Computer Networks Review Questions (A)
College Of Computer, Chongqing University, China
By Jean
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However, the contents must not be changed.
Contents
1. Introduction.......................................................................................................... - 3 - 2. The Physical Layer ..................................................................................................... - 6 - 3. The Data Link Layer ................................................................................................. - 11 - 4. The Media Access Control Sub layer ......................................................................... - 14 -
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1. Introduction
2. Give each classification of interconnected processors a typical example.
Answer:
(1) Personal area network (2) Local area network (3) Metropolitan area network (4) Wide area network (5) The internet
3. Please tell the roles that Service, Interface, Protocol and Layer play in the network. Answer:
Service – says what a layer does
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Interface – says how to access the service
Protocol–says how is the service implemented. ：A set of rules and formats that govern the communication between two peers A layer provides a service to the one above [vertical] A layer talks to its peer using a protocol [horizontal] 4. Please write down the layers in OSI reference model and TCP/IP reference model, and describe the functions of each layer in OSI reference model.
Answer: OSI model
(1) Physical layer: Concerned with bit transmission over physical channel.
(2) Data link layer: Provides reliable, error-free service on top of raw Layer 1 service, and breaks data into frames.
(3) Network layer: Deliver a packet to specified destination, and perform segmentation/reassemble
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(fragmentation/defragmentation) is control of routing in the subnet. Also responsible for congestion control and usage monitoring.
(4) Transport layer: Takes data from Session Layer, split it up into smaller units, and ensure that the units arrive correctly. Concerned with efficient provision of service. It determines the ‘type of service’ to provide to the Session Layer.
(5) Session layer: Allows establishment of sessions between machines. It is also responsible for dialogue control, token management and synchronization.
(6) Presentation layer: Concerned with representation of transmitted data. Deals with different data representations, and it also deals with data compression and encryption. (7) Application layer: Home to wide variety of protocols for specific user needs.
TCP/IP model
(1) Host-to-network layer (2) Internet layer (3) Transport layer (4) Application layer 5. Fill in the following blanks.
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(1) 1 Mbps = 1000 Kbps, 1 KB = 1024 bytes. (2) A measure of the width of a range of frequencies, which is measured in HZ、KHZ、MHZ, is called Signal bandwidth. (3) A rate of data transfer, which is measured in bits per second (bps), kbps，Mbps，Gbps，Tbps, is called Data bandwidth. (4) Network delay=Transmission delay + Processing delay + Queuing delay + Propagation delay 2. The Physical Layer
1. Try describing the characteristics of physical layer. Answer:
Mechanical characteristics: Mechanical specification of electrical connectors and cables.
Electrical characteristics: Electrical specification of transmission line signal level and impedance.
Functional characteristics: The function logic of electric signals generated on certain wires in the physical media.
Procedural characteristics: The procedures of protocols that control the sequence of events. 2. Fill the following blanks.
(1) Analog data transmission over analog channel needs carrier and modulation. - 6 -
(2) Digital data transmission over analog channel needs modulation. (3) Analog data transmission over digital channel needs sampling. (4) Digital data transmission over digital channel needs encoding. 3. Please find the channel communication mode according to the following figures.
(1)
(2)
(3)
Answer:
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(1) Simplex communication. (2) Half-duplex communication. (3) Full-duplex communication.
4. If a binary signal (two sampling levels) is sent over a 3-kHz channel whose signal-to-noise ratio is 30 dB, what is the maximum achievable data rate? Answer:
𝑆
10log10()=30
𝑁𝑆
=1000 𝑁Maximum
rate=𝑊log2(1+)=3000log2(1+1000)≈30𝐾𝑏𝑝𝑠
𝑁𝑆
data
5. Fill the following blanks.
(1) PCM is the most common technique for using digital signals to encode analog data. (2) There are four multiplexing techniques, they are Frequency Division Multiplexing, Time Division Multiplexing, Wavelength Division Multiplexing and Code Division Multiplexing. 6. Suppose a CDMA communication system. The address code of station A is (-1-1-1+1+1-1+1+1), of station B is (-1-1+1-1+1+1+1-1) and of station C is (-1+1-1+1+1+1-1-1). Now A sends bit 0, B sends bit 1 and C sends nothing. Then what signal C will receive?
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Answer:
The address code sent by A is: (+1+1+1-1-1+1-1-1) The address code sent by B is: (-1-1+1-1+1+1+1-1) C sends nothing.
Then the signal that C will receive is:
(+1+1+1-1-1+1-1-1)+ (-1-1+1-1+1+1+1-1)=(00+2-20+20-2) 7. What’s the difference between broadcast and switched communication network? Answer:
Broadcast communication networks: Information transmitted by any node is received by every other node in the network. Switched communication networks: Information is transmitted to a sub-set of designated nodes.
