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Lecture #8: Physical Basics Lecture #8: Physical Basics of Telecommunications.of Telecommunications.

C o n t e n t s C o n t e n t s Mathematical Results in SignalingMathematical Results in Signaling Physical Layer FunctionsPhysical Layer Functions Guided Transmission Media:Guided Transmission Media:

Electric Signal WiresElectric Signal Wires Light TransmissionLight Transmission

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Mathematical Results in Mathematical Results in SignalingSignaling

Signals’ presentation as periodical function of Signals’ presentation as periodical function of time: time: gg((tt). Period ). Period TT and frequency and frequency ff..

FourierFourier transform transform - a constant + endless - a constant + endless sum of sum of sinsin and and coscos expressions (harmonics): expressions (harmonics): derivation of derivation of sinsin coefficients coefficients derivation of derivation of coscos-coefficients -coefficients derivation of constantderivation of constant coefficientcoefficient

Power losses in data transmission.Power losses in data transmission. Selective harmonics’ amplitude deminissionSelective harmonics’ amplitude deminission

transmission subsidingtransmission subsiding filteringfiltering

Bandwidth - frequency interval of harmonics Bandwidth - frequency interval of harmonics (frequency components) in which the signal (frequency components) in which the signal is transmittedis transmitted

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Mathematical Results Mathematical Results in Signalingin Signaling

Impact of the number of harmonics on the Impact of the number of harmonics on the

transmitted signal shape.transmitted signal shape.

Boud rate and bit rateBoud rate and bit rate

Data rates and number of harmonics Data rates and number of harmonics

– 9.6 kb/s9.6 kb/s 2 harmonics2 harmonics

– 38.4 kb/s38.4 kb/s 0 harmonics0 harmonics (no transmission of (no transmission of

binary signal by phone twisted pair, that has a binary signal by phone twisted pair, that has a

cut-off at 3 kHz) cut-off at 3 kHz)

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Mathematical Results in Mathematical Results in SignalingSignaling

Maximum data rate of a Maximum data rate of a noiselessnoiseless channel channel – Nyquist’s theoremNyquist’s theorem: arbitrary signal passing : arbitrary signal passing

filter with frequency bandwidth filter with frequency bandwidth H H can be can be reconstructed by 2reconstructed by 2HH observations per second. observations per second. Faster observations are pointless, because higher Faster observations are pointless, because higher frequencies (>frequencies (>HH) are filtered. And vice-versa: ) are filtered. And vice-versa:

MAX(Data Rate) = 2MAX(Data Rate) = 2HH b/S b/S for two-level (i.e. for two-level (i.e. binary) signal.binary) signal.

MAX(Data Rate) = 2MAX(Data Rate) = 2H H loglog22VV for for VV-level discrete -level discrete signal.signal.

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Mathematical Results Mathematical Results in Signalingin Signaling

Maximum data rate of a Maximum data rate of a noisynoisy channel channel – Signal/Noise Ratio Signal/Noise Ratio RRSNSN==SS//NN ( (SS - signal power; - signal power;

NN - noise power) or usually - noise power) or usually

RRSN SN = 10 log= 10 log1010 S S //N N dBdBS/N=10S/N=10 S/N=100S/N=100 S/N=1000S/N=1000

RRSN SN =10dB=10dB RRSN SN =20dB=20dB RRSN SN =30dB=30dB

– Shannon’s theoremShannon’s theorem: arbitrary signal : arbitrary signal passing filter with frequency bandwidth passing filter with frequency bandwidth H H and signal-noise-ratio and signal-noise-ratio RRSNSN has has

MAX(Data Rate) = MAX(Data Rate) = H H loglog22(1(1+S/N+S/N)) b/Sb/S

Phone lines: Phone lines: HH=3000, =3000, RRSN SN =30 dB =30 dB

MAX(Data Rate) MAX(Data Rate) 30 kb/S 30 kb/S

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Physical LayerPhysical Layer The physical layer provides mechanical, The physical layer provides mechanical,

electrical, functional and procedural means electrical, functional and procedural means to activate, maintain and deactivate physical-to activate, maintain and deactivate physical-connections for bit transmission between connections for bit transmission between data link entities. data link entities.

Physical layer entities are interconnected by Physical layer entities are interconnected by means of a physical medium.means of a physical medium.

Data-circuitData-circuit

A communication path in the physical media for A communication path in the physical media for OSI between two physical entities, together OSI between two physical entities, together with the facilities necessary in the physical with the facilities necessary in the physical layer for the transmission of bits on it.layer for the transmission of bits on it.

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Physical layerPhysical layerServices provided to the data link layerServices provided to the data link layer

–physical-connections;physical-connections;–physical-service-data-units;physical-service-data-units;–physical-connection-endpoints;physical-connection-endpoints;–data-circuit identification;data-circuit identification;–sequencing;sequencing;– fault condition notification; andfault condition notification; and–quality of service parameters.quality of service parameters.

