Data and Computer Communications

Transcription

1 CT30A2003 Tietoliikennetekniikan perusteet Data and Computer Communications Chapter 3 Data Transmission Ninth Edition by William Stallings 1

2 Kommunikointijärjestelmä 2

3 Tiedonsiirto The successful transmission of data depends on two factors: quality of the signal being transmitted characteristics of the transmission medium

4 Terminologiaa data transmission occurs between a transmitter & receiver via some medium Johtimellinen/guided medium eg. twisted pair, coaxial cable, optical fiber Johtimeton/unguided / wireless medium eg. air, water, vacuum 4

5 Transmission Terminology

6 Transmission Terminology Simplex signals transmitted in one direction eg. Television Half duplex both stations transmit, but only one at a time eg. police radio Full duplex simultaneous transmissions eg. telephone

7 Analogue & Digital Signals 7

8 Frequency, Spectrum and Bandwidth time domain analoginen signaali various in a smooth way over time digitaalinen signaali maintains a constant level then changes to another constant level Jaksollinen/periodic signal Jakso toistuu s(t+t) = s(t), - <t< Jaksoton/aperiodic signal pattern not repeated over time 8

9 Periodic Signals 9

10 Sini-aalto s(t) = A sin(2πft +Φ) Amplitudi (A) maximum strength of signal volts Taajuus (f) rate of change of signal Hertz (Hz) or cycles per second period = time for one repetition (T) T = 1/f Vaihe (φ) relative position in time 10

11 Varying Sine Waves s(t) = A sin(2πft +Φ) 11

12 Aallonpituus (λ) is distance occupied by one cycle between two points of corresponding phase in two consecutive cycles assuming signal velocity v have λ = vt or equivalently λf = v especially when v=c c = 3*10 8 ms -1 (speed of light in free space) 12

13 Frequency Domain Concepts signal are made up of many frequencies components are sine waves Fourier analysis can shown that any signal is made up of component sine waves can plot frequency domain functions 13

14 Addition of Frequency Components (T=1/f) c is sum of f & 3f 14

15 Frequency Domain Representations freq domain func of Fig 3.4c freq domain func of single square pulse 15

16 Spectrum & Bandwidth

17 Tasavirtakomponentin vaikutus 17

18 Tiedonsiirto ja kaistanleveys any transmission system has a limited band of frequencies this limits the data rate that can be carried square wave have infinite components and hence bandwidth s(t) = A*4/π * Σ k sin(2πkft)/k but most energy in first few components limited bandwidth increases distortion have a direct relationship between data rate & bandwidth 18

19 19

20 Case I Kanttiaallon esitys kuvasta 3.7a Kolme taajuutta f = 1MHz B = 4MHz T= 1µs Bitit 0.5 µs välein Tiedonsiirtonopeus 2Mbps 20

21 Case II Kanttiaallon esitys kuvasta 3.7a Kolme taajuutta B = 8MHz f = 2MHz T= 0.5µs Bitit 0.25 µs välein Tiedonsiirtonopeus 4Mbps 21

22 Case III Kanttiaallon esitys kuvasta 3.4c Kaksi taajuutta f = 2MHz B = 4MHz T= 0.5µs Bitit 0.25 µs välein Tiedonsiirtonopeus 4Mbps 22

23 Bandwidth-Limited Signals 23

24 Bandwidth-Limited Signals (2) 24

25 Bandwidth-Limited Signals (3) Relation between data rate and harmonics. 25

26 Data Rate and Bandwidth any transmission system has a limited band of frequencies this limits the data rate that can be carried on the transmission medium limiting bandwidth creates distortions most energy in first few components square waves have infinite components and hence an infinite bandwidth There is a direct relationship between data rate and bandwidth.

27 Analog and Digital Data Transmission data entities that convey meaning signals & signalling electric or electromagnetic representations of data, physically propagates along medium transmission communication of data by propagation and processing of signals 27

28 Acoustic Spectrum (Analog) 28

29 Audio Signals freq range 20Hz-20kHz (speech 100Hz-7kHz) easily converted into electromagnetic signals varying volume converted to varying voltage can limit frequency range for voice channel to Hz 29

30 Digital Data as generated by computers etc. has two dc components bandwidth depends on data rate 30

31 Analog Signals

32 Digital Signals

33 Advantages & Disadvantages of Digital Signals cheaper less susceptible to noise but greater attenuation digital now preferred choice 33

34 Transmission Impairments signal received may differ from signal transmitted causing: analog - degradation of signal quality digital - bit errors most significant impairments are attenuation and attenuation distortion delay distortion noise 34

35 Attenuation where signal strength falls off with distance depends on medium received signal strength must be: strong enough to be detected sufficiently higher than noise to receive without error so increase strength using amplifiers/repeaters is also an increasing function of frequency so equalize attenuation across band of frequencies used eg. using loading coils or amplifiers 35

36 Attenuation Distortion

37 Delay Distortion only occurs in guided media propagation velocity varies with frequency hence various frequency components arrive at different times particularly critical for digital data since parts of one bit spill over into others causing intersymbol interference 37

38 Noise additional signals inserted between transmitter and receiver thermal due to thermal agitation of electrons uniformly distributed white noise intermodulation signals that are the sum and difference of original frequencies sharing a medium 38

39 Categories of Noise Intermodulation noise produced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium effect is to produce signals at a frequency that is the sum or difference of the two original frequencies

40 Categories of Noise Impulse Noise: caused by external electromagnetic interferences noncontinuous, consisting of irregular pulses or spikes short duration and high amplitude minor annoyance for analog signals but a major source of error in digital data Crosstalk: a signal from one line is picked up by another can occur by electrical coupling between nearby twisted pairs or when microwave antennas pick up unwanted signals

41 Channel Capacity Maximum rate at which data can be transmitted over a given communications channel under given conditions data rate in bits per second bandwidth in cycles per second or Hertz noise average noise level over path error rate rate of corrupted bits limitations due to physical properties main constraint on achieving efficiency is noise

42 Nyquist Bandwidth consider noise free channels if rate of signal transmission is 2B then can carry signal with frequencies no greater than B ie. given bandwidth B, highest signal rate is 2B for binary signals, 2B bps needs bandwidth B Hz can increase rate by using M signal levels Nyquist Formula is: C = 2B log 2 M so increase rate by increasing signals at cost of receiver complexity limited by noise & other impairments 42

43 Shannon Capacity Formula consider relation of data rate, noise & error rate faster data rate shortens each bit so bursts of noise affects more bits given noise level, higher rates means higher errors Shannon developed formula relating these to signal to noise ratio (in decibels) SNR db= 10 log 10 (signal/noise) Capacity C=B log 2 (1+SNR) theoretical maximum capacity get lower in practise 43