计算机网络复习Word文档下载推荐.docx
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PCMPulseCodeModulation脉冲编码调制
QAMQuadratureAmplitudeModulation正交振幅调制
QPSKQuadraturePhaseShiftKeying正交相移键控
ADSLAsymmetricDigitalSubscriberLine非对称数字用户环路
CRCCyclicRedundancyCheck循环冗余校验
VCVirtualCircuit虚电路
ATMAsynchronousTransferMode异步传输模式
PPPPointtoPointProtocol点对点协议
HLDCHigh-LevelDataLinkControl高级数据链路控制
RIPRoutingInformationProtocol路由信息协议
RTTRound-Trip
Time往返时延
CIDRClasslessInter-DomainRouting无类别域间路由
IPInternetProtocol网络之间互连的协议
TTLTimeToLive生存时间
MTUMaximumTransmissionUnit最大传输单元
ICMPInternetControlMessageProtocol网络控制报文协议
UDPUserDatagramProtocol用户数据报协议
ARPAddressResolutionProtocol地址解析协议
DHCPDynamicHostConfigurationProtocol动态主机配置协议
NATNetworkAddressTranslation网络地址转换
RPCRemoteProcedureCall远程过程调用
OSPFOpenShortestPathFirst开放式最短路径优先
BGP
BorderGatewayProtocol边界网关协议
TCPTransmissionControlProtocol传输控制协议
RTPReal-timeTransportProtocol实时传输协议
FTPFileTransferProtocol文件传输协议
SMTPSimpleMailTransferProtocol简单邮件传输协议
POP3PostOfficeProtocol-Version3邮局协议版本3
IAMPinternetmessageaccessprotocol因特网信息报文存取协议?
不知道对不对
IMAPInternetMailAccessProtocolInternet邮件访问协议
DNSDomainNameSystem域名系统
URLUniformResoureLocator统一资源定位器
HTTPHyperTextTransferProtocol超文本传输协议
WWWWorldWideWeb万维网
1、Basicandimportantconceptsornotations
●networkarchitecture,layers,protocols
●OSI/RM,TCP/IPreferencemodel,maintasksofdatalink/network/transportlayers,protocolsateachlayerofTCP/IPmodel
●PAN,LAN,WAN,VLAN,WLAN
●bandwidth,linkcapacity
●multiplexing,channelallocation
●Frame,Ethernetframeformat,MACaddress
●framingmethod:
character-count,byte-stuffing,bit-stuffing,flagbyte
●Connection/connectionlessservice,circuitswitching/packetswitching,
●ErrorControlcoding,Hammingdistance
●Hammingcode,even-parity/odd-parity
●CRC,generatorpolynomial,remainder
●stop-and-waitprotocols,slidingwindow,go-back-n,selective-repeat,piggyback
●CSMA/CD
●hiddenstationproblem,exposedstationproblem,CSMA/CA
●hub/repeater/switch/bridge/router/gateway.
●
routingalgorithm,Distancevectorrouting,linkstaterouting
●IPaddress,classfulandspecialaddressing,CIDR,subnetting/aggregation,mask/prefix
●routingtable/forwarding,
●packet,IPv4header
●NAT,3reservedprivateIPaddressranges
●TCP,port/portnumber,TCPheader,three-wayhandshake
●congestioncontrol,ECN/RED,AIMD/slowstart,congestionwindow,threshold
●DNSsystem,domainnameresolution
2、Computations
●Nyquistlaw,Shannonlaw,PCM
●FDM,CDMA
●bit-stuffing,byte-stuffing,character-count
●Hammingcode,CRC
●CSMA/CD,minimalframesize
●Distancevectorroutingalgorithm
●CIDR,aggregation
●forwardinginrouters,longestmatching
第五版:
P10611,16,20
11Whatistheprincipaldifferencebetweenconnectionlesscommunicationandconnection-orientedcommunication?
Connection-orientedcommunicationhasthreephases.Intheestablishmentphase,arequestismadetosetupaconnection.Onlyafterthisphasehasbeensuccessfullycompletedcanthedatatransferphasebestartedanddatatransported.Thencomesthereleasephase.
Connectionlesscommunicationdoesnothavethesephases.Itjustsendsthedata.
16把比特流转化为帧
OSI数据链路层TCP/IP:
链路层
决定哪条路径通过子网
OSI网络层TCP/IP互联网层
20WhatisthemaindifferencebetweenTCPandUDP.
