1、Chapter 2 The Physical Layer,College of Computer Science Chongqing University,Content,The Physical Layer Theoretical Basis for Data CommunicationsTransmission Media A Taxonomy of Communication Networks Example Networks,From Signals to Packets,0 0 1 0 1 1 1 0 0 0 1,Application,Presentation,Session,Tr
2、ansport,Network,Datalink,Physical,From Signals to Packets,The Physical Layer,The Physical Layer provides the mechanical,electrical,functional and procedural characteristics to establish,maintain,and release physical connections between data link entities.A physical-connection may involve intermediat
3、e open systems,each relaying bit transmission within the Physical Layer.Physical Layer entities are interconnected by means of a physical medium.,Four characteristics,Mechanical characteristics:mechanical specification of electrical connectors and cables,for example maximum cable lengthElectrical ch
4、aracteristics:electrical specification of transmission line signal level and impedanceFunctional characteristics:the function logic of electric signals generated on certain wires in the physical mediaProcedural characteristics:the procedures of protocols that control the sequence of events,RS-232,RS
5、-232(Recommended Standard 232)is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE(Data Terminal Equipment)and a DCE(Data Communications Equipment).It is commonly used in computer serial ports.,Mechanical characteristics,D
6、B25:male and female connector,DB9:male and female connector,Electrical characteristicsdata rates of up to 20 kbps,over cables of up to 15 metresnegative,bipolar logic in which a negative voltage is used to represent a logic 1,and a positive voltage represents a logic 0Functional characteristicsProce
7、dural characteristics,Content,The Physical Layer Theoretical Basis for Data CommunicationsTransmission Media A Taxonomy of Communication Networks Example Networks,Theoretical Basis for Data Communication,Data communications are the exchange of data between two devices via some form of transmission m
8、edium.Basic terminologyCommunication ChannelCommunication ratesData Encoding TechniquesMultiplexing Techniques,Basic terminology,DataInformationSignalAnalog dataDigital dataAnalog SignalsDigital Signals,Data Communication System Model,Theoretical Basis for Data Communication,Data communications are
9、the exchange of data between two devices via some form of transmission medium.Basic terminologyCommunication ChannelCommunication ratesData Encoding TechniquesMultiplexing Techniques,Communication Channel,Channel:a physical transmission mediumAnalog channel Digital channel,Types of communication cha
10、nnels,Data Transmission vs.Channel,Analog data transmission over analog channel:carrier and modulationDigital data transmission over analog channel:modulation,Analog data transmission over digital channel:samplingDigital data transmission over digital channel:encoding,Data Transmission vs.Channel,an
11、alog data,analog signal,modulator,analog data,digital signal,PCMcoder,digital data,analog signal,modulator,Channel Communication Mode,Simplex Communicationhalf-duplex communication:,Full-duplex communication,Data Transmission Mode,Baseband Transmission0,1 Passband Transmissionaural signalBroadband T
12、ransmissionanalog signal,Data Synchronization,Data synchronization:the process of establishing consistency among data from a source to a target data storage and vice versa and the continuous harmonization of the data over time Synchronous ModeBit synchronizationCharacter synchronizationFrame synchro
13、nization,Bit synchronization,Bit synchronization:find the beginning and the end of coming bits(symbols)at the receiverExternal synchronizationSelf-synchronizationExternal synchronization,Character synchronization,Frame synchronization,HDLC01111110(7E),Theoretical Basis for Data Communication,Data co
14、mmunications are the exchange of data between two devices via some form of transmission medium.Basic terminologyCommunication ChannelCommunication ratesData Encoding TechniquesMultiplexing Techniques,Communication rates,Communication rates have fundamental limitsFourier analysisBandwidth-limited sig
15、nalsMaximum data rate of a channel,Fourier Analysis,A time-varying signal can be equivalently represented as a series of frequency components(harmonics谐波):,a,b weights of harmonics,Signal over time,=,Bandwidth-Limited Signals,Having less bandwidth(harmonics)degrades the signal,8 harmonics,4 harmonic
16、s,2 harmonics,Lost!,Bandwidth,Lost!,Lost!,Maximum Data Rate of a Channel,Nyquists theorem relates the data rate to the bandwidth(B)and number of signal levels(V):Shannons theorem relates the data rate to the bandwidth(B)and signal strength(S)relative to the noise(N):Often expressed in decibels(db):=
17、10 log10(S/N),Max.data rate=2B log2V bits/sec,Max.data rate=B log2(1+S/N)bits/sec,How fast signalcan change,How many levelscan be seen,Theoretical Basis for Data Communication,Data communications are the exchange of data between two devices via some form of transmission medium.Basic terminologyCommu
18、nication ChannelCommunication ratesData Encoding TechniquesMultiplexing Techniques,Data Encoding Techniques,Digital Data,Digital Signals wired LANDigital Data,Analog Signals modemAnalog Data,Digital Signals codec,Digital Data,Digital Signals,Digital signal is a sequence of discrete,discontinuous vol
19、tage pulses.Bit duration:the time it takes for the transmitter to emit the bit.IssuesBit timing(synchronizing)Recovery from signalNoise immunitythe technique used in a number of LANs,NRZ(Non-Return-to-Zero)Codes(1),Uses two different voltage levels(one positive and one negative)as the signal element
20、s for the two binary digits.NRZ-L(Non-Return-to-Zero-Level)The voltage is constant during the bit interval.NRZ-L is used for short distances between terminal and modem or terminal and computer.,1 negative voltage0 positive voltage,NRZ(Non-Return-to-Zero)Codes(2),NRZ-I(Non-Return-to-Zero-Invert on on
21、es)The voltage is constant during the bit interval.NRZI is a differential encoding(i.e.,the signal is decoded by comparing the polarity of adjacent signal elements.),1 existence of a signal transition at the beginning of the bit time(either a low-to-high or a high-to-low transition)0 no signal trans
22、ition at the beginning of the bit time,Bi Phase Codes,Bi-phase codes require at least one transition per bit time and may have as many as two transitions.the maximum modulation rate is twice that of NRZ greater transmission bandwidth is required.Advantages:Synchronization with a predictable transiti
23、on per bit time the receiver can“synch”on the transition self-clocking.No ponentError detection the absence of an expected transition can used to detect errors.,Manchester encoding,There is always a mid-bit transition which is used as a clocking mechanism.The direction of the mid-bit transition repr
24、esents the digital data.Consequently,there may be a second transition at the beginning of the bit interval.Used in 802.3 baseband coaxial cable and CSMA/CD twisted pair.,1 low-to-high transition 0 high-to-low transition,Differential Manchester encoding,mid-bit transition is ONLY for clocking.Differe
25、ntial Manchester is both differential and bi-phase.Note the coding is the opposite convention from NRZI.Used in 802.5(token ring)with twisted pair.*Modulation rate for Manchester and Differential Manchester is twice the data rate inefficient encoding for long-distance applications.,1 absence of tran
26、sition at the beginning of the bit interval 0 presence of transition at the beginning of the bit interval,Bi-Polar Encoding,Has the same issues as NRZI for a long string of 0s.,1 alternating+1/2,-1/2 voltage 0 0 voltage,1,0,1,0,1,1,0,0,1,UnipolarNRZ,NRZ-Inverted(DifferentialEncoding),BipolarEncoding,DifferentialManchesterEncoding,Polar NRZ,ManchesterEncoding,