【期末考试复习】计算机网络英文版

【期末考试复习】计算机网络英文版
第一章计算机网络概述
●Identify the five components of a data communications system.
(数据通信系统5个组成部分：报文发送方接受方传输介质协议) message,sender,receiver,transmission medium and protocol.
●What are the three criteria necessary for an effective and efficient network?
performance, reliability,and security.
●What is an internet(互联网)? What is the Internet（因特网）?
★ An internet is an interconnection of networks which is a collection of networks connected by internetworking devices such as routers.
★The Internet is the name of a specific worldwide network that uses the TCP/IP protocol suite and is the largest internet in the world.
第二章数据和信号
●Which practical methods are used for analysis of a signal? And which is suitable
for network techniques? 研究电磁信号的方法是哪两种？适用于网络技术的？
①Time domain and frequency domain②frequency domain
●Why can't any digital signal be transferred with no distortion in communication
transmission?（数字信号为什么不能无失真传输？）
1.Because to receive an exact replica of the digital signal, all of the frequency components must be faithfully through the transmission medium, if certain harmonic cannot transfer successful, the signal we receive will be distorted. However, any
medium transfers signals only within certain frequency ranges, that is, has a limited bandwidth.
2.Transmission impairments of signals (attenuation, distortion and noise) always exist, so signal errors unavoidable.
●What's the bandwidth(带宽)? Explain briefly what correlation do bandwidth,
data rate and cost have in communication process?带宽、数据率、成本之间的关系？
The range of frequencies contained in a composite signal is its bandwidth.
In networking, we use the term bandwidth in two contexts with two different measuring values: Bandwidth in Hertz is the range of frequencies in a composite signal or the range of frequencies that a channel can pass, usually used to present the performance of analog channels and transmission media. (Another is) Bandwidth in bits per second is the speed of bit transmission in a channel,a link,or even a network, usually used to present the performance of links and communication devices of a network.(课本P84)
The greater the transmission bandwidth,the greater the data rate, and the higher the cost.
●Q3-3: How can a composite signal be decomposed into its individual frequencies?
复位信号如何分解成单独的频率成分？
If the composite signal is periodic, we can ues Fourier Series decompose it into a series of signals with discrete frequencies;
if the composite signal is nonperiodic, we can ues Fourier Transform decompose it into a combination of sine waves with continuous frequencies.
●Q3-4: Name three types of transmission impairment. 三种传
输减损？
Attenuation, distortion,and noise. 衰减、失真和噪声
●Q3-7: What does the Nyquist theorem have to do with communications?
奈奎斯特公式在通信中的作用？
Estimate the channel capacity of noise-free digital channels, , when the channel is greater than the data rate transmitted on channel capacity, it will be failure due to severe distortion.
☆The Nyquist theorem defines the theoretical maximum bit rate of a noiseless channel.
●Q3-8: What does the Shannon capacityhave to do with communications?
香农容量原理在通信中的作用？
Estimate the thermal noise interference channel capacity (theoretical upper limit), the data rate is higher, damage caused by noise will be more serious.
☆ The Shannon capacity defines the theoretical highest data rate for noisy channel. 第三章传输介质
●Briefly describe the main applications of three types of guided transmission
media. 三种重要的有向传输介质的应用
总的说：
Twisted-pair cable is used for voice and data communications. Coaxial cable is used in cable TV networks and traditional Ethernet LANs. Fiber-optic cable is used in backbone networks, cable TV networks, and Fast Ethernet networks.
分别说：
twisted-pair cable: 双绞线（广泛用于楼内布线）
/doc/5611869427.html,ed as telephone lines to provide voice and data channels –connecting
subscribers to the central telephone office.
