29/09/04 ian/modules/COM342/COM342_L3.ppt L3/1/61 COM342 Networks and Data Communications Ian McCrumRoom 5B18 Tel: 90 366364 voice

29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/COM342_L3.ppt L3/1/61

COM342Networks and Data Communications

Ian McCrum Room 5B18

Tel: 90 366364 voice mail on 6th ring

Email: [email protected]

Web site: http://www.eej.ulst.ac.uk

Lecture 3: The Physical layer


Today physical media:

• Serial and Parallel connections

• Connectors

• Cables Coaxial, twisted pair

• Optical fibers

• Radio waves


Modes of serial data transfer• Simplex communications

– Unidirectional data path from transmitter to receiver in the manner of radio broadcasts

• Half Duplex– Unidirectional at any one time in the manner of a conversation

over radio link with change of direction signaled by ‘over’.

• Full Duplex– two computers using two comms channels one for transmission

and one for reception both working simultaneously.


Parallel data transfer• Most data in the form of bytes or wider.

– Transfer all of the bits at the same time however one conductor for each bit, more copper etc. suitable for short distances and very high data rates, used inside computer where groups of conductors are called busses .

– synchronisation between each bit on different conductors becomes difficult specially as distance increases due to tiny differences between conductors and their environment.

Start End

Transmission -->


Serial slower but cheaperSerial-to-parallel conversion

Control

UnitControl

Signals

Control

ControlSignals

Signals

Transmit Buffer Register

Transmit Data Register

Receive Data Register

Receive Buffer Register

Serial data out

Serial data in

Parallel data in

Parallel data link

Parallel data link

Parallel data out


Connectors and cables• Standards… often specify details

• D-type 25way used for RS232 serial links in old days (and in the “official standard”) Modern usage dictated by PC design … 9 pin D-type connector– consider computer- modem cable with straight through cable connecting

DTE and DCE. Necessary because uni-directional line drivers all that were available in the old days…

• RJ45– telephone type connectors.

• Ribbon Cables and IDC connectors

• Network connectors and cables


Cables for data transmission

T w is te d p a ir

Centre conductorDielectric

Electrostatic shieldingJacket

PVC/Teflon Braid Foil BraidInsulation

Tin-plated solid copper core


Typical Coaxial connection

Network interface

Transceiver circuit

10base2 Cable

Maximum cable length 200m

Terminator Terminator

T-piece ConnectorSegment


Benefits of coaxial and Twisted pair

• Shielding against induced noise.

• Common mode rejection.

• Speeds of each (cat 5e 100m bits/sec)


Twisted Pair

(a) Category 3 UTP.(b) Category 5 UTP.


Coaxial Cable

A coaxial cable.


Fiber Cables

(a) Side view of a single fiber.(b) End view of a sheath with three fibers.


Fiber Optic Networks

A fiber optic ring with active repeaters.


1. Stepped-index fibre. In this type of fibre, the core has a uniform refractive index throughout. This generally has a core diameter of to . This is a multi-mode fibre.

Stepped-index fibre

Fibre optic cable is available in three basic forms:


Graded-index fibre. In this type of fibre, the core has a refractive index that gradually decreases as the distance from the centre of the fibre increases. This generally has a core diameter of . This is a multi-mode fibre.

Graded-index fibre


Mono-mode fibre. As the name suggests, the distinguishing characteristic of this fibre is that allows only a single ray path. The radius of the core of this type of fibre is much less than that of the other two, however it does have a uniform refractive index.

From, 1 to 3, we find that the cost of production increases, the complexity of transmitter and receiver increases, while the dispersion decreases. This latter property change means that the mono-fibre also has the potential to provide greater bandwidth. As it becomes cheaper to produce mono-mode fibre technology, we will see an increased use of this type of optical fibre


Fiber Optics

(a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.

(b) Light trapped by total internal reflection.


Transmission of Light through Fiber

Attenuation of light through fiber in the infrared region.


Fiber Cables

A comparison of semiconductor diodes and LEDs as light sources.


Optical fibre is a waveguide. The fibre (in its simplest form) consists of a core of glass of one refractive index, and a cladding of a slightly lower refractive index (Figure ). The fibre is then surrounded by a refractive sheath. Typical fibre dimensions are to diameter.

The basic structure of a fibre optic waveguide

http://www.cs.ucl.ac.uk/staff/S.Bhatti/D51-notes/node21.html#figfibre

http://www.cs.ucl.ac.uk/staff/S.Bhatti/D51-notes/node21.html


In simple terms, the action of a waveguide can be partially understood by considering the rays down the fibre. A light-wave entering the fibre is either refracted into the cladding, and attenuated, or is totally internally reflected at the core/cladding boundary. In this manner it travels along the length of the fibre. The maximum angle at which it may enter the guide and travel by total internal reflection is termed the acceptance angle It is also possible for the wave to follow a helical path down the guide. These rays are called skew-rays.


However, this view is too simple to explain all features of waveguide behaviour. In fact, it is not possible for the wave to take any ray down the guide. Only certain rays can be taken. These rays are called modes. For any particular frequency, there is a different ray. The modal action of a waveguide is a consequence of the wave nature of the radiation. A mono-mode fibre is a fibre that only has one acceptable ray-path per frequency. A multi-mode fibre has a number of possible rays that light of a particular frequency may take.


1 1

2

1 2 n

y

1Sin

2Sin=

1

2

n

n

1n

2n

1n

2n

Snell’s Law


1 1

2

1 2 n

y

1n

2n 2n

1n

as

then

1

2

2

1

n

n

Sin

Sin

1

2

2

1

n

n

Cos

Cos

12

Total Internal Reflection


From the diagram n1 is greater than n2

21so decreases as decreases

until as 21 , 02

for a finite value of 1 .

