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Fibre Optic Fibre Optic System DesignSystem Design

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To be discussed» Determining the fundamental link design

parameters – aquire information» Design loss calculations – power budget» Costing

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Design Design considerationsconsiderations

Transmission technology» Chosen around the data / IT requirements of the

organisation Transmission parameters

» Data rates» Bandwidth» Capacity» Transmission distances

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Future growth of transmission capacity» An extremely important issue» If there is free capacity available someone will

find a reason to use it !!!» The cost of extra fibres is insignificant in the total

cost of the installation» Therefore install many additional spare fibres

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Cable costs versus Cable costs versus distancedistance

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Distance (Km)

Cable Cost as %of Total Cost*

*This does not includecable installation costs

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System reliability» Quality of equipment? Refer to

– Consultants– Other users– Trade magazines

» Do not over design the system» Trade off between cost and reliability» Route cables through quiet areas» Mark all cables clearly at termination points

» Cont

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» Keep up to date documentation

» Route duplication?

» Consider other telecommunications technologies

Choice of wavelength

» One of the first design considerations

» Dictated by the application (FDDI uses 1300nm)

» Use only one over the entire site

» Generally use the shortest possible wavelength

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Cable selection and installation route» Multimode or monomode?» Network topology? (Ring, Star or Bus)

Repeaters or amplifiers ?» Needed for extra long cable runs » Require power supplies, enclosures,

maintenance etc» Best to avoid where possible» Amplifiers (analog) contribute noise » Repeaters (digital) are preferable

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Transmitters and receivers» Where possible use

– Those that adhere to international standards– From one manufacturer only

» Use LEDs where possible– Cheaper– Less affected by the environment– Less sensitive to vibration and stress

» Selection considerations– Maximum data rate– Wavelength of operation

» Cont

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– Max transmission distance– Losses in each link– Max dispersion over each link– Max rise time for each component– Max rise time for the system– Led or laser– NA and diameter of the fibres– Matching TX & RX modules to the fibre– Data encoding to be used– Commercial or industrial application– Reliability requirements– Link availability requirements

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System Design System Design ParametersParameters

Transmitter power– Measured at end of 2m attached fibre. .

Minimum transmit power– Quoted by manufacturer for TX operating life.– Use either peak or average power for all

measurements in system (not both )

Receiver sensitivity– Minimum input signal level for a BER of 10

-9 at a specified data rate (eg. 100 Mbps)

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System gain– Difference between TX output power and RX sensitivity

System losses– Natural fibre attenuation– Splicing– Connectors– Coupling – Dispersion– Ageing– Temperature – Physical stress– Damaged fibres

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Safety margin– To account for aging, environmental losses, design

errors and future in line splices– Add the manufacturers specified “receiver power

penalty” Accounts for jitter (phase variations in a digital signal),

bandwidth limitations, dispersion, clock recovery problems etc

– Recommend 5 to 10 dB

Dynamic range– Limit to the power into the receiver (to prevent

damage, distortion)– Difference between maximum receiver input power

and receiver sensitivity– Ensure sufficient link attenuation.

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Coupling losses Transmitter coupling losses

» LED has large surface area compared to fibre core.

– LED loses 15 dB with 50 m core– LED loses 35 dB with 8.5 m core

» Laser has small surface area compared to fibre core

– Laser loses very little

Receiver coupling losses» Negligible because photodiode is always

larger than fibre core

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Link loss budget– Safety margin minus the system gain– Maximum signal loss allowable for cable, splice

and connector losses

Fade margin– Link loss budget minus known losses (unused

system gain)

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Link Loss Budget and Safety Link Loss Budget and Safety MarginMargin

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ExampleExample

Power Budget Power Budget CalculationCalculation

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Bandwidth calculationsBandwidth calculations

Calculated using time responses of fibre and TX & RX components.

» Slowest response time allowed from system: where the system output pulse has risen to 90% of the input pulse value in 70% of the time

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Pulse duration And time Pulse duration And time responseresponse

T

T

T

NRZ

RZ

FilteredNRZ

T =1R

T =1

2R

Tr = 0.7T= rise time= Max response

Time of Link

90%

0.7T

T = Pulse DurationR = Data Rate

(a)

(b)

(c)

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Total system response time must be faster than 0.7 of the input signal period (pulse duration).» Maximum allowable bandwidth is inverse of the

calculated fastest system response time.

T s = 0.7 * T where Ts = system time response

And T = 1/R for NRZ where R = data rate

T = 1/2R for RZ

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System time response is sum of– Fibre time response– Transmitter time response– Receiver time response

» Fibre time response affected by chromatic and modal dispersion

» TX and RX rise times in specification sheets

Ts = ( Tt2 + Tr

2 + Tf2)1/2

where Tt = response of transmitter,

Tr = response of receiver

Tf = response of fibre

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Fibre response time (Tf)

Tf = (Tfm2

+ Tfc2 )½

Where Tfm = time response from modal dispersion (ns)

Tfc = time response from chromatic dispersion (ns)

Tfm = Dm x L where Dm = modal dispersion (ns/km)

L = Length (km)Dm = 350 Bandwidth of fibre(quoted)

ORDm = 1 00 Modal Bandwidth of fibre(quoted)

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Tfc = time response from chromatic dispersion (ps)

Tfc = Dc x x L where Dc = chromatic dispersion (ps/nm-km)

= spectral spread (nm) L = Length (km)

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ExampleExample

BANDWIDTH BANDWIDTH DESIGN DESIGN

CALCULATIONSCALCULATIONSVIDEOVIDEO