000 011111010 01111110
增加了首尾标志和 3 位填充，效率为 32/51=62.75%
2020 年 3 月 17 日
Q1. What type of service is provided to network layer by 10BaseT?
A. connectionless service
B. connectionless service with acknowledgement
C. frame-based connection-oriented service
D. byte-based connection-oriented service
Answer：A
Q2. 计算题
从信道上收到比特序列： 1101 0111 1110 0111 1110 1101 1011 1110 0010 1100 0101 1111 0101 1001 1111 1001，该比特序列中包含一个完整的帧，用十六进制写出该帧的内 容（不包含帧的首尾标志）。
Answer：D B E 5 8 B F 6
2020 年 3 月 13 日
Q1．Which is the maximum data rate over a 4-kHz channel whose signal-to-noise ratio is 30 dB, and QAM-16 modulation is used?
A. 8Kbps
B. 32Kbps
C. 40Kbps
D. 64Kbps
Answer：B
Q2. About routing (hunting for a path) in a circuit switching network, which is true?
A.There is no need for routing in circuit switching network.
B. Routing is required only at connection set up phase.
C. Routing is necessary during the whole procedure of communication.
D. Routing is necessary at circuit termination phase.
Answer：B
2020 年 3 月 10 日
Q1. Which multiplexing scheme is adopted in Internet?
A. Code Division Multiple Access
B. Frequency Division Multiplexing
C. Time Division Multiplexing
D. Wavelength Division Multiplexing
Answer：C
2020 年 3 月 6 日
Q1. Which of the following is not defined in network architecture?
A. Function of each layer
B. Protocols in each layer
C. Services provided by each layer
D. Implementation of certain service in each layer
Answer：D
Q2. Talking about bandwidith in Hz, which is true?
A. Twisted pair < Radio and TV < Coax < Fiber
B. Radio and TV < Twisted pair < Coax < Fiber
C. Radio and TV < Coax < Twisted pair < Fiber
D. Coax < Radio and TV < Twisted pair < Fiber
Answer：A
. 2020 年 3 月 3 日
Q1. For the following 2 statements, decide true or false.
- Layer N protocol must be used to implement the service provided by layer N.
- There is no need to consider the format of PDU when designing service
primitives.
A. True， True
B. True, False
C. False, True
D. False, False
Answer：A
Q2. In communications between 2 adjacent routers, which layer provides the function of data block exchange?
A. Physical layer
B. Data Link layer
C. Network layer
D. Transport layer
Answer：B
. 2020 年 2 月 28 日
Q1.（不定项） Which statement(s) is/are correct for a wifi network?
A. It is a broadcast network
B. It is a point-to-point network
C. It is an access network
D. It is an edge of 4G mobile communication network
Answer：AC
Q2. Which statement is NOT true?
A. A computer network can be organized as a set of layers.
B. A single function can be performed by the cooperation of adjacent 2 layers, thus the implementation is visible to both layers.
C. Each layer offers a certain service(s) to its upper layer.
D. The upper layer access the service provided through the interface.
Answer： B
. 2020 年 2 月 25 日
Q1. Which of the following is/are NOT a type of computer network?
A. Internet
B. a Wifi network in a Cafetiria
C. World Wide Web
D. 10 computers connecting to a router with cable
Answer：C