Computer Networks

Chapter 7 – Transmission Media

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Transmission Media Categories

The transmission medium is the physical path between the transmitter and receiver in a data transmission system

The nature of both, the signal and the medium determines the quality of transmission

The media can be divided into two categories: Guided media – physical medium exists Unguided media – the air is used as a

medium

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Twisted-Pair Cable

Insulated copper wires in spiral pattern. Widely used for analog and digital transmission One of the wires transmits the signal, the other is used as

ground reference The twist is introduced to reduce the interference

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Twisted Pair Cable - Applications

Most common medium for many applications Telephone network Between house and local exchange

(subscriber loop) Within buildings To private branch exchange (PBX) For local area computer networks, 10Mbps or

100Mbps, or 1000Mbps

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Unshielded vs. Shielded Twisted Pair Cable

Unshielded Twisted Pair (UTP) Ordinary telephone wire Cheapest Easiest to install Suffers from external electrical and

mechanical interference Shielded Twisted Pair (STP)

Metal braid or sheathing that reduces interference

More expensive Harder to handle (thick, heavy)

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UTP Categories

Category 3 up to 16MHz Voice grade found in most offices Used with 10BaseT, IBM Token ring; Arc Net

Category 4 up to 20 MHz, the use is same as Cat 3

Category 5 up to 100MHz Commonly pre-installed in new office buildings Used with 10BaseT, Fast Ethernet, Gigabit

Ethernet, ATM

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UTP Categories-cont.

Category 5E (enhanced) Up to 100MHz, similar use as Cat 5,

Category 6 Up to 250 MHz, similar use as Cat 5 Lower attenuation and longer distances

than Cat. 5 Category 6E – enhanced Cat 6 Category 7 (draft)

Up to 600 MHz Used for high speed transmissions

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UTP Connectors

Standard – RJ45 Can be inserted in

only one way Easy to manipulate

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Performance of UTP

The attenuation depends on how thick the conductors are and the frequency at which is used

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Coaxial Cable

Central core conductor, enclosed within an insulator sheath which is encased by an outer conductor covered by outer sheath.

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Coaxial Cable - Applications

Television distribution Cable TV (RG-59) Lately, only the last part is kept, the rest is

replaced by fiber Long distance telephone transmission

Can carry 10,000 voice calls simultaneously Being replaced by fiber optic

Short distance computer systems links Local area networks, 10Base2 (RG-58),

10Base5 (RG-11) Obsolete (rarely used today)

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Coaxial Cable - Connectors

The most common connectors used with coaxial cable are BNC connectors Ordinary BNC

connector to connect a single wire

T BNC connector – to connect two wires

BNC terminator – to terminate the end of the wire

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Coaxial Cable - Performance

Analog Amplifiers every few km Closer if higher frequency Up to 500MHz

Digital Repeaters every 1km Closer for higher data rates

The performance depends on the diameter of the cable and the frequency used

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Fiber-Optic Cable

Consists of three components: the light source (laser or light emiting diode) the medium (ultra-thin fiber of glass) the detector (generates electric pulse when

light falls on it) Light pulses sent down a fiber spread out

in length as they propagate. The attenuation of light through glass

depends on the wavelength of the light (0.85, 1.30 and 1.55 micron are used for communication)

Wavelength l = c/f , c is the speed of light

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Fiber-Optic Cable - Structure

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Optical Fiber - Transmission Characteristics

Act as wave guide for 1014 to 1015 Hz Portions of infrared and visible spectrum

Light Emitting Diode (LED) Cheaper Wider operating temperature range Last longer

Injection Laser Diode (ILD) More efficient Greater data rate

Wavelength Division Multiplexing

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Wavelength Division Multiplex (WDM)

Fiber 1 spectrum

Fiber 2 spectrum

Spectrum on the shared fiber

Shared fiber

Fiber 1

Fiber 2Fiber 3

Fiber 4

Prism Prism

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Fiber-Optic Cable – Propagation Modes

The density of the core remains constant from the center to the edges

The density of the core varies from the center to the edges

Uses step-index fiber and highly focused source of light

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Fiber-Optic Cable - Connectors