/doc/5611869427.html,ed as DSL (digital subscriber line) loop to provide high-data-rate Internet connection – the so called Family Broad Band.
/doc/5611869427.html,ed in LANs to provide high-data-rate baseband transmission (10 and 100 Mbps).
coaxial cable: 同轴电缆（不用了，目前主要用于有线电视）
1.Was widely used in analog telephone networks (10000 voice signals) and in digital telephone networks (data rate up to 600 Mbps).
However, it has largely been replaced with fiber-optic cable.2.Was widely used in cable TV networks
However, cable TV providers replaced most of coax with fiber-optic cable.
3.Was widely used in early traditional Ethernet LANs.
However, it has been replaced with UTP which has data rate up to 100 Mbps.
☆ Now, mainly used for CATV
fiber-optic cable: 光纤（广泛用于楼间布线和长途传输）
widely used for wirings between buildings or long-distance transmission.such as：
1.High-speed backbone networks because its wide bandwidth is cost-effective.
2.Cable TV networks use usually a combination of optical fiber and coaxial cable.
3.Backbone channels in LANs, CANs and W ANs
4.Lightning-proof networks
5.Secrecy networks against wiretapping
●What types of optical fiber transmission modes are there?
Which of them is
suitable for long distance transmission?
Types:sigal mode and multimode, and the multimode include two types:Step-index and Graded-index.
The sigal mode is suitable for long distance transmission.
第四章
●What is Encoding? Why must encoding be used for data transmission?
(1)【什么是编码？】Convert internal data of a sender into line signals suitable for transmission is called encoding.
(2)【数据发送时为什么要编码？】
1) Electromagnetic signals in information process devices are not allowed transmitting directly over communication lines.
2) The signal type of a device differs from the signal type of
a communication line.
3) The parameter requirements of device signals differs from the one of line signals.
4) Specific requirements in synchronization, efficiency , error control, etc..
★Because electromagnetic signals in information process devices are not allowed transmitting directly over communication lines, so encoding must be used for data transmission to converts internal data of a sender into line signals suitable for transmission.
2.3.4.机内信号与线路类型不一样；机内信号参数与线路传输的要求不一样；同步、效率、纠错等方面的特殊要求
●Which two techniques are used when analog data transmitted by digital signals?
Which of them is the most important? Why?
用数字信号传输模拟数据常用技术有哪两种？最常用的是？为什
么？
(1) Pulse Code Modulation(PCM) and Delta Modulation(DM).
(2) Pulse Code Modulation(PCM) is the most important.
(3) The principal advantage of DM over PCM is the simplicity of its implementation. In general, PCM exhibits better SNR characteristics at the same data rate.
●What processes will be involved when analog data are digitized by PCM
technique?
用PCM技术将模拟数据数字化时，要经过哪些处理步骤？
Sampling (PAM)---->quantization---->binary encoding---->digital to digital coding
●Q4-1 List three techniques of digital-to-digital conversion.
They are line coding,block coding and scrambling coding.
●Q4-4 Define baseline wandering and its effect on digital transmission.
(1)In decoding a digital signal, the receiver calculates a running average of the received signal power. This average is called the baseline.(2)The incoming signal power is evaluated against this baseline to determine the value of the data element.
(3)A long string of 0s or 1s can cause a drift in the baseline (baseline wandering) and make it difficult for the receiver to decode correctly.
●Q4-5 Define a DC component and its effect on digital transmission.
(Define) When the voltage level in a digital signal is constant for a while, the specturm creates very low frequencies(results of Fourier analysis). These frequences around zero,called DC(derect-current) components, that present problems for a system that can not pass low frequencies or a system that uses electrical
coupling.
(Effect) DC component means 0/1 parity that can cause base-line wondering. 直流分量的影响不仅仅是基线偏移，还有更重要的原因。

