COMPUTER NETWORKS (EC-321) PHYSICAL LAYER. OUTLINE Basis for Conventional Data Communication Transmission Media Wireless Transmission Communication Satellites

COMPUTER NETWORKS (EC-321)PHYSICAL LAYER

OUTLINE• Basis for Conventional Data Communication

• Transmission Media

• Wireless Transmission

• Communication Satellites

• Telephone System, Mobile Telephone System

BASIS FOR CONVENTIONAL DATA COMMUNICATION

• Fourier Analysis

• Bandwidth-Limited Signals

• The Maximum Data Rate of a Channel

FOURIER ANALYSIS

• Any periodic signal can be represented as a sum of sinusoids, known as a Fourier series:

• f is known as the fundamental frequency

• f = 1/T, where T is the period of the signal

• multiples of f are referred to as harmonics

• The values of the coefficients:

• The root-mean-square amplitudes, , are proportional to the energy of the corresponding frequency.

FOURIER ANALYSIS

• The spectrum of a signal is the range of frequencies that it contains

• The absolute bandwidth is the width of the spectrum.

• Most of the energy in a signal is contained in a relatively narrow band of frequencies. This band is referred to as the effective bandwidth, or just bandwidth.

• Fourier analysis helps us estimate how well a signal produced by a transmitter will be carried by a particular channel.

FOURIER ANALYSIS (EXAMPLE)

1

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1

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t tideal periodic signalreal composition(based on harmonics)

FOURIER ANALYSIS (EXAMPLE)

BANDWIDTH-LIMITED SIGNALS

• All transmission media have a finite bandwidth, which is determined by physical properties of the medium and the distance traversed.

• Thus, a given medium will filter out part of the signal (0 up to some frequency fc (cutoff)), namely, the higher frequency components. In addition, an artificial filter may be applied to the channel.

• In real, cutoff is not sharp ------ Bandwidth is 0 to freq where received power fallen by half

• Baseband and passband• Information carried not depend on starting and

ending freqs

EFFECT OF LIMITED BANDWIDTH SIGNALS

channel: voice-grade line (3000 Hz)

THE MAXIMUM DATA RATE OF A CHANNEL (CHANNEL CAPACITY)

• Impairments, such as noise, limit data rate that can be achieved

• For digital data, to what extent do impairments limit data rate?

• The maximum rate at which data can be transmitted over a given communication channel, under given conditions, is referred to as the channel capacity.

• We would like to make as efficient use as possible of a given bandwidth, i.e., we would like to get as high a data rate as possible at a particular limit of error rate for a given bandwidth.

THE MAXIMUM DATA RATE OF A CHANNEL (CHANNEL CAPACITY)

• Two Formulas

Problem: given a bandwidth, what data rate can we achieve?

Nyquist Formula

Assume noise free

Shannon Capacity Formula

Assume white noise

Nyquist Formula

• If the rate of signal transmission is 2B, then a signal with frequencies

no greater than B is sufficient to carry the signal rate.Given bandwidth B, highest signal rate is 2B

• If the signal consists of V discrete levels, Nyquist’s theorem states:

Maximum Data Rate = 2B log2 V bits /sec

• Nyquist’s formula indicates that, for a given bandwidth, the data rate can be increased by increasing the

number of different signal elements/levels.if all other things are equal, doubling the bandwidth doubles the data rate.


Signal-to-Noise Ratio:

• Ratio of the power in a signal to the power contained in the noise present at a particular point in the transmission.

• Typically measured at a receiver

• Signal-to-noise ratio (SNR, or S/N)

• A high SNR means a high-quality signal, low number of required intermediate repeaters

power noise

power signallog10)( 10dB SNR


• Shannon’s major result is that the maximum data rate or capacity of a

noisy channel whose bandwidth is B Hz and whose signal-to-noise

ratio is S/N, is given by:

Maximum number of bits/sec = B log2 (1 + S/N)

• Some remarks:• Given a level of noise, the data rate could be increased by increasing either signal

strength or bandwidth.

• As the signal strength increases, so do the effects of nonlinearities in the system

which leads to an increase in intermodulation noise.

• Because noise is assumed to be white, the wider the bandwidth, the more noise is

admitted to the system. Thus, as B increases, SNR decreases.

Example

• Consider an example that relates the Nyquist and Shannon formulations.

Suppose the spectrum of a channel is between 3 MHz and 4 MHz, and SNRdB

= 24dB. So,

B = 4 MHz – 3 MHz = 1 MHz

SNRdB = 24 dB = 10 log10(SNR) SNR = 251

• Using Shannon’s formula, the capacity limit C is:

C = 106 x 1og2(1+251) ≈ 8 Mbps.

• If we want to achieve this limit, how many signaling levels are required at

least?

By Nyquist’s formula: C = 2Blog2M

We have 8 x 106 = 2 x 106 x log2M M = 16.

Transmission Media

• Media are roughly grouped into

• guided media

• unguided media

• There are many options for sending data from point A to point B.• Guided media

• Magnetic

• Twisted pair

• Coaxial cable

• Power lines

• Fiber optics

• Unguided media

• Wireless

Guided media

• Magnetic Media

• One of the most common ways to transport data from one

computer to another is to write them onto magnetic tape or

removable media

• High Bandwidth and less cost

• Delay characteristics are poor

Never underestimate the bandwidth of a station wagon full of tapes

hurtling down the highway.

Guided media

• Twisted pair

• Consists of pair of twisted, insulated copper wires, about 1mm

thick. To gain bandwidth, sets of pairs are grouped into a single

cable.

