1、OSI Layer 1:Physical LayerMedia, Connections, and CollisionsOSI Layer1: Physical Layerp Type of Networkp LAN Mediap UTP for Ethernetp Media and signal Problems p Basic Knowledge of Data CommunicationType of NetworksPoint-to-pointType of Networksp Shared media environment pMultiple hosts can access t

2、he same medium. pIt means that they all share the same media - even though the wire might be UTP, which has four pairs of wirep Point-to-point network environment pOne device is connected to the other one via a linkpMost widely used in dial-up network connections, and is the one with which you are m

3、ost likely familiar. OSI Layer1: Physical Layerp Type of Networkp LAN Mediap UTP for Ethernetp Media and signal Problems p Basic Knowledge of Data CommunicationLAN Mediap Function is to transmit datap Transmission process is referred to as encodingp Cable types include STP, UTP, coaxial, fiber-optic

4、sLAN Media-STPp4-pair wiring combines shielding and cancellation via twisting of wirespnormally a 150-Ohm cablepreduces crosstalk EMI, RFIpaffords greater protection against all types of external interference than UTPLAN Media-ScTPpwrapped in a metallic foil shield or screenpusually 100 or 120 Ohm c

5、ablepBoth STP and ScTP prevent incoming electromagnetic waves from causing noise on data wires and also minimizes outgoing radiated electromagnetic wavesLAN MediaDisadvantages of STP and ScTPpcannot run cable as far as some other types of networking media without signal being repeatedpinsulation add

6、s considerably to size, weight, and cost of cablepshielding materials make terminations more difficult and susceptible to poor workmanshipLAN Media-UTPprelies solely on the cancellation effect, produced by twisted wire pairs to limit signal degradation caused by EMI and RFIphas four pairs of copper

7、wirephas impedance of 100 Ohmspeasy to install and is less expensive pcosts less per meter than any other type of LAN cablingpsmall external diameter does not fill up wiring ducts as rapidly as other types of cablepis installed using an RJ connector so potential sources of network noise are greatly

8、reduced and a good solid connection is practically guaranteedLAN MediaAdvantages of UTPLAN MediaDisadvantages of UTPpcable is more prone to electrical noise and interference than other types of networking mediapdistance between signal boosts is shorter for twisted-pair than for coaxial and fiber-opt

9、icsLAN Media-CoaxialpThinnet/thicknetplonger network runs without using repeaters than twisted-pairpless expensive than fiber but more expensive than twisted-pairLAN MediaFiber-Opticpconducts modulated light transmissionpnot susceptible to EMI or RFI and is capable of higher data rates than other ne

10、tworking mediapelectromagnetic waves are guided through optical fiberFiber-Optic Mode TypespSingle Modenalso called axial : light travels down the axis of the cablenfaster than multimode (up to 10 Gbps) because of the dispersion in multimodentypically used for WANSnsmaller in diameter than multimode

11、 (less dispersion)nuses ILD most often but also LEDpMultimodenlight enters the glass pipe at different angles and travels nonaxially, which means it bounces back and forth off the walls of the glass tubenlarger than single mode, used most often in LANSnsusceptible to greater dispersionWireless Commu

12、nicationpThe primary way of distinguishing between different electromagnetic waves is by their frequency.(frequency multiplexing)Wireless Transmission MethodsnLasersnoutput a coherent electromagnetic field in which all waves are at the same frequency and are aligned in a phasenInfrarednnormally a li

13、ne-of-sight technology but can be bounced or redirectedncannot go through opaque objectsnRadioncarry data signals that can pass through wallsnboth terrestrial and satellite radio technologiesOSI Layer1: Physical Layerp Type of Networkp LAN Mediap UTP for Ethernetp Media and signal Problems p Basic K

14、nowledge of Data CommunicationCable Specification and TerminationpThe standards for networking media were developed and issued by the following groups:nIEEEInstitute of Electrical and Electronics EngineersnULUnderwriters LaboratoriesnEIAElectronic Industries AlliancenTIATelecommunications Industry A

15、ssociationnANSIAmerican National Standards Institutep 一類線：主要用于語音傳輸，不用于數據傳輸 p 二類線：傳輸頻率1MHz，用于語音和最高4Mbps的數據傳輸，常見于令牌網 p 三類線三類線：EIA/TIA568標準指定電纜，傳輸頻率16MHz，用于語音傳輸及最高傳輸速率為10Mbps的數據傳輸，主要用于10BASE-T p 四類線：傳輸頻率為20MHz，用于語音傳輸和最高傳輸速率16Mbps的數據傳輸，主要用于令牌網和 10BASE-T/100BASE-T p 五類線五類線：增加了繞線密度，外套高質量絕緣材料，用于語音和數據傳輸(主要為