看看教材和课件，或看看原书作业答案。

●Q4-9 Define scrambling and give its purpose.
(Define) Scrambling is a way to avoid a long sequence of 0s in the original stream,Scrambling is a technique that does not increase the number of bits and does provide synchronization,Scrambling is a solution that substitutes long zero-lever pulse with a combination of other lever to provide synchronization.
(Purpose) substitutes long zero-lever pulse with a combination of other lever to provide synchronization.
☆ Its purpose is to provide synchronization without increasing the number of bits.
●Q4-10 Compare and contrast PCM and DM.
PCM is the most common technique to change an analog signal to digital data edit is very complex.Although DM is not perfect,the quantization error of DM is much less than that for PCM. In general, PCM exhibits better SNR characteristics at the same data rate.
☆ Both of them are use sampling to convert the analog signal to digital signal.PCM finds the value of the signal amplitude for each sample; DM finds the change from the previous sample(between two consecutive samples).
第五章
●Which modulation techniques are used in analog transmission? Which of them
is the most susceptible to noise? Which of them is the mechanism commonly used in all modern modems.
1.Amplitude shift keying ( ASK)
Frequency shift keying ( FSK )
Phase shift keying ( PSK)
Quadrature Amplitude Modulation(QAM)
2.The most susceptible to noise of the four digital-to-analog conversion techniques is ASK.
3.The mechanism commonly used in all modern modems of them is QAM.
●Q5-1 Define analog transmission.
(1) Analog transmission refers to the transmission of analog signals using a band-pass channel. Analog transmission is the only choice if we have a bandpass channel.
(2) While digital transmission is very desirable, a low-pass channel is needed.Baseband digital or analog signals are converted to a complex analog signal with a range of frequencies suitable for the channel.
●Q5-2 Define carrier signal and explain its role in analog transmission.
(Define) A sender produces an analog signal as fundamental wave to carry digital signals. The fundamental wave is called carrier signal.
(explain) Digital data are converted into modulated signals through changing one or more of three characteristics of a carrier signal: amplitude, frequency and phase.Most familiar application is for transmitting digital data through the public telephone network via a modem.
(A carrier is a single-frequency signal that has one if its characteristic (amplitude, frequency, or phase) changed to represent the baseband signal.
【In analog transmission, the sending device produces a
high frequency analog signal that acts as a base for the information signals (data). The base signal is called the carrier signal. It is to represent the baseband signal.】)
●Q5-3 Definne digital-to-analog conversion.
Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data. It is also called modulation of a digital signal. The baseband digital signal representing the digital data modulations the carrier to create a broadband analog signal.
●Q5-4 Which of characteristics of an analog signal are changed to represent the
digital signal in each of the following digital-analog conversions?
a.ASK changes the amplitude of the carrier.
b.FSK changes the frequency of the carrier.
c.PSK changes the phase of the carrier.
d.QAM changes both the amplitude and phase of the carrier.
QAM is the most efficient of these options and is the mechanism commonly used in all modern modems.●Q5-5 Which of the four digital-to-analog conversion techniques(ASK,FSK,PSK or QAM) is the most susceptible to noise?Defend your answer.
(Answer) The most susceptible to noise of the four digital-to-analog conversion techniques is ASK.
(Define) Noise usually affects the amplitude; therefore, ASK is the modulation technique most affected by noise than phase or frequency.
第六章
●Which of the four multiplexing techniques is (are) suitable for analog channels?
Which of them is (are) suitable for digital channels? （See P181 Questions Q6-3) Frequency-division and wavelength-division multiplexing are suitable for analog
channels
Synchronous time-division multiplexing and asynchronous time-division
multiplexing are suitable for digital channels.
●Which of the four multiplexing techniques is (are) suitable for electrical
links? Which of them is (are) suitable for optical links?
Frequency-division , Synchronous time-division and asynchronous time-division multiplexing
are suitable forelectrical links .
Wavelength-division multiplexing is suitable for optical links.
光传输信道都属于模拟信道，而时分复用只适用于数字信道。

●Describe the main differences between FDM and TDM.
In FDM signals generate by each sending device modulate different carrier
frequencies.
In TDM instead of sharing a portion of the bandwidth as in FDM,time is share.Each connections occupies a portion of time in the link.
一个是通过frequency (carrier modulation)实现复用，另一个是通过time (slot allocation) 实现复用。

