# How to Understand Tx Power vs. Rx Power of a Fibre Transceiver?

As we all know, single mode fibre optic transceiver is designed for long distance data transmission and multimode fibre optic transceiver is for short distance link. How to calculate the specific distance that a fibre optic transceiver can support at a certain occasion? What factors are crucial to the transmission distance? Do you have any idea of Tx power (transmit) and Rx power (receive) level of a fibre optic transceiver? This article will show you by introducing 10GBASE-SR SFP+ and 10GBASE-LR SFP module.

Tx Power and Rx Power of a Fibre Transceiver

First, let’s understand the most two important factors of the fibre optic transceiver: Tx power and Rx power. The optical Tx power is the signal level leaving that device and it should be within the transmitter power range. The Rx power is the incoming signal level being received from the far end device and it should fall within the receive power range.

10GBASE-SR SFP+ is a multimode fibre transceiver and can support the distance of 300 m over OM3 multimode fibre patch lead. While 10GBASE-LR SFP module is a single mode type and can run the network distance up to 10 km over single mode fibre patch leads. Before purchasing 10GBASE SFP+ module, you must carefully read the product details. The following lists the product details about 10GBASE-SR SFP+ and 10GBASE-LR SFP module from Fiberstore. As showing below, Tx power of this Cisco compatible 10GBASE-SR SFP+ is between -7.3 dBm and -1 dBm. The maximum receive power is below -11.1 dBm. With regard to Cisco compatible 10GBASE-LR SFP module, Tx power is from -8.2 to 0.5 dBm and the maximum Rx power is -14.4 dBm.

Tx Power and Rx Power of 10GBASE-SR SFP+

Tx Power and Rx Power of 10GBASE-LR SFP+ Module
Tx Power and Rx Power VS. Optical Power Budget

To calculate the specific distance of a fibre optic transceiver, we need to know its optical power budget (maximum allowable loss).

Optical power budget = Tx power – Rx power

Therefore, according to the Tx power and Rx power, we can calculate the maximum allowable loss of 10GBASE-SR SFP+ and 10GBASE-LR SFP module.

Optical Power Budget of 10GBASE-SR SFP+

Min Tx Power = -7.3 dBm
Max Rx Power = -11.1 dBm
Optical power budget (maximum allowable loss) = (-7.3 dBm) – (-11.1 dBm)=3.4 dBm

Optical Power Budget of 10GBASE-LR SFP+ Module

Min Tx Power = -8.2dBm
Max Rx Power = -14.4 dBm
Optical power budget (maximum allowable loss) = (-8.2 dBm) – (-14.4 dBm)=6.2 dBm

Tx Power and Rx Power VS. Transmission Distance

Except optical power budget, we have to 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 3 dB). The loss of each connector is 0.6 dB and 0.1 dB of each fusion splicing point. Suppose we use 10GBASE-LR SFP+ module to build a network 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 1310 nm, the cable loss is about 0.35 dB per kilometer.

The worst optical loss = Power Budget – Total Optical Power Loss=6.2 dB – 1.2dB (2×0.6 dB) – 0.4dB (0.1×4) – 3dB (safety factor) = 1.6 dB
Worst case distance = {Worst case OPB, in dB} / [Cable Loss, in dB/km]=1.6dB/0.35dB/km
So we can get 10GBASE-LR SFP+ module can support at least 4.57 km at this occasion. Data transmission distance is mainly influenced by the optical power budget and fibre cable loss. From the above content, the optical power budget of 10GBASE-SR SFP+ is smaller than that of 10GBASE-LR SFP+ module. The more the optical power budget, the further the fibre transceiver can support. What’s more, the cable loss of multimode fibre cable is larger than that of single mode fibre cable. Obviously 10GBASE-LR SFP+ module can support longer link distance than 10GBASE-SR SFP+.

Note: The optical power budget is based on a theoretical calculation, and is just for reference. The transmission distance should be calculated based on the power budget of fibre transceiver module tested on the switch and some practical attenuation.

Conclusion

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. Before purchasing a transceiver, you’d better calculate the optical power budget according to transceiver module details.

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