16、100/1000BASE-T)，是最常用的以太網電纜 p 超五類線超五類線：衰減小，串擾少，具有更高的衰減/串擾比和信噪比、更小的時延誤差，主要用于1000BASE-Tp 六類線六類線：傳輸頻率為1MHz250MHz，性能遠高于超五類標準，適用于高于1Gbps的應用p 七類線：帶寬為600MHz，可能用于今后的10G比特以太網。 Category of UTP pStraight CablepRollover CablepCrossover CableType of CableCreating a Straight-Through Cable123456789101112Straight ca

17、ble: use T568A on both ends or T568B on both ends100-Ohm balanced twisted-pair telecommunication outlet/connectorStraight CablepAlias: console cablepis used to connect a workstation or terminal to the console port of a router/switch to configure itpPin 1 on one end connects to Pin 8 on the other end

18、; then Pin 2 connects to Pin 7, Pin 3 to Pin 6 and so onRollover CableConfiguring Devices with ConsolenConnect the serial port (com) of computer by using RJ-45-to-DB-9 adapternStart up “super terminal”nUse “default configurations”Crossover Cableppairs 2 and 3 on one end of the cable will be reversed

19、 on the other endpT568-A on one end and T568-B on the otherpis considered to be part of the “vertical” cabling/ backbonepcan be used to pconnect two or more hubs or switches pconnect two isolated workstations to create a mini-LANOSI Layer1: Physical Layerp Type of Networkp LAN Mediap UTP for Etherne

20、tp Media and signal Problems p Basic Knowledge of Data CommunicationSignaling and Communications Problemsp Propagationntravel time; speed depends upon mediumnAs data transmission rates increase, you must sometimes take into account the amount of time it takes the signal to travel.p Attenuationnloss

21、of signal over distance due to surroundingsncan affect a network because it limits the length of network cabling over which you can send a messageSignaling and Communications ProblemspReflectionnCaused by discontinuities in the mediumnOccurs in electrical signals; can be a result of kinks in cable o

22、r poorly terminated cablesnNetworks should have a specific impedance to match the electrical components in the NICsnThe result of impedance mismatch is reflected energy.Signaling and Communications ProblemspNoisenunwanted additions to optical/electromagnetic signalsnCrosstalkelectrical noise from ot

23、her wires in a cablenEMI (electromagnetic interference) can be caused by electric motors.nCancellation of signals can be avoided through the twisting of wire pairs to provide self-shielding within the network media.Signaling and Communications ProblemsCollisions and Collision Domainsp A collision oc

24、curs when two bits propagate at the same time on the same network.p Collision domains are extended by adding repeaters and hubs.p Networks can be segmented by adding intelligent devices such as bridges, switches, and routers.Segmenting Collision DomainsCollisions and Collision DomainspWhen this dela

25、y limit is exceeded, the number of late collisions dramatically increase.pA late collision is when a collision happens after the first 64 bytes of the frame are transmittedpThese late collision frames add delay referred to as consumption delay.pAs consumption delay and latency increase, network perf

26、ormance decreases5-4-3-2-1 RuleFive sections of the network, four repeaters or hubs, three sections of the network are mixing sections (with hosts), two sections are link sections (for link purposes), and one large collision domain. OSI Layer1: Physical Layerp Type of Networkp LAN Mediap UTP for Eth

27、ernetp Media and signal Problems p Basic Knowledge of Data Communication數據通信基本知識o 數據通信的理論基礎n基本術語n信號處理n波特率與比特率o 數據通信技術n數據通信系統基本結構n數據表示和傳輸方式n信號的傳輸n數字信號編碼n多路復用n通信方式基本術語o 信號(signal)數據的電氣的或電磁的表現。 n “模擬的”(analogous)消息的參數的取值是連續的o 模擬信號是隨時間變化而平穩變化的連續波形式n “數字的”(digital)消息的參數的取值是離散的o 數字信號是離散信號，可能包含有限的幾個預定值 o 碼