4.P181-182 6.4.2 Questions：
Q6-3.Distinguish between a link and a channel in multiplexing.
In multiplexing,The word link refers to the physical path.The word channel refers to the
portion of a link that carriers a transmission between a give
pair of lines .One link can have many (n)channels.
●Q6-7.Which of the three multiplexing techniques is common for fiber-optic links?
It is the Wavelength -division multiplexing.(WDM)
because it allows the multiplexing of signals with a very high frequency
●Q6-9.Distinguish between synchronous and statistical TDM.
In synchronous TDM each input connection has an allotment in the output even if it is not sending data.Time slots preassigned to sources and fixed.Time slots allocated even if no data.
☆ In synchronous TDM, each input has a reserved slot in the output frame. This can be inefficient if some input lines have no data to send.
In statistical TDM slots are dynamically allocated to improve bandwidth efficient.can improve the efficient by remove the empty slots from the frame.Only when an input lines has a slot's worth of data to send is it given a slot in the output frame and the number of slots in each frame is less than the number of input lines.
第七章
●What's bit-level errors? Describe the basic principle and the requirement for
detection of bit-level errors.
★ The bit-level errors means one bit or more bits of a given data unit (such as a byte, character or packet) is（are）changed from 1 to 0 or from 0 to 1. The central concept in detecting or correcting errors is redundancy, i.e. send some extra bits with our data. These redundant bits, based on specific algorithm, are added by the sender and removed by the receiver. Their presence allows the receiver to detect or correct corrupted bits.
●Which techniques are commonly used in bit-level error detection? Which of
them is(are) implemented by hardware such as gate circuits ? Which of them is(are) implemented by software? Give their main applications.
(1) Parity Check, CRC and Checksum are commonly used in bit-level error detection.
(2) Parity Check and CRC are implemented by hardware such as gate circuits. Parity Check is used in peripherals and CRC is used in networks such as LANs and W ANs.
(3) Checksum is implenmented by software. Mainly used for the Internet.
●Which of FEC and ARQ is commonly used in computer networks?
ARQ is commonly used in computer networks.
●Define the Hamming di stance and the minimum Hamming distance.
(1) The Hamming distance between two words(of the same size) is the number of differences between corresponding bits.
(2) The minimum Hamming distance is the smallest Hamming distance between all possible pairs in a set of words.最小海明距的计算见P286
●Briefly describe the principle of CRC.CRC的工作原理
CRC is a most powerful of the redundancy checking techniques based on binary division.
At the encoder, the redundant bits are derived by dividing the augmented data unit with the predetermined divisor, and the remainder is the CRC. Then the CRC is appended to the end of a datawordand create the transmitted codeword.Also,to be valid, a CRC must have exactly one less bit than the divisor, and the
resulting codeword becomes exactly divisible by the same divisor.
At the decoder, the received codeword is divided by the same divisor. And the remainder is called syndrome. If there is no remainder, the data unit is assumed to be intact and is therefore accepted. Otherwise, it means that the data unit has been corrupted in transmission and therefore the remainder must be rejected.
The divisors used in CRC are normally called the generator polynomial(生成多项式) or simply the generator, which are given by some international organizations.
●Q10-4 In a codeword, we add two redundant bits to each 8-bits data word. Find
the number of
a.valid codewords.
b.invalid codewords
We have k=8, r=2, n=8+2=10.
a.The number of valid codewords is 2^k=2^8=256;
b.The number of invalid codewords is 2^n-2^k=2^10-2^8=768;
Q10-6 If we want to be able to detect two-bit errors, what should be the minimum Hamming distance?
The minimum Hamming distance d=s+1,since s=2,d=3.
●Q10-8 In CRC, if the dataword is 5 bits and the codeword is
8 bits, how many 0s
need to be added to the dataword to make the dividend? What is the size of the remainder? What is the size of the divisor?
(1) We need argment the datawords with three 0s.
(2) The size of the remainder is 3.
(3) The size of the divisor is 4.
(We have k=5 and n=8. The size of the dividend is the same
as the size of the codeword (8 bits). We need to augment the dataword with three 0s. The size of the remainder is r=n-k=8-5=3bits.
The size of the divisor is: r+1=4 bits)
第八章
●Why do we need the data link layer? And what is a "data link"?
The data link layer is needed because it is necessary to impose a layer of control in each communicating device that provides functions such as flow control, error control, addressing and link management.
