计算机网络复习主要知识点

TCP: Transmission Control Protocol 传输控制协议FDM: Frequency-division multiplexing 频分多路复用CSMA/CD: Carrier Sense Multiple Access/Collision Detect 载波监听多路访问/冲突检测方法FTP：File Transfer Protocol 文件传输协议IP：Internet Protocol 网络之间互连的协议NAT：Network Address Translation 网络地址转换UDP: User Datagram Protocol 用户数据报协议MAC: Medium Access Control 介质访问控制层CRC：Cyclical Redundancy Check 循环冗余码VLNA: Virtual Local Area Netwok 虚拟局域网URL：Uniform/Universal Resource Locator 统一资源定位符RIP：Routing Information Protocol 路由信息协议DNS: Domain Name System 域名系统HDLC: High-Level Data Link Control 高级数据链路控制DHCP: Dynamic Host Configuration Protocol 动态主机设置协议ICMP: Internet Control Message Protocol 网络控制报文协议SMTP: Simple Mail Transfer Protocol 简单邮件传输协议ARP: Address Resolution Protocol 地址解析协议ATM：Asynchronous Transfer Mode 异步传输模式CDMA: Code Division Multiple Access 码分多址CHAP.1Broadcast networks have a single communication channel that is shared by all the machines on the network. Short messages, called packets in certain contexts, sent by any machine are received by all others.When a packet with this code is transmitted, it is received and processed by every machine on the network. This mode of operation is called broadcasting.Some broadcast systems also support transmission to a subnet of the machines, something known as multicasting.Point-to-point networks consist of many connections between individual pairs of machines. To go from the source to the destination, a packet on this type of network may have to first visit one or more intermediate machines. Often multiple routes, of different lengths, are possible, so finding good ones is important in point-to-point networks.Point-to-point transmission with one sender and one receiver is sometimes called unicasting. (P15)Connection-oriented service is modeled after the telephone system. To talk to someone, you pick up the phone, dial the number, talk, and then hang up. Similarly, to use a connection-oriented network service, the service user first establishes connection, uses the connection, and then releases the connection. In most cases the order is preserved so that the bits arrive in the order they were sent.Connectionless service is modeled after the postal system. Each message carries the full destination address, and each one is routed through the system independent of all others. It ispossible that the first one sent can be delayed so that the second one arrives first. (P32)The physical layer is concerned with transmitting raw bits over a communication channel.The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer.The network layer controls the operation of the subnet. A key design issue is determining how packets are routed from source to destination.The basic function of the transport layer is to accept data from above, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end.(P38)TCP is a reliable connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet.UDP is an unreliable, connectionless protocol for applications that do not TCP’s sequencing or flow control and wish to provide their own.CHAP.2Maximum data rate = 2H log2 V bits/secMaximum data rate:最大数据传输率H：带宽V：离散级数Maximum number of bits/sec = H log2 (1+S/N)Maximum number of bits/sec:最大信道容量S/N:信噪比求分贝: dB = 10 log10S/NQSPK: V = 4T1 PCM: V = 27QAM-64: V = 64Two different switching techniques are used nowadays: circuit switching and packet switching. When you or your computer places a telephone call, the switching equipment within the telephone system seeks out a physical path all the way from your telephone to the receiver’s telephone. This technique is called circuit switching.CHAP.3The data link layer has a number of specific functions it can carry out. These functions include: Providing a well-defined service interface to the network layer.Dealing with transmission errors.Regulating the flow of data so that slow receivers are not swamped by fast senders.The methods of framingCharacter count: The method uses a field in the header to specify the number of characters in the frame.Flag bytes with byte stuffing: The method gets around the problem of resynchronization after an error by having each frame start and end with special bytes.Starting and ending flags, with bit stuffing: When the receiver sees five consecutive incoming 1 bits, followed by a 0 bit, it automatically dyestuffs the 0 bit.Physical layer coding violations.Error-controlThe usual way to ensure reliable delivery is to provide the sender with some feedback about what is happening at the other end of the line.This possibility is dealt with by introducing timers into the data link layer.To prevent this from happening, it is generally necessary to assign sequence numbers to outgoing frames, so that the receiver can distinguish retransmissions from originals.4. Hamming codes插入位置为20, 21,22, 23……2n位置1：校验1，跳1；位置2：校验2，跳2；位置4：校验4，跳4；……偶数个1为0，奇数个1为0.CHAP.4In the literature, broadcast channel are sometimes referred to as multi-access channel or random access channel.The problem of a station not being able to detect a potential competitor for the medium because the competitor is too far away is called the hidden station problem.When in fact such as a transmission would cause bad reception only in the zone between B and C, where neither of the intended receivers is located. This is called the exposed station problem. Manchester coding: 1：前高后低；0：前低后高。