Lecture 11. Digital Transmission Fundamentals

Transcription

1 CS4/MSc Compuer Neworking Lecure 11 Digial Transmission Fundamenals Compuer Neworking, Copyrigh Universiy of Edinburgh 2005 Digial Transmission Fundamenals Neworks consruced ou of Links or ransmission lines and swiches This lecure is abou digial ransmission lines Analogue ransmission sill used e.g. las mile of elephone sysem bu mos links are now digial Transmission media: Twised pair, coaxial cable, opical fibre, radio, infrared 2 1

2 Transmission Channels The wo imporan merics are: Laency - How long will i ake for 1 bi of informaion o be ransferred from one end of he link o he oher? Throughpu - How fas can bis be ransmied reliably? These depend on: amoun of energy pu ino ransmiing he signal he disance he signal has o raverse he amoun of noise inroduced he bandwidh of he ransmission medium Laency depends primarily on disance Focus on hroughpu/ransmission rae 3 A ransmission channel characerised by is effec on various frequencies Ampliude-response funcion, A(f), defined as raio of ampliude of he oupu signal o ha of he inpu signal, a a given frequency f Typical low-pass channel and an idealised channel of bandwidh W: 4 2

3 Signal Bandwidh A digial signal looks like a series of pulses. I can be shown (Fourier analysis) ha any signal is made up of componen sine waves a a number of frequencies The range of frequencies (specrum) deermines he signal bandwidh The faser he signal changes (dense, frequen pulses), he higher is bandwidh Channel bandwidh deermines if a signal can pass hrough reliably. If signal bandwidh > channel bandwidh here will be losses. Thus he channel bandwidh deermines "speed" of signal, herefore he maximum ransmission rae. 5 Nyquis Signalling Rae: r max = 2W pulses/second The maximum signalling rae ha is achievable hrough an ideal low-pass channel wih no inersymbol inerference Wih wo pulse ampliude levels ransmission rae = 2W bis per second Mulilevel ransmission possible if signal can ake 2 m ampliude levels ransmission rae = 2Wm bis per second In he absence of noise, bi rae can be increased wihou limi by increasing he number of ampliude levels Unforunaely, noise is always presen in a channel amoun of noise limis he reliabiliy wih which he receiver can correcly deermine he informaion ha was ransmied 6 3

4 Signal o Noise Raio SNR = AverageSignal Power Average Noise Power SNR (db) = 10 log 10 SNR 7 Shannon Channel Capaciy C = W log 2 (1 + SNR) bis/sec Reliable communicaion only possible up o his rae Example: Useful bandwidh of elephone line 3400 Hz purely because of added filers! assume a somewha opimisic SNR = 40 db C = 44.8 kbps only! In pracice, only 33.6 kbps possible inbound ino nework: SNR low because of 2x A-D conversion Wih digial connecion beween ISP - elephone nework speeds approaching 56 kbps can be achieved one A-D conversion is removed, SNR rises and more bandwidh is allocaed for downsream ransfer 8 4

5 Line Coding Compuers use binary code: 2-levels The channel ransfers symbols (could be > 2 levels) We need o do a mapping Facors o consider: Maximise ransfer rae - muliple levels are useful Minimise average ransmied power Synchronizaion - when should he receiver read he symbol? Combinaion of error deecion/correcion wih line coding Code complexiy (encoder/decoder hardware cos) Some media do no pass DC (consan) or low frequency signals (e.g. capacior in series) modulaion

6 Modulaion In realiy mos channels are no low-pass bu band-pass Need o pass he informaion by modulaing a sine signal of frequency f c Informaion Ampliude Shif Keying T 2T 3T 4T 5T 6 T Frequency Shif Keying Phase Shif Keying T 2T 3T 4T 5T 6 T 0 T 2T 3T 4T 5T 6 T 11 Signaling rae and Transmission Bandwidh Fac from modulaion heory: If Baseband signal x() wih bandwidh B Hz B f hen Modulaed signal x()cos(2πf c ) has bandwidh 2B Hz f c -B f c f c +B f 12 6

