计算机网络复习资料 - 图文

Computer network: a collection of autonomous computers interconnected by a single technology. The key distinction between a computer network and a distributed system is that in a distributed system, a collection of independent computers appears to its users as a single coheren system. 1.1.1 Business Applications of Nerworks

1.resource sharing (physical resources, software resources and information)

2.Client-Server model: a network with two clients and one server, involves requests and replies. Server: the service provider Client: the service requester 3.Home Network Applications: ①Access to remote information

②Person-to-person communication(peer-to-peer) ③Interactive entertainment ④Electronic commerce

4. In peer-to-peer system there are no fixed clients and servers. 1.2.1 Transmission Technology

1. Broadly speaking, there are two types of transmission technology that are in widespread use: ①Broadcast links(broadcasting, multicasting) ②Point-to-point links(unicasting)

2. Broadcast networks have a single communication channel that is shared by all the machines on the network .

3. Broadcast: when a packet with this code is transmitted, it is received and processed by every machine on the network.

4. Multicasting: some broadcast systems also support transmission to a subset of the machines, something known as multicasting.

5. Point-to-point networks consist of many connections between individual pairs of machines. 6. Unicasting: point-to-point transmission with one sender and one receiver. 7.Classification of interconnected processors(networks) by scale：

1.2.2 Local Area Networks ①Size: restricted, 1m~100m

②Transmission technology: broadcast links ③Topology: bus or ring

1.2.3 Metropolitan Area Networks ①Size: covers a city

②The best-known example of a MAN is the cable television network. 1.2.4 Wide Area Networks

①Size: often a country or continent

②store-and-forward or packet-switched subnet 1.2.5 Switching technology: ①Circuit switching ②Message switching ③Packet switching 1.2.6 internet & Internet

internetwork ( internet): A collection of interconnected networks. Internet: The specific and worldwide internet

①Subnet makes the most sense in the context of a wide area network, where it refers to the collection of routers and communication lines owned by the network operator. ②The combination of a subnet and its host forms a network.. ③An internetwork is formed when distinct networks are interconnected. 1.3.1 Protocol Hierarchies

①Most network are organized as a stack of layer, each on build upon the one below it. The purpose of each layer is to offer certain services to the higher layers. ②The rules and convention used in this conversation are called as the layer n protocol.

③A protocol is an agreement between the communicating parties on how communication is to proceed.

④Between each pair of adjacent layers is an interface.

⑤A set of layers and protocols is called a network architecture . 1.3.2 Design Issues for the Layers 1. Addressing （寻址） 2. Error Control 3. Flow Control

4. Multiplexing (多路复用） 5. Routing（路由）

1.3.3 Connection-Oriented and Connectionless Services

①Connection-Oriented service: establish a connection, use this connection (transfer data), release this connection

②Connectionless Service:

Each service can be characterized by a quality of service(reliable or unreliable) 1.4.1 The OSI Reference Model Seven layers of the OSI model:

①Physical layer: transmit raw bits over a communication channel.

②Data link layer: break up the input data into data frames and transmit them sequentially, flow

control. ③Network layer: control the operation of the subnet (routing decision and congestion control) ④Transport layer: accept data from above, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. (this layer is a true end-to-end layer). ⑤Session layer ⑥Presentation layer

⑦Application layer: contains a variety of protocols that are commonly needed by users. 1.4.2 The TCP/IP Reference Model

①Internet Layer: IP(Internet protocol)

②Transport: TCP(Transmission Control Protocol), UDP(User Datagram Protocol) 1.4.3 Comparing OSI and TCP/IP Models

Concepts central to the OSI model: Services, Interfaces,Protocols. 1.4.4 A Critique of the OSI Model and Protocols

Why OSI did not take over the world?(Bad timing, Bad technology, Bad implementations, Bad politics)

1.4.5 A Critique of the TCP/IP Reference Model Problems:

①Service, interface, and protocol not distinguished ②Not a general model

③Host-to-network “layer” not really a layer ④No mention of physical and data link layers

⑤Minor protocols deeply entrenched, hard to replace 2.1.1 Maximum Data Rate of a Channel 1. Nyquist Theorem:

Max data rate?2Hlog2Vbit/sec

2. Signal-to-noise ratio (S/N): the ratio of the signal power to the noise power.

dB?10lgS/N(decibel)

3. Shannon Theorem: The maximum data rate of a noisy channel whose bandwidth is H Hz, and whose signal-to noise ratio is S/N, is give by maximum number of bit/sec?Hlog2(1?S/N) 2.2 Guided Transmission Media: Twisted Pair, Coaxial Cable, Fiber Optics.