When a data link control protocol is used, the transmission medium between systems is referred to as a data link.
●Briefly describe the two main functions of data link layer.
The two main functions:
(1) Data link control (node-to-node communication)
Provide smooth and reliable transmission of frames between nodes.
- Framing (organize the transmitted bits)
- Flow control ( automatically regulate transfer rate)
- Error control ( handle frames lost or damaged)
(2) Media access control (not used if point-to-point)
How to share the link
- Addressing control
- Sending right control （Only used in broadcast networks,
e.g. LANs ）
●Briefly describe three version of ARQ.
(1)Stop-and-Wait ARQ,Send one frame at a time. (2)Design of Go-Back-N
ARQ,Send max W frames at a time,retranmit the error frames
and all succeeding
ones. 3)Selective Repeat ARQ ,Send max W frames at a time,retranmit rejected frames only.
●Briefly describe the main difference of three HDLC frames (I-,S- and
U-frames).
I-frame used to transport user data and control information relation to user data (piggybacking) S-frame used only to transport control information
U-frame used to transport link management information (Information carried by U-frames is intended for managing link itself.)
●Briefly describe the main functions of four S-frames of HDLC (RR,
RNR,REJ,SREJ).
RR stands for receiving ready. REJ stands for rejecting. REJ stands for receiving not ready. SREJ stands for selective-reject.
【RR: This kind of frame acknowledges the receipt of a safe and sound frame or group of frames.
RNR: It acknowledges the receipt of a frame or group of frames, and it announces that the receiver is busy and cannot receive more frames. It acts as a kind of congestion-control mechanism by asking the sender to slow down.
REJ: It is a NAK that can be used in Go-Back-N ARQ to improve the efficiency of the process by informing the sender, before the sender timer expires, that the last frame is lost or damaged.
SREJ: This is a NAK frame used in Selective Repeat ARQ. Note that the HDLC Protocol uses the term selective reject instead of selective repeat.】
6. P320-321 11.6.2 Questions：
●Q11-1. Define framing and give the reason it is needed.
The data-link layer needs to pack bits into frames ,so that each frame is distinguishable from another. Framing is divides a message into smaller frames for higher efficiency of flow and error control.
●Q11-8. Compare and contrast flow control and error control.
Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgement. Error control refers to a set of procedures used to detect and correct errors.
●Q11-18. Define piggybacking and its benefit.
Piggybacking is used to improve the efficiency of bidirectional transmission. When a frame is carrying data from A to B, it can also carry control information about the frame(s) received from B; when a frame is carrying data from B to A, it can also carry control information about the frames received from A.
●Q11-22. Compare and contrast HDLC with PPP.
HDLC is a bit-oriented protocol for communication over point-to-point and multipoint links. PPP is a byte-oriented protocol for point-to-point links.
第九章广域网使用的核心技术——交换技术
●List the switching methods descri bed in this chapter. What are the most
commonly today?
(1) Circuit switched techniques
(2) Message switched techniques
(3) Packet switched techniques
(4) Cell switched techniques
Circuit switching and packet switching are the most
commonly today.
●Which phases are required for communications in a circuit-switched network?
(Firstly, setup phase. When end system A needs to communicate with M, it needs to request a connection to M that must be accepted by all switches as well as by M itself.
Secondly, data transfer phase. After the dedicated path made of connected circuits (channels) is established, data transfer can take place.
Thirdly, teardown phase.)
●Briefly describe the similarities and differences between circuit-switched and
virtual-circuit networks.
☆ Similarities: a.Need three phases.
b.Allocated during setup.
c.All packets follow the same path established during setup phase.
Differences:
virtual-circuit networks :
Preplanned route established before any packets sent
Call request and call accept establish connection (handshake) Each packet contains a virtual circuit identifier instead of destination address
No routing decisions required for each packet
Clear request to drop circuit
Not a dedicated path (store-and-forward, not transparent connection)
circuit-switched networks:
Simpler devices (pure hardware)
Transparent connection and smaller delay (only signal delay)
Fixed bandwidth, not suitable to bursting heavy transfer traffic
Single rate
One-to-one communications
★这两种网络共同的地方是通信时都需要三个阶段，数据传输时都沿同一条预先建立好的线路进行，最重要的―异‖至少有三个：电路交换传输时报文不分包连续传输，而且是―透明直传‖，独占线路资源，所以，线路利用率低而通信资费高，但延迟最小；虚电路传输时采用―存储-转发‖技术，整个报文要分成一个个―分组‖，不独占线路资源，所以线路利用率高而通信资费低，但延迟不确定。

