Assume that over there (points to the far left-paw corner of "The Cattery") is an Adva FSP 150CCf-825, powered up and operational.
Two Adva SFP transceiver modules are available --
- SFP/LS/1310S/SM/LC (Part No. 0061003002)
- SFP/GBE/1310S/SM/LC (Part No. 0061003008)
The first being up to 100Mbps capable over an up to 10km single mode optical fibre link and the second being up to 1000Mbps capable over an up to 10km single mode optical fibre link.
An Adva patch cable adaptor pack, F150/BT-/SFP/GBE1310S/01 (Part No. 2061993008-02), length 2m, as supplied to the BT Group is available. This consists of a pair of LC/UPC connectors linked by two lengths of 2m of single mode fibre to a pair of SM/UPC connectors. (Two simplex fibres, side by side, making a duplex link.) Also in the pack is an adaptor, a pair of female SM/UPC to female SM/UPC connectors, back-to-back.
The second of the two SFP transceivers, described above, is inserted into a vacant SFP position (LAN-5) of the FSP 150CCf-825 and the LC/UPC connectors of the patch cable adaptor pack are plugged into the SFP transceiver.
Assume that over there (points to the diagonally opposite corner of "The Cattery" to that where the FSP 150CCf-825 is sited) is a Planet GT-802S media convertor. It has two SM/UPC ports, described as 1000Base-LX, and an 8P8C Ethernet socket. The former are capable of operating over a pair of up to 10km (simplex) fibres to provide a duplex link and the latter is capable up to 1000Mbps.
The GT-802S data sheet states --
Fiber (sic) Maximum Distance: 10km
Optic Wavelength: 1310nm
Max. Optical Launch Power: -3dBm
Min. Optical Launch Power: -9.5dBm
Max. Input Power: -3dBm
Min. Input Power: -20dBm
Optical Link Budget: 4.9dB
A pair of single mode fibre patch cables, length 4.5m, are connected to the two ports of the GT-802S and are run to meet the SC/UPC connectors of the patch cable adaptor from the FSP 150CCf-825.
Tempting as it may be to link the two ends with the provided two SC/UPC female to SC/UPC female adaptor, attenuators must be inserted to reduce the power such that the receive optics (at each end of the duplex link) are not overloaded.
The question is: What value attenuator should be inserted into each (simplex) leg of the (duplex) link?
The total fibre length is 6.5m. The optics at each end of the link are good for up to 10km. Assuming that the attenuation of an optical fibre is proportional to its length, let's consider reducing the received power by a thousand. Performing the calculation --
10 x log10(10-3) = -30
Hence I assume that
at least a 30dB attenuator should be inserted in each leg of the link. But that "assumes a link length" of 10m. (We've reduced the initial 10km by a thousand.) To err on the safe side, let's "assume a link length" of 5m. Recalculating --
10 x log10(5 x 10-4) = -33
So the second, and final, question (for those who have followed my logic
and are still reading): Would 33dB attenuation, in each leg of the link, be approximately correct? (Remembering that this is a thought experiment and, thus, I do not possess the means to measure the relevant light levels.)