7 Quadraure Ampliude Modulaion (QAM) QAM uses wo-dimensional signaling A k modulaes in-phase cos(2πf c ) B k modulaes quadraure phase cos(2πf c + π/4) = sin(2πf c ) Transmi sum of inphase & quadraure phase componens A k x Y i () = A k cos(2πf c ) cos(2πf c ) + Y() B k x Y q () = B k sin(2πf c ) Transmied Signal sin(2πf c ) l l Y i () and Y q () boh occupy he bandpass channel QAM sends 2 pulses/hz 13 Signal Consellaions Each pair (A k, B k ) defines a poin in he plane Signal consellaion se of signaling poins (-A,A) B k (A, A) B k A k A k (-A,-A) (A,-A) 4 possible poins per T sec. 2 bis / pulse 16 possible poins per T sec. 4 bis / pulse 14 7

8 Properies of Transmission Media Two broad caegories: Guided (wired) and unguided (wireless) Guided media can have unlimied ransmission capaciy: Can always add more wires in parallel e.g. high-speed Eherne Guided media need righ of way o be deployed. Unguided do no, so neworks can be se up faser True mobile communicaion erminals mus be wireless Signal level in wireless can be mainained for longer disances (lower aenuaion per meer) 15 Properies of Copper wire pairs wising reduces suscepibiliy o crossalk and inerference» shielded (STP) or unshielded (UTP) can pass a relaively large range of frequencies: Aenuaion (db/mi) gauge 24 gauge 22 gauge 19 gauge f (khz) sill consiues overwhelming proporion of access nework wiring Caegory 5 cable specified for ransmission up o 100Mbps 4kHz bandwidh on elephone lines due o insered filers» loading coils added o provide flaer response and beer fideliy 16 8

9 Opical fibre ligh core cladding jacke relies on oal inernal reflecion of ligh waves: θ c core and cladding have differen refracive indices: n core > n cladding originally 20 db per km, now 0.25 db per km signals can be ransmied more han 100 km wihou amplificaion Loss (db/km) Rayleigh scaering waer absorpion peak Infrared absorpion Wavelengh (µm) 17 Opical fibre (2) Mulimode fibre - muliple rays follow differen pahs: Single-mode fibre - all rays follow a single pah: refleced pah direc pah larger core of mulimode fibre allows use of lower-cos LED ransmiers single-mode fibre designed o mainain spaial and specral inegriy of opical signals over longer disances» and have much higher ransmission capaciy Regeneraors conver opical signal o elecrical for amplificaion Purely opical amplifiers developed recenly 18 9

10 Elecromagneic specrum 19 Radio Transmission 3 khz o 300 GHz aenuaion varies logarihmically wih disance» varies wih frequency and wih rainfall subjec o inerference and mulipah fading» inerference he main reason for igh regulaory conrols on radiaed power applicaions: Blueooh, , Saellie ec

11 21 Muliplexing Sharing expensive resources beween several informaion flows Frequency-division Time-division Frequency-division muliplexing: used when he bandwidh of he ransmission line is greaer han ha required by a single informaion flow muliplexer modulaes signals ino appropriae frequency slo and ransmis he combined signal: 0 A W f 0 B W f A B C f 0 C W f 22 11

12 Time-Division Muliplexing ransmission line organised ino equal-sized ime-slos an individual signal assigned o ime-slos a successive fixed inervals 0T 0T 0T A 1 A 2 3T B 1 B 2 3T C 1 C 2 3T 6T 6T 6T A 1 B 1 C 1 A 2 B 2 C 2 0T 1T 2T 3T 4T 5T 6T Wavelengh Division Muliplexing (WDM and DWDM): he equivalen of frequency division muliplexing in he opical domain o make use of he enormous bandwidhs available here 23 12