CS3502, Data and Computer Networks: the physical layer-3

CS3502,Data and Computer Networks:the physical layer-3

impairments to transmission • three categories 1. attenuation/attenuation distortion • loss of signal power through distance • attenuation varies with frequency 2. delay distortion • guided media only • velocity varies with frequency • limiting factor on bandwidth, especially with digital signals 3. noise

impairments to transmission noise • thermal noise - heat; electron agitation • intermodulation noise • unwanted combining of signals at diff. frequencies • crosstalk • common on t.p. • 2 nearby paths coupled electrically • impulse

encoding techniques • using signals to send information: main purpose of physical layer • 4 major cases 1. digital signals to send digital data 2. digital signals to send analog data 3. analog signals to send digital data 4. analog signals to send analog data

encoding techniques • what needs to happen 1. X and R must be synchronized 2. X emits a signal 3. R receives and interprets signal • factors affecting transmission • data rate • S/N : signal-to-noise ratio • encoding technique

encoding techniques • desirable properties of an encoding scheme • synchronization capability - the ability to stay synchronized, or to get re-synchronized • error detection capability • immunity to noise - the ability to separate noise from the transmitted signal

encoding techniques • digital data - digital signals • simple binary methods: NRZ-L, NRZ-I(M), NRZ-S • voltage level constant throughout bit time • simple, but no synchronization capability • most vulnerable to noise • used only for low-moderate data rates, short distances NRZ-L: high 0, low 1 NRZ-M: change on 1, not on 0 NRZ-S: change on 0, not on 1 • examples - diagram... what is the baud rate? • -M, -S are differential methods

encoding techniques • digital data - digital signals; better methods multilevel binary, bipolar AMI these hold 0 voltage for binary 0, then alternate between + and - for binary 1 Pseudoternary reverse of bipolar AMI biphase methods - require at least 1 transition in each bit time • increase reliability in presence of noise • increased synch. capability • increased ability to detect errors

encoding techniques • biphase methods: always a transition in the middle of the bit time manchester • down for 1, up for 0 differential manchester • change at start of bit indicates a 0 • Q: what is the baud rate? • Q: can you think of a way to increase the data rate but not the baud rate?

encoding techniques • digital data, analog signals • analog signal: a continuously varying electomagnetic wave Q: why use analog signals for digital data? what are 3 critical and widely used analog media ? • also may want to mix digital, analog data

encoding techniques • digital data, analog signals carrier signal - a constant analog signal, transmitted from sender to receiver example: the dial tone indicates a live connection; a carrier wave • bits encoded by varying 1 or more of 3 properties • modulation: ASK, FDK, PSK

encoding techniques • ASK: amplitude shift keying (diagram) • carrier • ASK

encoding techniques • ASK Summary • unaltered carrier contains no data • can be used in optical fiber • for other media, only good for lower bit rates • less efficient, relatively than other methods; more susceptible to errors, because • higher amplitudes attenuate more rapidly than lower ones • more susceptible to interference

encoding techniques • FSK: frequency shift keying : diagram • FSK: summary • less error prone • used for high frequency (coax, microwave,etc) • also used on voice lines • radio

encoding techniques • PSK: phase shift keying - differential • binary PSK : phase shift of 1/2 period indicates 1; no shift indicates 0; (diagram) • QPSK: use of 4 angles for higher bit rates

encoding techniques • PSK - summary • more efficient that ASK, FSK • can be further enhanced with more signal levels • number of angles media dependent • example: 2400 bauds, 9600 bps; PSK, ASK together (12 angles, 2 amplitudes) • show how to combine these techniques for higher bit rates; eg, ASK-FSK, ASK-PSK, FSK-PSK

encoding techniques • analog data, digital signals • 2 main techniques : pulse code modulation (PCM), delta modulation (DM) • why? • voice over optical fiber • TV channels, movies, pictures over internet • principle: the sampling theorem • theorem statement (see text) • note: based on exact samples

encoding techniques • PCM: pulse code modulation • samples of the analog data taken • each sample quantized • samples transmited as digital signal • received samples used to reconstruct analog data • example: voice channels • samples taken 8000/sec • quantized to 7 bits • synch. bit added -> 8 bits • 8 x 8000 = 64000 bps, standard digital voice channel

encoding techniques • delta modulation • similar idea (digital samples of analog data) • reduction in number of bits transmitted • periodically sends a sample • send a “1” or “0” indicating “up” or “down” • the up or down is by a fixed amount • less accurate than PCM • if intervals not chosen to match signal, or if signal varies, leads to less accuracy. • less widely used, but could be alternative

encoding techniques • analog data, analog signals • basis original telephone network; (ie, used analog electical signals to transport analog voice signals); still in local loops to large degree • cable TV; (FDM - frequency division multiplexing) • broadcast radio • major techniques • amplitude modulation • frequency modulation • phase modulation

CS3502, Data and Computer Networks: the physical layer-3