Computer and Data Networks, 2. Physical layer and data transmission 1 ©Dr.Z.Sun

2. Physical Layer and Data Transmission

Computer and Data Networks, 2. Physical layer and data transmission 2 ©Dr.Z.Sun

Outline

n Data transmission basics and theory, n Transmission median Telephone systems for computer connectionsn Circuit, message and packet switchingn Multiplexingn Virtual circuits versus circuit switching

Computer and Data Networks, 2. Physical layer and data transmission 3 ©Dr.Z.Sun

Theoretical basis for data communication

n Fourier analysis (19th Century): any periodic function can be constructed by summing a number of sines and cosines

n T is period, f=1/T, root-mean-square amplitude

n All transmission facilities diminish different Fourier components by different amount, thus introducing distortion

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The maximum data rate of a channel

n The Nyquist theorem noiseless channel is:Max. data rate = 2Hlog2V bit/s

where H is bandwidth, V discrete signal levels.Example: 3 kH, binary signal can be transmitted up to 2 x 3000 log2 (2) = 6,000 bit/s = 6 Kbit/s

n The Shannon theorem for noise channel:Max. rate = Hlog2 (1+S/N) bit/s

where S/N is signal-to-noise ratio.Example: S/N=30 & 3 kH -> 3000 log2 (1+103) = 30,000 Kbit/s

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Transmission Media

n Magnetic media (tape, floppy, CD): computer data and music

n Twisted pair: Telephone and computer connectionsn Coaxial cables: LAN and cable TVn Fibre optics: high speed networking for LAN and WANn Wireless transmission: mobile phone, wireless LAN,

satellite

Computer and Data Networks, 2. Physical layer and data transmission 6 ©Dr.Z.Sun

Wireless transmission

n Fundamental relation between f, λ and c is

λ f = c=> df/dλ = -c/λ2

∆f = c∆λ/λ2

Wheref - Frequency in Hzλ - Wavelength c - Speed of light (3x108 m/s)n Example:

λ = 1.3 x 10-6,∆λ = 0.17 x 10 -6 , => ∆f = 30 THz

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Telephone system

Three major partsn Local loops (twist

pair, analogsignalling)

n Trunks (fibre optics or microwave, mostly digital

n Switching office

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Connecting computer using telephone line

n Local loops are still analogue

n Transmission impairments due to attenuation, delay distortion and noise

n Square waves (in digital) have a wide spectrum

n Modem is the solution

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Digital Subscriber Lines (2)

Operation of ADSL using discrete multitone modulation.

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Digital Subscriber Lines (3)

A typical ADSL equipment configuration.

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Modemsn Get around the problems associated

with DC signal, especially on telephone lines, AC signalling is used

n Amplitude, frequency, frequency shift and phase shift modulation

n Example: transmit multiple bit per baud 9600 bit/s over a 2400 band line

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Multiplexing: FDM

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PCM and TDM

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Switching - circuit, packet and message

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Packet Switching

A comparison of circuit switched and packet-switched networks.

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Space switch and Time switch

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N-ISDN

n ISDN services - voice and datan Reference points: R, S, T, Un Interface: basic rate, Primary

rate, Hybrid

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Virtual circuit versus circuit switching n In a circuit switching network, a

connection means a physical path from source to destination

n In a virtual circuit switching network, a connection means a logical path sharing the same physical path with the others.

Computer and Data Networks, 2. Physical layer and data transmission 19 ©Dr.Z.Sun

Summary

n Basics and theory: Nyquist and Shannon theoremsn Transmission media: Twist pair, cable, optical and

wirelessn Telephone systems for computer connection using

modemn Multiplexing: FDM and TDMn Switching: circuit, message and packetn Space and time switchesn Virtual circuit and circuit switching