●Briefly describe the similarities and differences between datagram and virtual-
circuit networks.
☆① similarities : use store and forward ；data transmitted in small packets
② difference:
Datagram:：no call setup(establish) and disconnect phases；Each packet treated independently ；Packets can take any practical route (use routing)；Packets may arrive out of order or loss
Virtual circuit：need three steps: establish/transfer/disconnect；Provide sequencing and error control；No routing decisions to make；Loss of a node looses all circuits through that node ★这两种网络共同的地方是都要将每个报文分成一个个―分组‖，而且都使用―存储-转发‖技术，主要不同在于一个通信时都需要三个阶段（要写出三个阶段名称），一个只有一个阶段；一个所有分组都沿同一路径传输，所以顺序到达，不会乱序，而另一个各分组是自行其道，可能乱序或丢失分组。

如果说的完整点，还应该补充一下这些网络技术所工作的协议层有何不同。

5. P231 8.
6.2 Questions
Q8-4 Compare and contrast a circuit-swithed network and a packet-switched network.
In a circuit-switched network,data are not packetized; data flow is somehow a continuation of bits that travel the same channel during the data transfer phase.
In a packet-switched network data are packetized; each packet is somehow an independent entity with its local or global addressing information.
第十章
1、本章所介绍的各种广域网类型中，使用电话线接入的是哪些类型？目前所谓的“家庭宽带”用的是哪一种？
(1) 使用电话线接入的是拨号上网(PSTN),―一线通‖ISDN和ADSL。

(2) 目前所谓的―家庭宽带‖用的是ADSL。

(Public Switched Telephone Network (公用交换电话网)、Integrated Service Digital Network (综合业务数字网)、Asymmetric Digital Subscriber Line (非对称数字用户线))
2、常见的外网有电话通信网、有线电视网、分组交换网、数据专用网等。

常用的ADSL接入的是什么网？FRN接入的是什么网？HFC 接入的是什么网？DDN接入的是什么网？
(1)ADSL接入的是电话通信网。

(2) FRN接入的是分组交换网。

(3) HFC接入的是有线电视网。

(4) DDN接入的是数据专用网(数据数字网)。

3.ADSL使用哪种复用技术来提高数据传输速率？频分复用技术（FDM）
【飘过两个关键词：正交振幅调制（QAM）、离散多音频技术】
4.WAN的接入线路有哪些种类？
WAN的接入线路有电话线，电视电缆，电力线，无线和光纤。

(通过电话线接入（借助于各类modem）；通过电视电缆接入（借助于cable modem）；通过电力线接入；通过无线接入；通过光纤接入)
5.什么是FTTH？什么是ONU？
★ FTTH是―光纤到家‖的意思。