• twisting the wires reduces EMI

• used for both analog and digital transmission

• various grades of cable are used

• data rate up to 10Gbps

• used for many decades in the phone system, still dominate local

loops

• Very commonly used in LANs today

Guided media

• Twisted pair (EMI)

Guided media

• Twisted pair (Terminology)

• The cables are marked as 'XbaseY'. For example, 10base10, where

X means the data rate in Mbps, base means 'baseband'. Baseband

means a transmission medium of signals and Y means

the category of cabling.

Guided media

• Coaxial cable

• It has better shielding and greater bandwidth than unshielded

twisted pairs

• so it can span longer distances at higher speeds

• Two kinds of coaxial cable are widely used

• 50-ohm

• 75-ohm

Guided media

• Coaxial cable (50-ohm)

• 50-ohm, also called baseband

• up to 1-2 Gbps over 1 km

• previously widely used in long-haul

• telephone lines (now largely replaced by fiber)

• May still be used in some LANs

Guided media

• Coaxial cable (75-ohm)

• 75-ohm cable, also called broadband

• Originally, analog transmission and cable TV

• about 450 MHz for 100 km

• usually, the cable is divided into 6 MHz channels

• amplifiers are required to boost signals

Guided media

• Power lines

• Used at low data rates by electricity companies for years

• Use in the home: data signal is superimposed on a low-frequency power

signal.

• Di culties with household wiring:ffi

• It doesn’t carry high-frequency signals well

• Its electrical properties vary

• It can act as an antenna

Guided media

• Optical Fibre

• Composition• ultra-thin (2 to 125 µm) fiber of glass that can transmit light pulses in one direction

• three concentric sections

• core: innermost; one or more very thin strands of glass or plastic

• cladding: surrounds each fiber, glass or plastic coating with optical properties

much di erent from core (specifically, lower index of refraction)ff

• jacket: plastic surrounding one or more cladded fibers, to protect against

moisture, abrasion, crushing, etc.

GUIDED MEDIA• Optical Fibre (Optical Transmission and Reception)

• Internal reflection occurs with indices of refraction of core and cladding and

angle of incident

• Refractive index n= C/V

• Core has high ‘n’ compare to cladding

GUIDED MEDIA• Optical Fibre (Optical Transmission and Reception)

• Multiple angles (modes) may propagate

• multimode fiber - multiple modes propagate

• single-mode fiber - only one mode propagates

• which is more expensive?

• Light sources

• Light Emitting Diode (LED)

• Injection Laser Diode (ILD)

• Reception - photodiode

GUIDED MEDIA• Advantages of Optical Fibre

• Higher bandwidth

• data rates of multiple Gbps over 10s of kilometers demonstrated

• Small size and light weight compared to copper

• Much lower attenuation (0.1dB/km), implying greater spacing of repeaters

• Immunity to electromagnetic interference

• do not radiate energy

• cause no interference

• very di cult to tapffi

GUIDED MEDIA

Attenuation of light through fiber in the infrared region

UNGUIDED MEDIA

Wireless Transmission

• The Electromagnetic Spectrum• Radio Transmission• Microwave Transmission• Infrared Transmission• Light Transmission

THE ELECTROMAGNETIC SPECTRUM

• Electromagnetic Waves• Moving electrons create electromagnetic waves that can

propagate through free space.• Wave characteristics

• Frequency• Wavelength

• Wireless communication is based on broadcast/receipt of waves using electronic circuits attached to antennas.

• In a vacuum, all waves propagate at the same speed• Fundamental relation among frequency, wavelength, and

speed:

THE ELECTROMAGNETIC SPECTRUM• Radio, microwave, infrared, and visible light are good for transmission

• UV, X-rays, gamma rays would be good due to their higher frequencies, but..• hard to produce and modulate• do not propagate well through buildings• interfere with our ability to live


Frequency Bands and Bandwidth• Carrying capacity is proportional to the bandwidth of the media• Computing a frequency band:• c = fλ• df/dλ = − c/λ2

• Then ∆f = c∆λ/λ2

• Given the width of a wavelength band ∆λ, you can compute corresponding frequency band ∆f.


Radio Transmission• Description

• frequencies in 30 MHz to 1 GHz• radio is omnidirectional, whereas microwaves are focused• line of sight is not required• Tight licensing needed due to interference

• Uses• packet radio (often a defence application)• ALOHA-type networks• cellular telephone• personal communication network (PCN)• wireless LANs (growing rapidly)


Use of Frequencies• Most transmissions use a narrow frequency band: ∆f/f

<< 1• Wide band variations:

• Frequency hopping spread spectrum• Hard to detect, impossible to jam• the receiver will not be stuck on an impaired frequency for long

enough to shut down communication • Good resistance to multipath fading and narrowband interference

• Good for ISM band (crowded part of spectrum)

• Direct sequenced spread spectrum• Ultra-WideBand (UWB)


Use of Frequencies


Use of Frequencies


Radio Transmission Characteristics• VLF, LF, and MF bands follow the ground• HF and VHF bands bounce off ionosphere


Terrestrial Microwave• Description

• parabolic dishes mounted on towers• “line of sight” distance from one another• Ex: 100 ft towers can be 80 km apart• for long distance transmission, use relay• towers in a point-to-point fashion

• Uses • long-haul telecommunications service, as alternative to

coax cable for television and voice• communication between buildings (closed-circuit TV)• digital data in small regions (radius < 10 km)

Documents

COMPUTER NETWORKS (EC-321) PHYSICAL LAYER. OUTLINE Basis for Conventional Data Communication Transmission Media Wireless Transmission Communication Satellites