2.3 Wireless Transmission: Radio Transmission, Microwave Transmission, Infrared and Millimeter Waves, Satellite communication. 2.5.1 Structure of the Telephone System

Three Major Components of the Telephone System:

①Local loops(Analog twisted pairs going to houses and businesses)

②Trunks(Digital fiber optics connecting the switching offices)

③Switching offices(Where calls are moved from one trunk to another) 2.5.2 The Local Loop: Modems,ADSL & Wireless. Conversion is done by the modems and codecs 1. Modems

比较：baud, symbol, bit rate

A connection that allows traffic in both direction simultaneously is called full duplex . A connection that allows traffic either way, but only one way at a time is called half duplex . A connection that allows traffic only one way is called simplex . 2. Digital Subscriber Lines (DSL)

ADSL(Asymmetric DSL), DMT (Discrete MultiTone), NID(Network Interface Device), DSLAM(DSL Access Multiplexer)

2.5.3 Multiplexing: transport many conversations over a single physical trunk

1. FDM(Frequency Division Multiplexing): the frequency spectrum is divided into frequency bands, with each user having exclusive possession of some band .

2. TDM(Time Division Multiplexing): the user take turns, each one periodically getting the entire bandwidth for a little burst of time.

PCM(Pulse Code Modulation): samplingquantizingcoding For telephone channel , the sampling rate is 8000/ps (125usec/sample), quantizing levels is 256, coding bits is 8.

The T1 carrier (1.544 Mbps) (used in North American and Janpan) The E1 carrier (2.048 Mbps) 3. WDM(Wavelength Division Multiplexing)

2.5.4 Switching: Circuit Switching & packet switching(store-forward switching) 1. Circuit Switching(线路连接) delay:

数据块长度(比特)①发送时延(sending delay): 发送时延?

信道数据传输率(比特/秒)信道长度(米)②传播时延(propagation delay): 传播时延?

信号在信道上的传播速率(米/秒)③排队时延(queuing delay): 总时延 = 发送时延+传播时延+排队时延

2. Packet Switching(封装交换):

A comparison of circuit switched and packet-switched networks:

2.5.5 The Mobile Telephone System (self-study) First-Generation Mobile Phones: Analog Voice

Second-Generation Mobile Phones: Digital Voice (CDMA) Third-Generation Mobile Phones: Digital Voice and Data 2.6.1 CDMA-Code Division Multiple Access

1. In CDMA, each bit time is subdivided into m short intervals called chips. 2. Each station is assigned a unique m bit code called a chip sequence. To transmit a “1” bit, a station sends its chip sequence. To transmit a “0” bit, it sends the one’s complement of its chip sequence.

3. Each station has its own unique chip sequence. All chip sequences are pairwise orthogonal, that mean the normalized inner product of any two distinct chip sequence, S and T (written as S?T), is 0.

1mS?T?SiTi?0, S?T?0 ?mi?14. The normalized inner product of any chip sequence with itself is 1.

1m1m21mS?S??SiSi??Si??(?1)2?1

mi?1mi?1mi?15. When two or more station transmit simultaneously, their bipolar signals add linearly.

6. To recover the bit stream of an individual station, the receiver must know that station’s chip sequence in advance. It does the recover by computing the normalize inner product of the received chip sequence and the chip sequence of the station whose bit stream is trying to recover. 习题：

3. Television channels are 6MHz wide. How many bits/sec can be sent if four-level digital signals are used? Assume a noiseless channel.

解：maximum×data rate?2Hlog2V?2?6?log24Mb/s?24Mb/s

4. If a binary signal is sent over a 3kHz channel whose signal-to-noise ratio is 20dB, what is the maximum achievable data rate?

解：由dB?10lgS/N，得S/N?10根据香浓定理，得知该信道的容量为

20/10?100

maximum number of bit/sec?Hlog2(1?S/N)?3log2(1?100)?19.98kbps 又由奈奎斯(Nyquist)定理，发送二进制信号的3kHz信道的最大数据传输速率为

2?3log22?6kbps

28. Ten signals, each requiring 4000Hz, are multiplexed on to a single channel using FDM. How much minimum bandwidth is required for the multiplexed channel? Assume that the guard are 400Hz wide.

解：There are ten 4000Hz signals. We need nine guard bands to avoid any interference. The minimum bandwidth required is 4000×10 + 400×9=43,600Hz.

【译文】根据题意，可知有10个4000Hz信号。我们需要9个保护频带，以避免任何干扰。所需的最小带宽为4000×10+400×9 =43600Hz.）

29. Why has the PCM sampling time been set at 125μsec?

解：A sampling time of 125μsec corresponds to 8000 samples per second. According to the Nyquist theorem, this is the sampling frequency needed to capture all the information in a 4kHz channel, such as a telephone channel. (Actually the nominal bandwidth is somewhat less, but the cut off is not sharp.)

【译文】一个125μsec采样时间对应8000个样本。根据奈奎斯特定理，采样频率，这是需要捕获所有在4kHz的渠道，如电话信道的信息。为获取一个4kHz 通道中的全部信息，需要每秒8000次得采取频率。（其实，标称带宽稍差，但切断不锋利。）

42. 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

本文来源：https://www.bwwdw.com/article/4otv.html