Answer:
Thefirstone,TCP(TransmissionControlProtocol),isareliableconnection-orientedprotocolthatallowsabytestreamoriginatingononemachinetobedeliveredwithouterroronanyothermachineintheInternet.
Thesecondprotocol,UDP(UserDatagramProtocol),isanunreliable,connectionlessprotocolforapplicationsthatdonotwantTCP'
ssequencingorflowcontrolandwishtoprovidetheirown.
TCP是transmissioncontrolprotocol传输控制协议,UDP是userdatagramprotocol用户数据电报协议。
TCP是面向连接的协议,允许字节流无错误地从一台机器上互联到别的机器上。
UDP是不可靠的,面向无连接的协议,适用于不需要TCP协议的那种流量控制和自我实现的应用程序。
P2072,3,4,26,27,28,41,44
2Anoiseless4-kHzchannelissampledevery1msec.Whatisthemaximumdatarate?
Ifanarbitrarysignalhasbeenrunthroughalow-passfilterofbandwidthB,thefilteredsignalcanbecompletelyreconstructedbymakingonly2B(exact)samplespersecond.
Anoiselesschannelcancarryanarbitrarilylargeamountofinformation,nomatterhowoftenitissampled.Justsendalotofdatapersample.Forthe4kHzchannel,make8000samples/sec.Ifeachsampleis16bits,thechannelcansend128kbps.Ifeachsampleis1024bits,thechannelcansend8.192Mbps.Thekeywordhereis‘‘noiseless.’’Withanormal4kHzchannel,theShannonlimitwouldnotallowthis.
PCM(PulseCodeModulation)脉码调制它将4千赫兹的电话信道的模拟信号转化为数字信号。
一条普通的电话线路带宽为4KHz,根据奈奎斯特定理:
只要每秒4000*2=8000次的采样频率就能获取一个4KHz的信道中的全部信息,再高的采样频率也无意义,所以PCM的采样时间为1/8000=125us。
3Ifabinarysignalissentovera3-kHzchannelwhosesignal-to-noiseratiois20dB,whatisthemaximumachievabledatarate?
20dB=10lgS/N∴S/N=100
Shannonlimit=Blog2(1+S/N)=3000*log2(1+100)=19.975Kbps
Nyquistlimit=2Blog2V=2*3000*log22=6Kbps
Daterate≤19.975KbpsandDaterate≤6Kbps(同时满足)
4Wehaveachannelwitha1MHzbandwidth.TheSNRforthischannelis63;
whatistheappropriatebitrateandsignallevel?
Shannonlimit=Blog2(1+S/N)=106*log2(1+63)=6Mbps
Nyquistlimit=2Blog2V=2*106*log2V=6MbpsV=8
26WhatisPCM?
WhyhasthePCMsamplingtimebeensetat125usec?
PCMmeanspulsecodemodulation.Itistheanalog-to-digitalsignal模拟数字信号convertingtechniquefortransmittinginformationfromthe4-KHztelephonechannel.电话通道
AccordingtotheNyquisttheorem,thesamplingfrequencyneededtocapturealltheinformationina4-kHzchannelis8KHz.Thatistosay,asamplingtimeis1sec/8000=125μsec.
27
28
41SupposethatA,B,andCaresimultaneouslytransmitting0bits,usingaCDMAsystemwiththechipsequencesofFig.2-45.Whatistheresultingchipsequence?
编码方法:
用要发送的bit串转化成双极性码,”1”对应”+1”,”0”对应”-1”,不发送的用全“0”表示,然后按顺序同各个站的码片序列相乘,最后将各站的结果序列(混合)相加,就是在空中发射的信号。
A:
“0””-1”,-1*(-1-1-1+1+1-1+1+1)=(+1+1+1-1-1+1-1-1);
B:
“0””-1”,-1*(-1-1+1-1+1+1+1-1)=(+1+1-1+1-1-1+1+1);
C:
“0””-1”,-1*(-1+1-1+1+1+1-1-1)=(+1-1+1-1-1-1+1+1)
D:
不发送,(00000000);
4个序列相加得:
(+3+1+1-1-3-1-1+1).
