What are the two most important factors of the fibre optic transceiver
The two most important factors of the fibre optic transceivers are TX power and RX power. The TX power is the signal level leaving the device, while RX power is the incoming signal level being received from the far/end device. Both the signal should fall within the permissible range i.e., TX within the transmitter power range and RX within the receivable power range.
Single mode fibre optic transceiver is designed for longer distance data transmission, while multimode fibre optic transceiver is for the shorter distance link.
TX Power and RX Power vs Optical Power Budget
Optical Power Budget (OPB in this section for easy reading purpose only) is the maximum allowable loss. And, it is calculated by the following
Optical Power Budget = TX Power – RX Power
i) Let’s calculate the OPB of 10GBASE-SR SFP+
Min TX Power = -7.3dBm
Max RX Power = -11.1dBm
OPB (or Maximum Allowable Loss) = (-7.3dBm) – (-11.1dBm) = 3.4dBm
ii) OPB for 10GBASE-LR SFP+
Min TX Power = -8.2dBm
Max RX Power = -14.4dBm
OPB = (-8.2dBm) – (-14.4dBm) = 6.2dBm
TX Power & RX Power vs Transmission Distance
We have to also consider other factors including the link length, fibre optical connectivity components, fusion splicing points and some unpredictable fibre attenuation caused by fibre patch lead bending (usually, the attenuation is about 3dB). The loss of each connection is 0.6dB and 0.1dB of each fusion splicing point. Supposing that we are using 10GBASE-LR SFP+ module to build a networking covering 2 connectors, 4 fusion splicing points. This module is interfaced with LC single mode fibre patch cord. If the single mode fibre cable has the wavelength of 1310nm, the cable loss is about 0.35dB per kilometer.
Therefore, the Worst Optical Loss = OPB - Total Optical Power Loss
6.2dB - 1.2dB {2 * 0.6dB} – 0.4dB {0.1 * 4} – 3dB {safety factor} = 1.6dB
Worst Case Distance = {Worst Case Distance} = {Worst Optical Loss, in dB} / {Cable Loss, in dB/km} = 1.6dB/0.35dB/km = 4.57km
In conclusion, 10GBASE-LR SFP+ can support at least 4.57km with the above scenario. Data transmission distance is mainly influenced by the Optical Power Budget and the fibre cable loss.
From the above content, the OPB of 10GBASE-SR SFP+ is smaller than that of 10GBASE-LR SFP+ module. *Thus, the more the OPB, the further the fibre transceiver can support*. Also, the cable loss of multimode fibre cable is larger than that of single mode fibre cable. In the nut shell, 10GBASE-LR SFP+ module can support longer distance than 10GBASE-SR SFP+.
While the OPB is based on a theoretical calculation, and is just for reference. The transmission distance should be calculated based on the power budget of the fibre transceiver module tested on the switch and some practical attenuation.
TX Power and RX Power level of a transceiver are the main factors of transmission distance. The more the optical power budget, the better the transceiver.
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