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COM342 Networks and Data Communications. Lecture 3: The Physical layer. Ian McCrum Room 5B18 Tel: 90 366364 voice mail on 6 th ring Email: [email protected] Web site: Today physical media:. Serial and Parallel connections Connectors

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com342 networks and data communications

COM342Networks and Data Communications

Lecture 3: The Physical layer

Ian McCrum Room 5B18

Tel: 90 366364 voice mail on 6th ring

Email: [email protected]

Web site:

today physical media
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.


Serial slower but cheaper


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


Typical Coaxial connection

benefits of coaxial and twisted pair
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.


Fibre optic cable is available in three basic forms:

  • 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


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


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.





Snell’s Law


1 2 n




Total Internal Reflection


1 2 n


From the diagram n1 is greater than n2


decreases as



until as

for a finite value of


is now the critical angle

beyond which Total Internal Reflection

occurs and



Light Acceptance cone

1 2




when Snell is applied therefore the light

acceptance cone is


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
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
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

variety see fig 2 11 for spectrum
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
Communication Satellites
  • Geostationary Satellites
  • Medium-Earth Orbit Satellites
  • Low-Earth Orbit Satellites
  • Satellites versus Fiber

communication satellites1
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
Communication Satellites (2)

The principal satellite bands.

communication satellites 3
Communication Satellites (3)

VSATs using a hub.


(a) Relaying in space.

(b) Relaying on the ground.

telephone system for data comms
Telephone system for data comms:
  • Why telephone system for data communications
  • Structure of PSTN
  • How it can carry digital data

public switched telephone network
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
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
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

  • 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 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
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

  • 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
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.



Energy Distribution for

Human Speech

~3,400 Hz

20 kHz

O Hz

300 Hz

Bypass Filter

(a) A binary signal

(b) Amplitude modulation

(c) Frequency modulation

(d) Phase modulation


modems 2
Modems (2)

(a) QPSK.

(b) QAM-16.

(c) QAM-64.

modems 3
Modems (3)

(a) V.32 for 9600 bps.

(b) V32 bis for 14,400 bps.




Trunk Line




156 Mbps


622 Mbps


2.5 Gbps


10 Gbps


40 Gbps

speeds are multiples of 51.84 Mbps


1. Normally, One Ring is Used in Each Ring







Rings Can Be

Wrapped if a

Trunk line

Is Broken.

Still a Complete






digital subscriber lines
Digital Subscriber Lines

Bandwidth versus distanced over category 3 UTP for DSL.