Fibre Optic System DesignIDC

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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 considerationsq

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

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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 distance100 90 80 70 *This does not include cable installation costs

Cable Cost as % 60 of Total Cost*50 40 30 20 10

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100

1000

10000

Distance (Km)

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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 technologiesq

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 wavelengthIDC

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

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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 operationIDC

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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 requirementsIDC

System Design Parametersq q

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 )

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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 penaltyq

Accounts for jitter (phase variations in a digital signal), bandwidth limitations, dispersion, clock recovery problems etc

Recommend 5 to 10 dB q

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 lossesq

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 qIDC

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

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Fade margin Link loss budget minus known losses (unused system gain)

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Link Loss Budget and Safety Margin

-15 Min TX power (62.5/125/250) -20Optical Power -25 (dBm)Connector and splice losses Link loss budget Cable attenuation

Effective dispersion losses

-30Safety margin

-35

Receiver Sensitivity

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7Distance (Km)

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Maximum Cable Distance

Example Power Budget CalculationIDC

Bandwidth calculationsq

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 responseT T = Pulse Duration R = Data Rate

(a)T= NRZ T

1 R

(b)RZ T90%

T=

1 2R

(c)IDC0.7T

Filtered NRZ

Tr = 0.7T = rise time = Max response Time of Link

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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 T = 1/2R for RZIDC

where R = data rate

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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 + Tr2 + Tf2)1/2where Tt = response of transmitter, Tr = response of receiverIDC

Tf = response of fibre

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Fibre response time (Tf)(Tfm2 + Tfc2 )

Tf

=

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) OR Dm = 1 00 IDC Modal Bandwidth of fibre(quoted)

Tfc = time response from chromatic dispersion (ps) Tfc = Dc x x Lwhere Dc = chromatic dispersion (ps/nm-km)

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

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Example BANDWIDTH DESIGN CALCULATIONS VIDEO

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