GEN7iB
493
APPLICATION SPECIFIC USAGE
APPLICATION SPECIFIC USAGEH
Standard GHS systems use VCSEL 850 nm optical transmitters/receivers; they have an
optical power budget of 8 dB. Calculating the maximum length of optical cable can be
done in the following manner:
Optical budget GHS system 850 nm: 8 dB
Maximumbercablelength: L
optical
(km)
Fiber cable loss: -3.5 dB/km
Number of couplers: c
Coupler loss: 0.75 dB
Safety margin for aging and repair: -3 dB
L
km
optical
+ (
*
0.75 + (
35.
()


This formula also applies to Single Mode systems
For example, if two couplers are used in the cable, c = 2, the maximum length would be
L
optical
= 1 km
Thesecondlimitingfactorforcablelengthisbercablebandwidth.
Bandwidthlimitiscausedbylightpulsedispersionintheopticalber;thisonlyaffects
MultiModebersystems.
ThislimitistheproductoftheGHSsystem'smaximumsignalingspeedandthedened
bercablebandwidth.
GHSsignalingspeedoveropticalber: 1000 MHz
OMclassdenedbandwidth: BW
Maximumbercablelength: L
BW
(km)
L
BW
=
(km)
For example, if an OM2-type cable is used, the maximum length will be L
BW
= 0.5 km
Themaximumopticalberlengththatcanbeusedinasetupistheshortestoutcome
of L
optical
or L
BW
Ifthetwoexamplesabovearefollowed,theopticalberlengthmustbelimitedto
L
BW
= 0.5 km
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