Common connectors

ST- used in cable TV

SC – used in computer networks

MT-RJ – a new connector with a size of RJ-45

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Fiber-Optic Cable - Characteristics

Advantages Greater capacity (data rates of hundreds of Gbps Smaller size & weight (easier to put in the ground than

cooper cables) Lower attenuation Electromagnetic isolation (not susceptible to electric

interference) Greater repeater spacing (10s of km at least)

Disadvantages High cost Difficult and expensive to install and maintain Light is unidirectional – one cable needed for each

direction

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Fiber –Optic Cable - Applications

Used with wavelength division multiplex (WDM) for long distance transmission of voice channels and data signals

Cable TV Local Area Networks, 100Base FX,

(Fast Ethernet) and 1000Base X (Gigabit Ethernet)

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Electromagnetic Waves in the Air

Besides through guided media, electromagnetic waves can spread through the atmosphere and outer space

Hz 104 108 1012 1016 1022

RadioMicrowaveInfrared UV X-ray Gamma ray

Visible light

Frequency spectrum of electromagnetic waves

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Wireless Transmission

Wireless transmission is used in all types of Wireless communication Mobile devices Satellite communication

The frequencies used by the signal and the power of the signal are most important for this type of transmission Frequencies 3KHz to 1 GHz are usually

called radio waves Frequencies between 1 and 300 GHz are

called microwaves

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Antennas Antennas are used for both, transmission

and reception of wireless signals To exchange information the antennas need to

be tuned to the same frequency

Two types of antennas Omnidirectional Directional

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Wireless Spectrum

Radio waves Micro waves

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Problems with Wireless Transmission

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Radio Waves

At low frequency, radio waves pass through obstacles well, but the power falls off sharply with distance (AM radio)

At high frequency, radio waves tend to travel in streight lines and bounce off obstacles

At all frequencies radio waves are subject to inerference from electrical equipment

The governments license the users of radio transmitters

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Radio Waves (cont.)

Radio waves are omnidirectional Signal spreads in all directions Can be received by many antennae Convenient for broadcasting

Frequencies used 30MHz to 1GHz

Applications Radio, Television and Paging systems

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Microwaves

Microwaves are unidirectional Focused beam Careful alignment required

Frequencies used 2GHz to 40GHz

Applications Wireless LANs, Satellite communication

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Infrared Transmission

A short range communication system – one room

Line of sight must be provided Frequencies used

3 x 1011 to 2 x 1014 Hz Application

PC-to-PC short range transmission

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Spread Spectrum

A type of wireless transmission in which signals are distributed over several frequencies simultaneously Developed to provide secure wireless

transmission (for military purposes) Used in wireless LAN to reduce

propagation effects (multi-path interference and others due to the higher frequencies)

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Satellite Microwave

Microwave onto which the data is modulated is transmitted to the satelite from the ground

Satellite receives on one frequency, amplifies or repeats the signal and transmits it back to earth using on board circuit known as transponder.

A typical satelite channel has extremely high bandwidth (500 MHz)

Satelites for communication purposes require geo-stationary orbit (Height of 35,784km)

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Satellites

A single satellite usually contains multiple transponders (typically 6-12)

Each transponder consists of a radio receiver and transmitter and uses a different radio frequency (i.e., channel) Multiple communications can proceed simultaneously

and independently

The degree of collimation of the microwave beem can be: coarse, so that the signal can be picked in a large

geografic area focused, so that it can be picked up over a limited

area

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Geosynchronous Satellites

Place in an orbit that is exactly synchronized with the rotation of the earth Geostationary Earth Orbit (GEO)

Distance required for geosynchronous orbit is 36,000 km or 20,000 miles.

The entire 360-degree circle above the equator can only hold 45-90 satellites. This is because satellites need to be

separated to avoid interference

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Network Connection accross an Ocean via Satelite

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Application of Satelites

Television Long distance telephone Private business networks Internet when there is no other

connection

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Satelites vs. Fiber

Satelite advantages: Sites that are not connected can easily use

the satelite by installing a ground station

Satelite disadvantages: Very large propagation delay (due to big

distances) Very low security Quality of transmission can become

questionable due to external influences