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 2012ITCN 2

This week : We shall examine network hardware – Physical layer types – Copper – Wireless – Optical – Electricity 1 November 2012ITCN 3

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 1 November 2012ITCN 4

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 1 November 2012ITCN 5

RJ45 Connectors 1 November 2012ITCN 6

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 1 November 2012ITCN 7

Coaxial Cable 1 November 2012ITCN 8

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 1 November 2012ITCN 9

Total Internal Reflection 1 November 2012ITCN 10

Fibre Optic Cabling 1 November 2012ITCN 11

Unguided Transmission The previous methods were guided, i.e. followed a cable Unguided methods – Radio – Terrestrial Microwave – Satellite microwave 1 November 2012ITCN 12

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 1 November 2012ITCN 13

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 1 November 2012ITCN 14

Volts, Current, Resistance 1 November 2012ITCN 15 Current flows in closed loopsCurrent flows in closed loops Circuit must be formed from conductors e.g. copperCircuit must be formed from conductors e.g. copper Voltage causes current to flowVoltage causes current to flow Resistance and impedance resist flow of electronsResistance and impedance resist flow of electrons

Water Analogy 1 November 2012ITCN 16 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 1 November 2012ITCN 17

Analogue and Digital Signals Analogue is “wavy”, can have ANY value Seen in nature Amplitude, frequency, time period used to measure waves Digital has discrete values Seen in computers etc. 1 November 2012ITCN 18

Electrical Circuits Complete loop must exist Volts push electrons round circuit 1 November 2012ITCN 19

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 1 November 2012ITCN 20

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 1 November 2012ITCN 21

Signals in time domain 1 November 2012ITCN 22

Propagation = spreading Electricity travels approximately 2/3 speed of light ~2 x 10 8 metres per second Finite time for signals to travel Must be considered especially in high data- rate systems 1 November 2012ITCN 23

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. 1 November 2012ITCN 24

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 1 November 2012ITCN 25

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 1 November 2012ITCN 26

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 1 November 2012ITCN 27