28、元(code)在使用時間域（或簡稱為時域）的波形表示數字信號時，代表不同離散數值的基本波形。理論基礎:信號處理 模擬信號可以被分為簡單信號和復合信號簡單信號（正弦波）不能被分解為更簡單的模擬信號復合信號可以被分解為多個正弦波復合模擬信號的分解：傅立葉分析傅立葉分析任何一個周期為T的有理周期性函數 g(t)可分解為若干項（可能無限多項）正弦和余弦函數之和： f = 1/T 基本頻率；an, bn ：n次諧波項的正弦和余弦振幅值111g(t) = c +sin(2)cos(2)2nnnnanftbnft理論基礎:信號處理o 數字信號一般是非周期性的，通常在傳輸介質上表現為方波o 一個數字信號可以分

29、解為無窮多個被稱為諧波的簡單正弦波，每個諧波都具有不同的頻率與相位o 在介質上發送數字信號時，其實質是在發送無窮多的簡單諧波，如果某些分量未能忠實地通過介質傳輸，則在接收端將產生信號畸變o 由于介質本身的限制，信號畸變是難以完全避免的o任何實際的信道都不是理想的，在傳輸信號時會產生各種失真以及帶來多種干擾。 o碼元傳輸的速率越高，或信號傳輸的距離越遠，在信道的輸出端的波形的失真就越嚴重數字信號通過實際的信道 o 有失真，但可識別o 失真大，無法識別 實際的信道（帶寬受限、有噪聲、干擾和失真）發送信號波形接收信號波形發送信號波形實際的信道（帶寬受限、有噪聲、干擾和失真）接收信號波形無噪聲信道的最

30、高傳輸速率o 1924 年，奈奎斯特(Nyquist) 指出在假定無噪聲的信道上，為避免碼間串擾，傳輸比特率的上限值為： C=W log2L bpsn W 為信道的帶寬（以 Hz 為單位）n L 為表示數據的信號電平的數量o 在任何信道中，碼元傳輸的速率是有上限的，否則就會出現碼間串擾的問題，使接收端對碼元的判決（即識別）成為不可能。o 如果信道的頻帶越寬，也就是能夠通過的信號高頻分量越多，那么就可以用更高的速率傳送碼元而不出現碼間串擾 噪聲信道的最高傳輸速率o 香農(Shannon)用信息論的理論推導出了帶寬受限且有高斯白噪聲干擾的信道的極限、無差錯的信息傳輸速率o 信道的極限信息傳輸速率

31、C 可表達為C = W log2(1+S/N) bps n W 為信道的帶寬（以 Hz 為單位）；n S 為信道內所傳信號的平均功率；n N 為信道內部的高斯噪聲功率n S/N稱為信噪比 香農公式的含義 o 信道帶寬或信道的信噪比越大，極限傳輸速率越高o 只要信息傳輸速率低于信道的極限信息傳輸速率，就一定有辦法實現無差錯的傳輸 o 若信道的帶寬 W 或信噪比 S/N 沒有上限（實際不可能），則其極限信息傳輸速率 C 也沒有上限o 實際能夠達到的傳輸速率比香農極限傳輸速率低不少o 請注意：對于頻帶寬度已確定的信道，即使信噪比不能再提高，且碼元速率已達上限，也有辦法提高傳輸速率。這就是用編碼的方法

32、讓每個碼元攜帶更多比特的信息量 理論基礎:波特率和比特率n 波特率（baud）和比特率（bit）n波特率（調制速率）：信號每秒鐘變化的次數n比特率：每秒鐘傳送的二進制位數。n 波特率與比特率的關系取決于信號值與比特位的關系n例：每個信號值表示為位，則比特率是波特率的倍；每個信號值表示為位，則比特率和波特率相同n對于比特率為a bps的信道，發送位所需的時間為 8/a秒，若位為一個周期，則一次諧波的頻率是：f = a/8 Hz數據通信系統的模型 傳輸系統輸入信息輸入數據發送的信號接收的信號輸出數據源點終點發送器接收器調制解調器PC 機公用電話網調制解調器數字比特流數字比特流模擬信號模擬信號 輸入

33、漢字顯示漢字數據通信系統源系統目的系統傳輸系統輸出信息PC 機數據通信技術：數字數據編碼p 基帶：基本頻帶，指傳輸變換前所占用的頻帶，是原始信號所固有的頻帶p 基帶傳輸:在傳輸時直接使用基帶數字信號(不轉換為模擬信號，即不調制)n基帶傳輸是一種最簡單最基本的傳輸方式，一般用低電平表示“0”，高電平表示“1”n適用范圍：低速和高速的各種情況。n限制：因基帶信號所帶的頻率成分很寬，所以對傳輸線有一定的要求將數字數據轉換到模擬信號：調制010011100基帶信號調幅調頻調相將數字數據轉換到數字信號：線路編碼oLine coding means to convert the binary data i

