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计算机网络复习重点(重点终结版)

物理拓扑结构(physical topology) 网状拓扑结构(mesh topology) 各台设备之间都有一条专用的点到点的链路,每台设备必须有 n-1 个 I/O 端口 星型拓扑结构(star topology) 每台设备拥有一条仅与中央控制器连接的点到点专用链路 总线拓扑结构(bus topology) 环状拓扑结构(ring topology) 协议和标准:  协议是用来管理数据通信的一组规则。协议规定了通信的内容、通信的方式和通信 的时间。  标准是由标准化组织协商达成一致的规则。 因特网模型(Internet model) 物理层 ; 数据链路层 ; 网络层 ;传输层 ;应用层An exchange using the Internet modelPhysical layerNote:The physical layer is responsible for transmitting individual bits from one node to the next.Data link layer

Note: The data link layer is responsible for transmitting frames from one node to the next.Network layerNote:The network layer is responsible for the delivery of packets from the original source to the final destination.Transport layerNote:The transport layer is responsible for delivery of a message from one process to another.Application layer

Note:The application layer is responsible for providing services to the user.Summary of duties=========================================================== Position of the physical layerNote: Frequency and period are inverses of each other.Note: A digital signal is a composite signal with an infinite bandwidth.  The bit rate and the bandwidth are proportional to each other.  The analog bandwidth of a medium is expressed in hertz; the digital bandwidth, in bits per second.  Digital transmission needs a low-pass channel.  Analog transmission can use a band-pass channel.noiseless channel

noisy channelAttenuationDistortionNoise

===========================================================Unipolar: Unipolar encoding uses only one voltage level.Polar: Polar encoding uses two voltage levels (positive and negative).NRZ-L:In NRZ-L the level of the signal is dependent upon the state of the bit. NRZ-I:In NRZ-I the signal is inverted if a 1 is encountered.

RZ encodingA good encoded digital signal must contain a provision for synchronization.Manchester encodingIn Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation.Differential Manchester encodingIn differential Manchester encoding, the transition at the middle of the bit is used only

for synchronization. The bit representation is defined by the inversion or noninversion at the beginning of the bit.BipolarIn bipolar encoding, we use three levels: positive, zero, and negative.Bipolar AMI encodingFrom analog signal to PCM digital codeAccording to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency How many bits should be sent for each sample? 1 bit for the sign and X bits for the value.===========================================================Bit rate is the number of bits per second. Baud rate is the number of signal units per second. Baud rate is less than or equal to the bit rate. ASK

Relationship between baud rate and bandwidth in ASKIn ASK the baud rate and bit rate are the same. An ASK signal requires a minimum bandwidth equal to its baud rate FSK

In FSKthe baud rate and bit rate are the same. BW=fc1-fc0+Nbaud PSKThe 4-PSK methodThe 8-PSK characteristics

Relationship between baud rate and bandwidth in PSKA telephone line has a bandwidth of almost 2400 Hz for data transmission.The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BWt = 2 x BWm.

The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BWt = 10 x BWm.The bandwidth of a stereo audio signal is usually 15 KHz. Therefore, an FM station needs at least a bandwidth of 150 KHz. The FCC requires the minimum bandwidth to be at least 200 KHz (0.2 MHz). ============================================================

multiplexingFDM is an analog multiplexing technique that combines signals. FDM process&&FDM demultiplexing exampleWDMWDM is an analog multiplexing technique to combine optical signals(光信号) . TDM TDM is a digital multiplexing technique to combine data.

In a TDM, the data rate of the link is n times faster, and the unit duration is n times shorter.Radio waves are used for multicast communications, such as radio and television, and paging systems. Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs. Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation. ============================================================ ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses. The existing local loops can handle bandwidths up to 1.1 MHz.语音通信带宽为 4k

Data Link Layer奇偶性校验CRC 校验(循环冗余码校验)校验和校验 Simple parity check can detect all single-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd. Two-dimensional parityBinary division in a CRC generatorChecksum

Positions of redundancy bits in Hamming codeStop-and-Wait ARQPiggybackingGo-Back-N ARQ

Go-Back-N ARQ: sender window sizeIn Go-Back-N ARQ, the size of the sender window must be less than 2m; the size of the receiver window is always 1. Selective Repeat ARQIn Selective Repeat ARQ, the size of the sender and receiver window must be at most one-half of 2m.

