Network Media. Copper, Optical, Fibre(Physical Layer Technologies)

Introduction to Computer Networking

Last Week :

Many devices exist for networking Hubs, switches, routers, modems, cables All have a bandwidth Measured in bps Mbps Gbps etc. Throughput is moderated by real world Layered models help to understand complex

communications structures

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This week :

We shall examine network hardware

– Physical layer types– Copper– Wireless– Optical– Electricity

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Computer Networks

These are formed from nodes interconnected with cabling

Nodes (computers generally) communicate via the cabling

Sometimes cables are not used– Radio, microwave, satellite links– different bandwidths

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Twisted Pair

This is insulated copper wires twisted together

Used extensively in LANs ( Cat 5 ) Short distance runs can carry 100 Megabits

per second Longer distances carry lower data rates Picks up interference

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RJ45 Connectors

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Coaxial Cable

Consists of two conductors, an inner cable surrounded by a dielectric

Meshed wire shielding covers the dielectric Supports high data rate Does not pick up interference easily

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Coaxial Cable

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Fibre Optic

This is strands of glass used to carry signals in the form of light pulses

Supports extremely high data rates – greater than 6 Tbits per second

Expensive & hard to join Low attenuation

– Great distances may be covered– transatlantic links possible

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Total Internal Reflection

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Fibre Optic Cabling

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Unguided Transmission

The previous methods were guided, i.e. followed a cable

Unguided methods– Radio– Terrestrial Microwave– Satellite microwave

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Radio

Used by mobile telephones, wireless LANs, local links

Sends signals using electromagnetic means Susceptible to interference

– frequency hopping helps here Cheaper because there is no need for long

cables Wireless LANs now exist

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Microwave

Highly directional, high frequency radio beams

Uses parabolic reflectors in line-of-sight positions– Often on towers or rooftops

Frequencies over 10 GHz are attenuated by rainfall – Loss of signal power

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Volts, Current, Resistance

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•Current flows in closed loopsCurrent flows in closed loops

•Circuit must be formed from conductors Circuit must be formed from conductors e.g. coppere.g. copper

•Voltage causes current to flowVoltage causes current to flow

•Resistance and impedance resist flow of Resistance and impedance resist flow of electronselectrons

Water Analogy

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•Higher water level causes greater flowHigher water level causes greater flow

•Water is analogous to electronsWater is analogous to electrons

Oscilloscopes

Used to measure and observe current flow Sophisticated tool - expensive

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Analogue and Digital Signals

Analogue is “wavy”, can have ANY valueSeen in natureAmplitude, frequency, time period used

to measure wavesDigital has discrete valuesSeen in computers etc.

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Electrical Circuits

Complete loop must exist Volts push electrons round circuit

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Ground or Earth

Connection to our planet for safety purposes Can also mean reference point, the 0 volts

level Used when making electrical measurements Voltage is measured between 2 points in a

circuit

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Grounding Equipment

Case of equipment is connected to earth for YOUR safety

If case becomes live due to a fault, easy path provided for electrons to flow

Your body resists flow more than earth connection

Most electricity flows to earth Should trip breaker or blow fuse

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Signals in time domain

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Propagation = spreading

Electricity travels approximately 2/3 speed of light

~2 x 108 metres per second Finite time for signals to travel Must be considered especially in high data-

rate systems

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Dispersion, Jitter, Latency

Dispersion is spreading out of a pulse – limits data rate

Jitter is uncertainty of arrival time of a signal

Latency is delay due to signal propagation, repeaters, switches etc.

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Attenuation, Reflection

Attenuation is weakening of signal as it crosses a medium

Due to resistance and impedance Reflection occurs when impedances are not

matched Some of signal reflected back along cable or

fibre Can damage equipment or corrupt data

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Noise

Unwanted elements introduced into signal

Has many origins – thermal noise; AC power noise; crosstalk; EMI/ RFI

Twisting cables and shielding limits EMI/ RFI. Keep cables short where possible

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Conclusion

3 Types of physical layer channels Wire- copper Wireless Fibre optic Volts push the electrons Measure with oscilloscope or meter Noise, jitter, attenuation, latency affect Tx

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