8. Compare the delay in sending an x -bit message over a k -hop path in a circuit-switched network and in a (lightly loaded) packet-switched network. The circuit setup time is s sec, the propagation delay is d sec per hop, the packet size is p bits, and the data rate is b bps. Under what conditions does the packet network have a lower delay? Answer:
Circuit-switched delay=𝑠++𝑘𝑑
𝑏𝑥
Packet-switched delay=
𝑝𝑏
∙𝑘+𝑘𝑑+∙(−1)=∙(𝑘−1)+
𝑏
𝑝
𝑏
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𝑝𝑥𝑝
𝑘𝑑+ 𝑏
𝑥
Suppose Circuit-switched delay > Packet-switched delay 𝑠++𝑘𝑑 > ∙(𝑘−1)+𝑘𝑑+
𝑏
𝑏
𝑏
𝑥
𝑝
𝑥
𝑝(𝑘−1)<𝑠𝑏
So under the condition of 𝑝(𝑘−1)<𝑠𝑏 does the packet network have a lower delay.
9. What are Manchester encoding and differential Manchester encoding? And describe their features. Answer:
(1) With Manchester encoding, each bit period is divided into two equal intervals. A binary 1 bit is sent by having the voltage set high during the first interval and low in the second one. A binary 0 is just the reverse: first low and then high. This scheme ensures that every bit period has a transition in the middle, making it easy for the receiver to synchronize with the sender. A disadvantage of Manchester encoding is that it requires twice as much bandwidth as straight binary encoding because the pulses are half the width.
(2) Differential Manchester encoding is a variation of basic Manchester encoding. In it, a 1 bit is indicated by the absence of a transition at the start of the interval. A 0 bit is indicated by the presence of a transition at the start of the interval. In both
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cases, there is a transition in the middle as well. The differential scheme requires more complex equipment but offers better noise immunity.
3. The Data Link Layer
1. A bit stream 1101011011 is transmitted using the CRC method. The generator polynomial is 𝑥4+𝑥+1. Show the actual bit string transmitted. Answer:
2. A channel has a bit rate of 4 Kbps and a propagation delay of 20ms. For what range of frame sizes does the (simplistic) stop-and-wait
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protocol give an efficiency of at least 50 percent? Answer:
𝑥
≥50% −33𝑥+2×20×10×4×10𝑥≥320𝑏𝑖𝑡
When the frame size is 320bit, the stop-and-wait protocol can give an efficiency of at least 50 percent.
3. If Go-Back-N ARQ protocol is used to control the traffic, and with 7bit frame coding segment. Show the largest length of the sliding window. Answer:
𝑊𝑇≤2𝑛−1=127
The largest length of the sliding window is 127.
4. Go-Back-N ARQ protocol is used to control traffic in data link layer, the sender has sent frames numbered 0 to 6. When the timer is timeout, the ACK frame of data frame number 1 does not return, so how many frames should the sender resend? Answer: 6
5. Please describe the three modes of communication in HDLC in brief. Answer:
NRM(Normal Response Mode)
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-refers to the standard primary-secondary relationship
-secondary device must have permission from the primary device before transmitting
ARM(Asynchronous Response Mode)
-secondary may initiate a transmission without permission from the primary whenever the channel is idle
-do not alter the primary-secondary relationship in any other way ABM(Asynchronous Balanced Mode)
-all stations are equal and therefore only combined stations connected in point-to-point are used
-Either combined station may initiate transmission with the other combined station without permission.
6. As following are control fields of three frame in HDLC, please find them.
Answer:
(a) an information frame (b) a supervisory frame (c) an unnumbered frame
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4. The Media Access Control Sub layer
1. Suppose the maximum distance between the nodes of a 10Mbps CSMA/CD local area network is 2.5km, the propagation speed in the medium is 2×108𝑚/𝑠m/s. Find the shortest frame length of the network. Answer:
The shortest length of the network=2×
2.5×1032×108
×10×106.
2. Please illustrate the hidden and exposed terminals in wireless LAN according to the following figure.
Answer: Hidden terminals
-A sends to B, C cannot receive A
-C wants to send to B, C senses a “free” medium (CS fails) -collision at B, A cannot receive the collision (CD fails) -A is “hidden” for C Exposed terminals
-B sends to A, C wants to send to another terminal (not A or B)
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-C has to wait, CS signals a medium in use
-but A is outside the radio range of C, therefore waiting is not necessary -C is “exposed” to B 3. Fill in the following blanks.
(1) Project group 802 under IEEE to set standards relating to physical and logical links of nodes in a network, data link layer is divided into MAC sublayer and LLC sublayer, the shortest frame is 64B. (2) A Fast Ethernet switch port data transfer rate is 100Mbps, if the port can support full-duplex transmission of data, the port bandwidth is 200Mbps. (3) In the Ethernet switch forwarding decisions, the address of the PDU is the destination physical address. 4. Please describe what different types virtual local area network has. Answer: Port-based VLAN MAC-based VLAN Protocol-based VLAN IP Address-based VLAN
5. If A wants to send data to B, please describe the virtual carrier sensing.
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Answer:
(1)A first sends a Request-to-Send (RTS) to B, after receiving the Clear-to-Send (CTS) , then sends data (2)On receiving RTS, B responds CTS
(3)Hidden node C overhears CTS and keeps quiet Transfer duration is included in both RTS and CTS (4)Exposed node D overhears a RTS but not the CTS D’s transmission cannot interfere at B
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