44ACDMAreceivergetsthefollowingchips:
(-1+1-3+1-1-3+1+1).AssumingthechipsequencesdefinedinFig.2-45,whichstationstransmitted,andwhichbitsdideachonesend?
解码方法:
用接收到的码片同各个站的码片序列进行“点积”(又称为“内积”),也就是按位相乘,结果序列的所有位相加,除以码片序列的长度,”1”意味着原先编码时是”1”,”-1”表示编码”0”,“0”表示没有静默,没发送。
(-1+1-3+1-1-3+1+1)⊙(-1-1-1+1+1-1+1+1)=(1-1+3+1-1+3+1+1)/8=8/8=1,即:
A发送了bit“1”;
(-1+1-3+1-1-3+1+1)⊙(-1-1+1-1+1+1+1-1)=(1-1-3-1-1-3+1-1)/8=-8/8=-1,即:
B发送了bit“0”;
(-1+1-3+1-1-3+1+1)⊙(-1+1-1+1+1+1-1-1)=(1+1+3+1-1-3-1-1)/8=0/8=0,即:
C没有发送信息。
P2722,7,8,14,15
2
A是错的应为00000101
7An8-bitbytewithbinaryvalue10101111istobeencodedusinganodd-parityHammingcode.Whatisthebinaryvalueafterencoding?
p1p2a3p4a5a6a7p8a9a10a11a12
=?
?
1?
010?
1111
P1+a3+a5+a7+a9+a11=奇?
+1+0+0+1+1=奇
P2+a3+a6+a7+a10+a11=奇?
+1+1+0+1+1=奇
P4+a5+a6+a7+a12=奇?
+0+1+0+1=奇
P8+a9+a10+a11+a12=奇?
+1+1+1+1=奇
得汉明码为011001011111。
若传输后,接收方收到的为011111011111,检验:
P1:
0+1+1+0+1+1=偶p2:
1+1+1+0+1+1=奇
P4:
1+1+1+0+1=偶p8:
1+1+1+1+1=奇
8
?
14
15Abitstream10011101istransmittedusingthestandardCRCmethoddescribedinthetext.Thegeneratorpolynomialisx3+1.Showtheactualbitstringtransmitted.Supposethethirdbitfromtheleftisinvertedduringtransmission.Showthatthiserrorisdetectedatthereceiver'
send.
生成式为1001。
消息比特加上3个0后成为10011101000,除以1001后余式为100,因此实际传输的比特串为10011101100。
左起第三比特颠倒后接收到的多项式为10111101100,除以生成式,余式为100,而不是0,这就使接收方明白发生了错误。
P36913,14,15,16,22
13
15A1-km-long,10-MbpsCSMA/CDLAN(not802.3)hasapropagationspeedof200m/µ
sec.Repeatersarenotallowedinthissystem.Dataframesare256bitslong,including32bitsofheader,checksum,andotheroverhead.Thefirstbitslotafterasuccessfultransmissionisreservedforthereceivertocapturethechannelinordertosenda32-bitacknowledgementframe.Whatistheeffectivedatarate,excludingoverhead,assumingthattherearenocollisions?
(长为1km的CSMA/CD局域网(不是802.3网),数据率为10Mbps,传播速度为200m/µ
s。
系统中不用中继器。
数据帧长256比特,其中包括32比特头部、检验和和其它开销。
一次成功传输后的第一个时隙保留给接收方捕获信道,以便让它发送一个32比特的确认帧。
假设不发生冲突,扣除开销,有效的数据率为多大?
)
CSMS/CD局域网是分时隙的(timeslotted)。
每次发送之前必须侦听信道,至少花费一个时隙(timeslot)才能确保不冲突!
时隙长度取决于最大往返传播延时。
Single-triptakes1km/200m/µ
sec=5usec,round-trippropagationdelayisτ=5X2=10usec,sothelengthoftimeslotshouldbe10usec.信号从电缆一端传导到另一端用时1000/(200/10^-6)=5us,往返需5*2=10us,因此时隙长度为10us。
transmitterseizescable(10μsec):
beforetransmission,thetransmitterchecksthecable,andifthecableisidle,startstotransmitatthebeginningofnexttimeslot发前“听信道”要用一个时隙
transmitdata(25.6μsec)传输延时
Delayforlastbittogettotheend(5.0μsec)单程传播延时
receiverseizescable(10μsec):
similartothetransmitter同1.
acknowledgement
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