the physical layer read chapter 2 csc 3352 computer communications chapter 22 putting the bits on...

The Physical Layer

Read Chapter 2

CSC 3352 Computer Communications Chapter 2 2

Putting the bits on the wire…


Terminology (1)

TransmitterReceiverMedium Guided medium

e.g. twisted pair, optical fiber Unguided medium

e.g. air, water, vacuum


Terminology (2)

Point-to-point Direct link

Only 2 devices share link

Multi-point More than two devices share the link


Terminology (3)Simplex One direction

e.g. Television

Half duplex Either direction, but only one way at a

time e.g. police radio

Full duplex Both directions at the same time

e.g. telephone


Continuous Signals

Continuous signal: Intensity Varies in a smooth way over time, no breaks...


Discrete signal: Maintains a constant level then changes to another constant level

Discrete Signals


Periodic Signals

Periodic signal: The same pattern is repeated over time...


Amplitude (A) maximum strength of signal…Volts

Frequency (f) Rate of change of signal, Hertz (Hz) or

cycles per second

The period Period = time for one repetition (T) T = 1/f

Phase () Relative position in time

Amplitude, Frequency and Period...


Amplitude, Frequency and Period...


Wavelength ()

: Distance occupied by one cycle. It is also the distance between two points of corresponding phase in two consecutive cyclesIn a vacuum space: c = 3*108 ms (speed of light in free

space) = cT f = c


Baud

Definition: the unit baud measures the number of changes per second in the signal.

OR, how many times the signal changes its amplitude per second...


Spectrum & Bandwidth

Spectrum range of frequencies contained in

signal

Absolute bandwidth width of spectrum

Effective bandwidth Often just bandwidth Narrow band of frequencies containing

most of the energy


Data Rate and Bandwidth

Any transmission system has a limited band of frequencies

This limits the data rate that can be carried


The Maximum data rate of a channel

Nyquist:maximum data rate = 2 H log2 V bits/s

Shannon (Nyquist is an optimist)maximum data rate = H log2 (1+S/N)S/N…..


Analog and Digital Data Transmission

Data Entities that convey meaning

Signals Electric or electromagnetic

representations of data

Transmission Communication of data by

propagation and processing of signals


Data

Analog Continuous values within some interval e.g. sound, video

Digital Discrete values e.g. text, integers


SignalsMeans by which data are propagatedAnalog Continuously variable Various media: wire, fiber optic, space Speech bandwidth 100Hz to 7kHz Telephone bandwidth 300Hz to 3400Hz Video bandwidth 4MHz

Digital Use only two levels of voltage


Data and SignalsUsually use digital signals for digital data and analog signals for analog dataCan use analog signal to carry digital data Modem

Can use digital signal to carry analog data Compact Disc audio


Transmission Impairments

Signal received may differ from signal transmittedAnalog - degradation of signal qualityDigital - bit errorsCaused by: Attenuation Distortion Noise


AttenuationSignal strength falls off with distance

Loss of energy due to the propagation of bits in long distance…

Depends on medium

Received signal strength: must be enough to be detected must be sufficiently higher than noise to

be received without error


Distortion

Only in guided mediaPropagation velocity varies with frequencyBits do not arrive in the initial order...


NoiseAdditional signals inserted between transmitter and receiver

Thermal: due to thermal agitation of electrons

Crosstalk: a signal from one line is picked up by another

Impulse: high amplitude


What To Do Against Attenuation, Distortion and Noise?

A media adapted to the distance/bit rate

Encoding schemes (we will see it later)


Media for Networks

Twisted pair 2 twisted insulated copper wires Twesting waires reduces interferences Ex.: the telephone system Category 3: Some megabits per sec. (for only

few Kms, may need repeaters for longer distances)

Category 5: more twists per centimeter, less crosstalk, longer distance, more suitable for high speed computer communications…


Media for NetworksCoaxial cable See fig. 2-3 on page 84 Has a better shielding so it can span longer

distance… Higher speed… Good noise immunity… Baseband: 2Gbps for 1km, less speed for

more kms (amplifiers are requiered), used in both voice and data transmission…

Broadband: oriented cable TV, mainly analog transmission, accuracy is not required, long distance…


Media for NetworksFiber optics See page 87 for an introduction to optical

transmission system… 3 main elements: the light source, the transmission

medium and the light detector A pulse of light=1 bit, absence=0bit The transmission media is an ultra-thin fiber of

glass How to transmit… Several Gbps for 30kms, 1 repeater per 30kms…


Future is With Fiber

Bigger bandwidths

Attenuation/distance is lower than copper

Not sensitive to electromagnetic interference

Lighter and thinner than copper

Difficult to tap


Wireless Links: The spectrum of telecommunication

UV

100 1014104 108 1012

Infrared

1016 1022

GammaRadio Microwa. X-Ray

104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016F(Hz)

Band LF MF HF VHF UHF

Twisted pair

Coax

Satellite

Microwave

Fiber

Maritime

AMRadio

FMRadio

TV


Different Bands of the Spectrum

Radio Transmission

Microwave Transmission

Infrared and Millimeter Waves

Lightwave Transmission


Relation Spectrum, Frequency, Data rate...