1 is now the critical angle cbeyond which Total Internal Reflectionoccurs and

1

21

n

nCosc


y

n 1 2

1n1n

2n2n

c

m

Lig

ht A

ccep

tanc

e co

ne

1

21

n

nCoscas

then

cm Sinn

Sin 1

1

when Snell is applied therefore the light acceptance cone is m2


1n

2n

cl

c

a

Propagation of light by total internal refection

See attenuation profile Fig 2.6 A.T. and then

Fig 2.7 for fibre construction


Copper v F.O.

• repeaters 5km

• reactive

• E.M. R.F. problems

• bulky

• tappable

• repeaters 30km• relatively inert• no E.M. R.F.

problems• >bandwidth in

duct• no tapping


Wireless Tx

• Wavelength* frequency = speed of light• therefore Atlantic 252 where the 252

refers to the frequency in kilohertz .. leads to the wavelength being 1190m long where the speed of light is taken to be 300,000,000 m/s

c

f



Variety see Fig 2.11 for spectrum

• Radio VLF,LW,MW 9kHz bandwidth, long dist, earth hugging Fig 2.12

• Radio HF,VHF various bandwidths, straight lines and ionosphere bounce up to 60MHz

• Microwave line of sight, large bandwidths (418MHz)

• Infra Red line of sight, good for LAN in rooms

• Light - building to building good bandwidth Fig 2.13





Communication Satellites• Geostationary Satellites

• Medium-Earth Orbit Satellites

• Low-Earth Orbit Satellites

• Satellites versus Fiber


Communication Satellites

Communication satellites and some of their properties, including altitude above the earth, round-trip delay time

and number of satellites needed for global coverage.


Communication Satellites (2)

The principal satellite bands.


Communication Satellites (3)VSATs using a hub.


Globalstar

(a) Relaying in space.

(b) Relaying on the ground.



Telephone system for data comms:

• Why telephone system for data

communications

• Structure of PSTN

• How it can carry digital data


Public Switched Telephone Network

• It exists everywhere and is relatively cheap to establish contact

• It is slow and error prone.• It is improving rapidly and costs are falling• allows access for many home users to

Internet and enables home working.• Vast investment• Relies on Circuit switching


PSTN Structure

• pairs of handsets therefore a conductor per pair, n houses implied n conductors! Fig2.14a

• first manual centralised switching office with jumpers being placed by operators Fig2.14b

• the interconnection of switching offices(cities) led to the same problem one conductor per office pair same problems as fig 2.14a

• hierarchy developed as in fig 2.14c



PSTN Structure Fig 2.15

• Subscriber linked to local exchange by local loop by a pair of copper wires, distance can be small or up to many kilometres.

• thus a local call is switched with the local exchange.• Local exchanges are connected by trunk lines in an

ascending hierarchy.• medium and long distance calls are carried on multiplexed

high bandwidth links and managed through switching higher up the hierarchy.

• International connections demand interfaces and standardisation


Transmission• Local loops consist of twisted pairs and signalling is

analogue.• trunks are higher bandwidth and employ co-

axial(ageing), microwave and fibre optics. This uses multiplexing for analogue(ageing) and digital signals.

• Amplification of an analogue signal can also amplify the noise arising as it propagates thus noise can predominate over a long connection.

• Amplification of a digital signal is merely the regeneration of the original digital signal, thus only noise is that which was originally present.



Digital v Analogue• Digital -Predictable attenuation therefore regenerators can be

reliably sited to restore the signal to either 0 or 1, therefore no loss of signal even over long distances c.f. international telephone calls.

• Analogue amplification is imperfect and cumulative over long distances.

• Many sources can produce digital signals using the same connections

• Data rates are increasing

• digital is cheaper

• digital more readily maintained.


Transmission and reception

• Attenuation, loss in signal strength, increases as a proportion to the length of conductor. dB/km. varies with wavelength distorts wave shape.

• delay distortion also varies with wavelength, overlaps different bits, can limit bandwidth.

• noise, random and burst.

• crosstalk


Modem

• MOdulator DEModulator

• Change a wave in such a manner that the changes represent another signal

• recognise the changes in the received wave and deduce what the modulating signal was.

• falling prices.

• high speeds.


Modulation techniques• amplitude modulation

• frequency modulation

• phase modulation frequency shift keying

• combination Quadrature amplitude modulation QAM

• constellation patterns upto 64 points for 6 bits per baud

• compression (more later)

• echos supression and cancellation

• full and half duplex

• in-band signalling.


Signal

Energy Distribution forHuman Speech

O Hz 300 Hz ~3,400 Hz 20 kHz

Bypass Filter


Modems

(a) A binary signal

(b) Amplitude modulation

(c) Frequency modulation(d) Phase modulation





Modems (2)

(a) QPSK.(b) QAM-16.(c) QAM-64.


Modems (3)

(a) V.32 for 9600 bps.

(b) V32 bis for 14,400 bps.

(a) (b)


Trunk Line Speed

SONET/SDH*

OC3/STM1 156 Mbps

OC12/STM4 622 Mbps

OC48/STM16 2.5 Gbps

OC192/STM64 10 Gbps

OC768/STM256 40 Gbps

speeds are multiples of 51.84 Mbps


SONET/SDH Ring

SONET/SDH Ring

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

2.Rings Can BeWrapped if a

Trunk lineIs Broken.

Still a CompleteLoop.

1. Normally, One Ring is Used in Each Ring

Break


Digital Subscriber Lines

Bandwidth versus distanced over category 3 UTP for DSL.

Documents

29/09/04 ian/modules/COM342/COM342_L3.ppt L3/1/61 COM342 Networks and Data Communications Ian McCrumRoom 5B18 Tel: 90 366364 voice