HDLC 高级数据链路控制 HDLC frameBit stuffing(比特填充)is the process of adding one extra 0 whenever there are fiveconsecutive 1s in the data so that the receiver does not mistake the data for a flag.Point-to-Point12.1PPP frame

Transition statesMultiple-access protocolsFigure 13.5CSMA persistent strateg13.3 Channelization FDMA:In FDMA, the bandwidth is divided into channels. TDMA:In TDMA, the bandwidth is just one channel that is timeshared. CDMA:In CDMA, one channel carries all transmissions simultaneously.

Figure 13.16CDMA multiplexer============================================================ =============================================================*****************************************************************=A repeater connects segments of a LAN. A repeater forwards every frame; it has no filtering capability. A repeater is a regenerator, not an amplifier. Hubs A bridge has a table used in filtering decisions. A bridge does not change the physical (MAC) addresses in a frame. AMPS ( 高级移动电话系统 , 北美 ) is an analog cellular phone system using FDMA. D-AMPS, or IS-136, is a digital cellular phone system using TDMA and FDMA. GSM is a digital cellular phone system using TDMA and FDMA. IS-95 is a digital cellular phone system using CDMA/DSSS and FDMA.

GSMThe Iridium system has 66 satellites in six LEO orbits, each at an altitude of 750 km. Iridium is designed to provide direct worldwide voice and data communication using handheld terminals, a service similar to cellular telephony but on a global scale. Teledesic has 288 satellites in 12 LEO orbits, each at an altitude of 1350 km. ============================================================ Frame Relay operates only at the physical and data link layers. Frame Relay does not provide flow or error control; they must be provided by the upper-layer protocols. Virtual connection identifiers in UNIs and NNIs18.21An ATM cellNetwork Layer

Switching at the network layer in the Internet is done using the datagram approach to packet switching. Communication at the network layer in the Internet is connectionless.分类Figure 19.17Network addressIn classful addressing, the network address is the one that is assigned to the organization. A network address is different from a netid. A network address has both netid and hostid, with 0s for the hostid.

Network LayerProtocols: ARP, IPv4, ICMPv4, IPv6, and ICMPv6 An ARP request is broadcast; an ARP reply is unicast. Figure 20.7IP datagramThere is no flow control or congestion control mechanism in IP. IPv6 addressIPv4-IPv6Three transition strategies双协议栈隧道技术数据包头转换RIP Updating AlgorithmReceive: a response RIP message 1. Add one hop to the hop count for each advertised destination. 2. Repeat the following steps for each advertised destination: 1. If (destination not in the routing table) 1. Add the advertised information to the table. 2. Else 1. If (next-hop field is the same) 1. Replace entry in the table with the advertised one. 2. Else 1. If (advertised hop count smaller than one in the table)

1. Replace entry in the routing table. 3. Return.Figure 21.20Shortest-path calculationDijkstra Algorithm1. Start with the local node (router): the root of the tree. 2. Assign a cost of 0 to this node and make it the first permanent node. 3. Examine each neighbor node of the node that was the last permanent node. 4. Assign a cumulative cost to each node and make it tentative. 5. Among the list of tentative nodes 1. Find the node with the smallest cumulative cost and make it permanent. 2. If a node can be reached from more than one direction 1. Select the direction with the shortest cumulative cost. 6. Repeat steps 3 to 5 until every node becomes permanent.

Transport LayerUDP is a connectionless, unreliable protocol that has no flow and error control. It uses port numbers to multiplex data from the application layer.

Application LayerDNS can use the services of UDP or TCP, using the well-known port 53. FTP uses the services of TCP. It needs two TCP connections. The well-known port

21 is used for the control connection, and the well-known port 20 is used for the data connection. HTTP uses the services of TCP on well-known port 80.RTP uses a temporary even-numbered UDP port.RTCP(实时传输控制协议) message typesRTCP uses an odd-numbered UDP port number that follows the port number selected for RTP.SecurityIn cryptography, the encryption/decryption algorithms are public; the keys are secret. In polyalphabetic substitution, the relationship between a character in the plaintext and a character in the ciphertext is one-to-many. Public-key algorithms are more efficient for short messages.对称密匙加密公匙加密

Digital signature does not provide privacy. If there is a need for privacy, another layer of encryption/decryption must be applied. Signing the whole documentSigning the digestA symmetric key between two parties is useful if it is used only once; it must be created for one session and destroyed when the session is over. The symmetric (shared) key in the Diffie-Hellman protocol is K = Gxy mod N. The AH protocol provides source authentication and data integrity, but not privacy. ESP provides source authentication, data integrity, and privacy. Transport Layer Security(TLS)Handshake protocolApplication Layer Security Pretty Good Privacy(PGP) A packet-filter firewall filters at the network or transport layer.(Packet-filter firewall 分 组过滤防火墙) A proxy firewall filters at the application layer.(Proxy firewall 代理防火墙)