The frequency band is derived from .f = c

Deriving gives the frequency band corresponding to a wavelength and wavelength interval is then :…


The Spectrum: A cake to Share

WARC (International)

FCC ( U.S. agency)


Radio Transmission

Easy to generateTravel long distance, penetrate buildingsOmnidirectionalProperties depend on frequency low frequencies, penetrates buildings

well but sensitive to distance (1/r3). High frequencies, travel in straight

lines, absorbed by rain

Sensitive to interferences


Microwave Transmission

Travel long distance, (Does not penetrate buildings)Not Omnidirectional. Travel in straight lines (transmitter and receiver must be carefully aligned)Absorbed by rain

ISM band


Infrared and Millimiter Waves

Do not go thru solid walls

Good for indoor wireless LANs


Using the Telephone System for Computer Networks

Why?

What is the telephone system?

How does it work?

How can computers use it for communicating?


The Telephone System

Phone

Local Loop

End office

Toll office (We may have other levels of hierarchy…)


The Telephone SystemLocal loop analog twisted pair one per user Low bandwidth

Inter switches trunks Use to be analog but is now digital (see

page 109) fiber High bandwidth Shared


Computer Modem

End office

CodecDigital

Short digital connectionRS 232

Local loopAnalog

Local loop for a Computer


Transmission impairments

(Local Loop)

Attenuation

Delay distorsion

Noise


Modulation

Amplitude modulation

Frequency modulation (FSK)

Phase modulation (PSK)

Constellation patterns (see ex. in page 111)


Full/Half duplex

Other problems with telephone lines: Echo effects: a person hears its own words

after a short delay… Echo suppressors: a device that detects

human voice from one end and suppresses signals from the other end… switching… half duplex connection…

Echo cancelers… Estimates the echo and subtracts it from the original signal…Full duplex connection


Interface Computer (DTE)-Modem (DCE)

The RS 232 is a good example of physical layer protocol

DataTerminal Equipment

DataCircuit-Term.

Equipment

Protective ground (1)Transmit (2)

Receive(3)Request To Send (4)

Clear To Send(5)Data Set Ready(6)

Common Ground (7)

Carrier Detect(8)Data terminal Ready(20)


Interface Computer (DTE)-Computer (DTE)

With a null modem device we can use RS 232 to connect 2 DTE…

DTE DTEShield (1)

Transmit (2)Receive(3)

Request To Send (4)Clear To Send(5)

Data Set Ready(6)

Common Ground (7)

Carrier Detect(8)Data terminal Ready(20)


Computer Modem

End office

Codec

Digital

Digital

RS 232Local loopAnalog

Review

We looked in detail into thisWe focus now on trunks


Trunks: transporting many conversations on the same “wire”

Frequency Division Multiplexing Wavelength Division Multiplexing

(Fiber)

Time Division Multiplexing


Frequency Division Multiplexing

Let the signals we need to transmit working in frequency.

All signals are raised in frequency by n.’ such that ’ > . The extra is called guard band.

All signals are added


Frequency Division Multiplexing(Example)

The phone voice is limited to Hz frequency.

All signals are raised in frequency by n.’ with ’=4000 Hz (two 500Hz guard band)

All signals are added


Frequency Division Multiplexing

12

4000Hz

1 group (60 to 108KHz)

51 super group

51 mastergroup


Time Division Multiplexing

Works only for digital signals

Allocate time slots to every signal

Each signal has the exclusive use of the line for a given time slot


TDM (Example)

1200 bits/s

1200 bits/s

2400 bits/s


PCM : Transporting Digital Voice

A codec digitalizes 24 voice channels, sends-

receives 8 bits per channel,

If a standard voice sampling ratio (8000bps) is

used, the codec will perform 192bits every 125Msec

In fact it is 192bits+1checkbit=193bits/125Msec

What should be the data rate to transport the

samples of the 24 channel? 1.544Mbps (this is the

capacity of a T1 career)


A word on ISDN

Read section 2.5

Idea : integrated voice and digital service Voice channel 2 and more digital channels in

multiples of 8, 16, or 64 Kbps


Cellular Radio :the American system

Mobile phones: Two way system The AMPS was invented by Bell Labs in 1982 Based on cell architecture (see figure in next slide) Moving in the space… Security issues


Cellular Radio :the European system

The Global Systems for Mobile communications (GSM). The standard of digital mobile systems in Europe (and north africa)

Incompatible with USA and Japan

A new frequency (1.8GHz)

Uses FDM and TDM

Uses a smart card to store the serial number and phone number — » less vandalsime…

56Chapter 2CSC 3352 Computer Communications

Cellular Radio : Architecture

Public Switching Telephone System

A cellulaire area

MSCMSC


Conclusion

The Physical layer is the basis of all networks… 2 fundamental limits: the Nyquist limit and the Shannon limit… Guided media and unguided media… The telephone system is a major playor in this layer…ISDN and mobile links are the future…

the physical layer read chapter 2 csc 3352 computer communications chapter 22 putting the bits on...

Documents