34、nto a form that can travel on a physical communications link such as an electrical pulse on a wire, a light pulse on an optical fiber, or an electromagnetic wave in space數據通信技術：編碼方式 編碼方式可分為三類： 單極性編碼單極性編碼 極化編碼 NRZ(Non-Return to Zero，不歸零制碼不歸零制碼) RZ(Return to Zero，歸零制碼歸零制碼) 雙相位編碼 曼徹斯特碼曼徹斯特碼 差分曼徹斯特碼差分曼徹

35、斯特碼 雙極性編碼 傳號交替反轉碼傳號交替反轉碼(AMI) 雙極性8連0替換碼(B8ZS) 3階高密度雙極性碼(HDB3)單極性編碼n 原理：n 用0電平表示“0”，正電平表示“1”n 缺點：n 難以分辨一位的結束和另一位的開始n 發送方和接收方必須有時鐘同步n 若信號中“0”或“1”連續出現，信號直流分量將累加，單極性編碼的直流分量問題嚴重n 結論：容易產生傳播錯誤00 1 0 0 1 0極化編碼不歸零制碼(NRZ: Non-Return to Zero)n 不歸零電平編碼n 原理：用負電平表示“0”，正電平表示“1”（或相反）n 缺點：n 難以分辨一位的結束和另一位的開始n 發送方和接收方

36、必須有時鐘同步n 盡管不會如單極性編碼嚴重，但若信號中“0”或“1”連續出現，信號直流分量仍將累加00 1 0 0 1 0極化編碼不歸零制碼(NRZ: Non-Return to Zero)n 不歸零反相編碼n 原理：信號電平的一次翻轉代表比特1，無電平變化代表0n 不歸零反相編碼優于不歸零電平編碼：由于每次遇到“1”（或“0”）都要發生躍遷，因此可以根據電平躍遷進行有限的同步0 1 1 0 1 1極化編碼：歸零制碼（RZ: Return to Zero）n 原理：用負電平表示“0”，正電平表示“1”（或相反），比特中位跳變到零電平，從而提供同步n 優點：信號本身帶有同步信息，經濟性好，且不易

37、出錯n 缺點：需要采用三個不同電平，兩次信號變化來編碼1比特，因此增加了占用的帶寬00 1 0 0 1 0n原理：每一位中間都有一個跳變，從低跳到高表示“0”，從高跳到低表示“1”n請思考：“00” “01”曼徹斯特編碼下應怎樣表示？ n優點：克服了NRZ碼的不足。每位中間的跳變既可作為數據，又可作為時鐘，能夠自同步；同時只采用兩個電平，跳變減少，比RZ碼效率更高極化編碼：曼徹斯特碼（Manchester）00 1 0 0 1 0極化編碼：差分曼徹斯特碼（Differential Manchester）n 原理：n 每一位中間跳變：表示時鐘n 每一位位前跳變：表示數據n 有跳變表示“0”，無跳

38、變表示“1”n 優點：時鐘、數據分離，便于提取001101NRZ曼徹斯特差分曼徹斯特極化編碼：差分曼徹斯特碼（Differential Manchester）雙極性編碼：雙極性傳號交替反轉碼（AMI）n 與RZ相同的是: 采用三個電平：正、負與零n 與RZ不同的是: 零電平表示“0”，正負電平的躍遷表示“1”，實現對“1”電平的交替反轉。n 優點：n 對每次出現的“1”交替反轉，使直流分量為0n 盡管連續“0”不能同步，但連續“1”可以同步00 1 0 0 1 0數據通信技術：多路復用n 多路復用技術n由于一條傳輸線路的能力遠遠超過傳輸一個用戶信號所需的能力，為了提高線路利用率，經常讓多個信號共用一條物理線路物理鏈路復用器分解器信號信號復用器分解器信號信號信道多路復用時分復用TDM(Time Division Multiplexing)o 時分復用是將時間劃分為一段段等長的時分復用（TDM ）幀，每個時分復用的用戶在每個 TDM 幀中占用固定序號的時隙。o 每一個用戶所占用的時隙是周期性地出現o TDM 信號也稱為等時(isochronous)