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Guided Transmission MediaGuided Transmission Media Parameters of the transmission Parameters of the transmission

media:media: bandwidthbandwidth costcost delaydelay carry distance carry distance support devicessupport devices general supportgeneral support durability, noise protectiondurability, noise protection spread and popularityspread and popularity

Guided media:Guided media: conductor wiresconductor wires fiber opticsfiber optics

Unguided media:Unguided media: radio wavesradio waves LASER rays LASER rays

Light Amplification by Stimulated Emis-sion of Radiation

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Twisted PairTwisted Pair Pair of conductor wires that are helical Pair of conductor wires that are helical

twistedtwisted Reduction of the interference and induction Reduction of the interference and induction

between neighbor pairsbetween neighbor pairs bandwidth: up to 1 Gb/S (not in phone lines)bandwidth: up to 1 Gb/S (not in phone lines) cost: lowcost: low delay: delay: carry distance: carry distance: 101022 -10 -104 4 m without amplificationm without amplification support devices: support devices: analog and digital transmissionanalog and digital transmission general support: general support: durability, noise protection : durability, noise protection : spread and popularity: phone systems (POTS spread and popularity: phone systems (POTS

- Plain Old Telephone Service)- Plain Old Telephone Service)

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Baseband Coaxial CableBaseband Coaxial Cable StructureStructure Impedance 50 Impedance 50 ; other properties:; other properties:

bandwidth: up to 10 Mb/Sbandwidth: up to 10 Mb/S cost: lowcost: low delay: delay: carry distance: carry distance: 101022 -10 -1044m without m without

amplificationamplification support devices: support devices: analog and digital analog and digital

transmissiontransmission general support: general support: noise protection: better than twisted noise protection: better than twisted

pair pair spread and popularity: LAN, cable spread and popularity: LAN, cable

TVTV

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Broadband Coaxial Broadband Coaxial CableCable The bandwidth up to The bandwidth up to 300-500 MHz300-500 MHz - for - for

analog transmission and digital data modems analog transmission and digital data modems on both ends.on both ends.

Modems allow transmission of >1b/S for each Modems allow transmission of >1b/S for each 1Hz of the bandwidth or 1Hz of the bandwidth or

The cable bandwidth is split into multiple The cable bandwidth is split into multiple 6 6 MHzMHz channels channels

Larger areas need analog amplification that Larger areas need analog amplification that defines transmission direction (as the defines transmission direction (as the amplifier has input and output) amplifier has input and output)

Bi-directionalBi-directional transmission needs: transmission needs: dual cable connection between both endsdual cable connection between both endssingle cable and frequency splittingsingle cable and frequency splitting

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Broadband Coaxial Broadband Coaxial CableCable

Frequency splitting in single cableFrequency splitting in single cable– Subsplit system: 5-30 MHz for input signal Subsplit system: 5-30 MHz for input signal

and 40-300 MHz outputand 40-300 MHz output– Midsplit system: 5-116 MHz for input Midsplit system: 5-116 MHz for input

signal and 168-300 MHz outputsignal and 168-300 MHz output Parameters:Parameters:

bandwidth: up to bandwidth: up to 10 Mb/S10 Mb/S cost: lower than baseband coaxcost: lower than baseband coax carry distance: carry distance: 101044 -10 -105 5 mm without without

amplification (analog signal)amplification (analog signal) support devices: analog and digital support devices: analog and digital

transmissiontransmission noise protection: better than twisted pair, noise protection: better than twisted pair,

worse than baseband coaxworse than baseband coax spread and popularity: cable TV (widely spread and popularity: cable TV (widely

installed), perspective for installed), perspective for MANMANss

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Transmission Media - Transmission Media - Fiber OpticsFiber Optics

Evolution of the system speed scissors:Evolution of the system speed scissors: early computer age: bottleneck is in early computer age: bottleneck is in

intercomputer communications: data intercomputer communications: data processing is faster than data transmissionprocessing is faster than data transmission

mature computer age: bottleneck shifted mature computer age: bottleneck shifted to data processing as the communications to data processing as the communications became fasterbecame faster

ExampleExample: fiber optics transmits more than : fiber optics transmits more than 100 100 Tb/STb/S but the converters between electrical and but the converters between electrical and optical signals limit the speed to optical signals limit the speed to 10 Gb/S10 Gb/S..