ONU是―光网络单元‖，使用ONU 可以将光信号转换为电信号馈送到用户家中，也可以将用户的电信号转换为光信号馈送到外部光网络。

6、什么是EPON？其功能是什么
★ EPON 就是―以太网无源光网络‖(Ethernet Passive Optical Network)，利用PON的拓扑结构实现以太网的光纤接入。

EPON主要分成三部分，即OLT（Optical Line Termination,光线路终端）、ODN（Optical Distribution Network，光分配网络）和ONU（optical network unit,光网络单元）组成。

?其下行采用broadcast方式，上行采用TDM技术。

★其中OLT位于局端，可以广播方式向ONU发送以太网数据。

ONU位于用户端，可以是路边、小区、住宅楼，或者是家中、办公室中，用于选择接收OLT发送的广播数据；并在OLT分配的发送窗口中向上行方向发送。

在OLT(光线路终端)和ONU(光网络单元)之间的光分配网络(ODN)没有任何有源电子设备。

P432 14.6.2 Questions
Q14-6 What is DSL technology? What are the services provided by the telephone companies using DSL? Distinguish between a DSL modem and a DSLAM.
(1) DSL (Digital subscriber line) technology is one of the most promising for supporting high-speed digital communication over the existing local loops, it is a set of technologies, each differing in the first letter (ADSL, VDSL, HDSL, and SDSL). The set is often referred to as xDSL, where x can be replaced by A, V, H, or S.
(2) Telephone companies developed digital subscriber line (DSL) technology to provide higher-speed access to the Internet.(3) ADSL uses a device called ADSL modem at the customer site. It uses a device called a digital subscriber line access multiplexer (DSLAM) at the telephone company site.
第十一章
1. Define protocol.
A protocol is synonymous with rules for data communication.it defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively. When communication is simple, we may need only one simple protocol;when the communication is complex, we need a protocol at each layer, or
protocol layering.A protocol defines what is communicated, how it is communicated, and when it is communicated. This provides accurate and timely transfer of information between different devices on a network.
2. Why are protocols needed? (P27 Q1-11)
A protocol defines what is communicated, in what way and when. This provides accurate and timely transfer of information between different devices on a network.
3. There are three important basic protocols in network communication. What are they? Which is(are) used in the Internet?
(1) They are OSI(Open Systems Interconnection)model,IEEE 802 LAN protocols and TCP/IP(Transmission Control/Internet)protocol suite.
(2) TCP/IP is used in the Internet.
4. Define the layer protocol and the layer service.
1)The layer protocol defines the implementing methods and techniques of the functions in that layer.
2)The layer service defines the communication functions provided by a lower entity (service provider) for an upper entity (service user) in a layer architecture.
5. What's a network model? Which network models have been discussed in this chapter? Which of them is(are)) called Reference Model?
(1) A network model refers to a layered architecture used to represent the whole process of network communication. It must include three items: How many layers ? (layering); What to do in each layer? (service); How to do in each layer? (protocol).
(2) OSI model,IEEE LAN model and TCP/IP model.
(3) OSI model is called Reference Model.
6. Define encapsulation and virtual communication in the OSI Model.
★ Encapsulation means when user data are passed down along a layered architecture, control information is added to user data at each layer as header or trailer (except physical layer). （所谓―封装‖是指信息通过各层向下传递时，每层的软件负责加上它的报头(header)或报尾(Trailer)信息(除了物理层外)。

（该章课件第55号幻灯片）
Virtual communication refers to information exchange in data transmission between peer-to-peer protocol layers (rather than between communication entities) of communication parties.（虚通信指通信双方对等协议层之间（而非通信实体之间）在数据传输过程中产生的信息交换。

）（该章课件第63号幻灯片）第十二章
1.What is network topology? List their types. Which of them are (is) usually used for WANs.
(1) As network term,network topology refers to the way in which a network is laid out physically. Two or more devices。