Network Media. Copper, Optical, Fibre (Physical Layer Technologies) Introduction to Computer...
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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
1 November 2012 ITCN2
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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