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Fiber OpticsFiber Optics Optical transmission system:Optical transmission system:

– light sourcelight source– light transmission media light transmission media – light detectorlight detector

Light/electricity conversion is done by the light Light/electricity conversion is done by the light source and detector: light pulse codes “1” and source and detector: light pulse codes “1” and generates electrical pulse in th detectorgenerates electrical pulse in th detector

Media: Media: glass fiberglass fiber (variant “ (variant “fibrefibre”) - ”) - unidirectionalunidirectional transmission (direction determined transmission (direction determined by the positions of the source and the detector)by the positions of the source and the detector)

Physical ground of the light transmission:Physical ground of the light transmission:total internal reflectiontotal internal reflection; reflection angle, ; reflection angle, boundary refraction; single- and multi-mode boundary refraction; single- and multi-mode fibers.fibers. 2/5

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Fiber OpticsFiber Optics

Glass transparency equals that of the clear airGlass transparency equals that of the clear air Light attenuation Light attenuation AA[dB] per linear km[dB] per linear km,,

AA(()) diagram, transmission bands: diagram, transmission bands:– 0.850.85mm-6-6((m):m): A A=0.8 i.e. 85% transmission/km=0.8 i.e. 85% transmission/km– 1.3 m1.3 m-6 -6 :: A A=0.2 i.e. 95% transmission/km =0.2 i.e. 95% transmission/km – 1.85 m1.85 m-6-6 : : A A=0.18 i.e. 96% transmission/km=0.18 i.e. 96% transmission/km– bandwidth 30 THz for the three bandsbandwidth 30 THz for the three bands..

A single soliton nonlinear Schrodinger surface

GaAscrystal

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% of% oftransmissiontransmission

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A[dB]A[dB] 2020 1010 77 5.25.2 44 33 2.22.2 1.51.5 0.950.95 0.450.45 00

Light pulses’ shape: solitons (a solitary wave Light pulses’ shape: solitons (a solitary wave that propagates with that propagates with little loss of energylittle loss of energy and and retains its shape and speed after collidingretains its shape and speed after colliding with another such wave)with another such wave)

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Fiber CablesFiber Cables

Single core (Single core ( 50 m 50 m-6-6) cables and ) cables and Multiple core cables (Multiple core cables ( 10 m 10 m-6-6) ) Cable Interconnections:Cable Interconnections:

– terminating connectors with fiber plugs - 20% light terminating connectors with fiber plugs - 20% light

losses losses

– mechanical junction - 5% light losses, personnelmechanical junction - 5% light losses, personnel

– termofusing - less than 1% losses, special termofusing - less than 1% losses, special

equipmentequipment

Light sources: Light sources: LEDLEDs or s or crystal laserscrystal lasers Light sensors: Light sensors: photo-diodesphoto-diodes

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Fiber Optic NetworksFiber Optic Networks Networks based on fiber optic connections Networks based on fiber optic connections

can cower the range between LANs and WANscan cower the range between LANs and WANs Topology always based on Topology always based on point-to-pointpoint-to-point

connections e.g. ring with T-connector for connections e.g. ring with T-connector for each node:each node:– passive interfacepassive interface: main light conductor (fiber) and : main light conductor (fiber) and

LED/photodiode junctions for each station; high LED/photodiode junctions for each station; high reliability, short distance, restricted number of reliability, short distance, restricted number of computers in the networkcomputers in the network

– active interfaceactive interface: main light fiber has a break at : main light fiber has a break at each station and the signal is regenerated by the each station and the signal is regenerated by the repeater; repeaters are electrical (wired interface repeater; repeaters are electrical (wired interface to the computer) or optic (fiber interface to to the computer) or optic (fiber interface to computer); reliability depends on the junctions, computer); reliability depends on the junctions, unrestricted size in length and stations number, unrestricted size in length and stations number, long interstation distance (km) long interstation distance (km)

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Fiber Optic NetworksFiber Optic Networks

Passive star topologyPassive star topology - modified ring with - modified ring with fiber interface to computer. The passive fiber interface to computer. The passive star is the point where every light pulse star is the point where every light pulse of the incoming fibers illuminates any of of the incoming fibers illuminates any of the outgoing fibers to the computers. the outgoing fibers to the computers.

Its properties resembles those of passive Its properties resembles those of passive ring topology (limited distance and ring topology (limited distance and number of stations and independent number of stations and independent reliability to the state of each station)reliability to the state of each station)

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Fiber vs. Wire Fiber vs. Wire Advantages:Advantages:

Lower attenuationLower attenuation

Wider bandwidth Wider bandwidth

No interference between the No interference between the

lineslines

Power independence throughout Power independence throughout

the routethe route

Better protection & security Better protection & security

against tapsagainst taps

Lighter weight, non corrosiveLighter weight, non corrosive

Lower installation cost for new Lower installation cost for new

routersrouters

Drawbacks:Drawbacks:

Unidirectional Unidirectional

transmission transmission

doubled doubled

conductors or conductors or

occupied bandsoccupied bands

More expensive More expensive

interfacesinterfaces

Requires Requires

additional staff additional staff

qualificationqualification

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21

Average harmonic frequency

For 3kHz bandwidth

Bandwidth /

Average harmo-

nic frequency =

Number of har-

monics sent

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Twisted pairTwisted pair

(a)(a) Category 3 UTP. Category 3 UTP.(b)(b) Category 5 UTP. Category 5 UTP.

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