1、Chapter 2 The Physical Layer,College of Computer Science Chongqing University,Content,The Physical Layer Theoretical Basis for Data CommunicationsTransmission Media A Taxonomy分类 of Communication Networks Example Networks,From Signals to Packets,0 0 1 0 1 1 1 0 0 0 1,Application,Presentation,Session,
2、Transport,Network,Datalink,Physical,From Signals to Packets,The Physical Layer,The Physical Layer provides the mechanical,electrical,functional and procedural characteristics to establish,（机械特性，电气特性，功能特性，过程特性）maintain,and release physical connections between data link entities.A physical-connection
3、may involve intermediate open systems,each relaying bit transmission within the Physical Layer.Physical Layer entities are interconnected 连通的by means of a physical medium.,Four characteristics,Mechanical characteristics:mechanical specification of electrical connectors and cables,for example maximum
4、 cable lengthElectrical characteristics:electrical specification of transmission line signal level and impedance阻抗Functional characteristics:the function logic of electric signals generated on certain wires in the physical mediaProcedural characteristics:the procedures of protocols that control the
5、sequence of events,RS-232,RS-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.,
6、Mechanical characteristics,DB25: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（两极的）logic in which a negative voltage is used to represent a logic 1,and a positive voltage represents a logic
7、0Functional characteristicsProcedural 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
8、 via some form of transmission medium.Basic terminologyCommunication ChannelCommunication ratesData Encoding TechniquesMultiplexing Techniques,Basic terminology,DataInformationSignalAnalog dataDigital dataAnalog SignalsDigital Signals,Data Communication System Model,Theoretical Basis for Data Commun
9、ication,Data communications are 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 ch
10、annel,Types of communication channels,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
11、,Data Transmission vs.Channel,analog 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 Tra
12、nsmissionaural signalBroadband Transmissionanalog 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 synchronizationCharac
13、ter synchronizationFrame synchronization,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 Basi
14、s for Data Communication,Data communications 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 limitsFouri
15、er analysisBandwidth-limited signalsMaximum 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 t
16、he signal,8 harmonics,4 harmonics,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):O
17、ften expressed in decibels(db):=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 transmissi
18、on medium.Basic terminologyCommunication ChannelCommunication ratesData Encoding TechniquesMultiplexing Techniques,Data Encoding Techniques,Digital Data,Digital Signals wired LANDigital Data,Analog Signals modem调制解调器Analog Data,Digital Signals codec编码解码器,Digital Data,Digital Signals,Digital signal i
19、s a sequence of discrete,discontinuous 不连续voltage 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 po
20、sitive and one negative)as the signal elements for the two binary digits.NRZ-L(Non-Return-to-Zero-Level)The voltage is constant during the bit interval间隔.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
21、)Codes(2),NRZ-I(Non-Return-to-Zero-Invert on ones)The voltage is constant during the bit interval.NRZI is a differential encoding(i.e.,the signal is decoded by comparing the polarity极性 of adjacent signal elements.),1 existence of a signal transition at the beginning of the bit time(either a low-to-h
22、igh or a high-to-low transition)0 no signal transition 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.Advan
23、tages:Synchronization with a predictable transition per bit time the receiver can“synch”on the transition self-clocking.No ponentError detection探测 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 mec
24、hanism.The direction of the mid-bit transition represents 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 encod
25、ing,mid-bit transition is ONLY for clocking.Differential 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 transition 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,
