Know more about Tx Power vs. Rx Power of a Fibre Transceiver?

Tx Power and Rx Power of a Fibre Transceiver.
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.

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.
Take 10GBASE-SR SFP as an example, its transmit power range is -7.3 dBm~1 dBm, and the receiver power range is below -11.1 dBm. When we see an RX sensitivity around -12 dBm or lower, there is typically some sort of fault in the cable plant (bad splice, dirty connector, poorly seated jumper, etc.) that causes excessive signal loss.

When designing an optical link, one of the factors to consider is the optical power budget (maximum allowable loss). According to the TX power and RX sensitivity, we can calculate the maximum allowable loss of the SFP modules. As illustrated above, 10Gbase-SR SFP Power Budget (dBm) = TX Power (-7.3 dBm) - RX Sensitivity (-11.1 dBm)=2.8dBm.

The more the optical power budget, the further the fiber transceiver can support. The 10GBASE-SR SFP can support the distance of 300 m over OM3 multimode fiber patch cable and the 10GBASE LR SFP module, which enables the 10G network at lengths up to 10 km over single mode fiber patch cable.

The above picture shows it is on a healthy link if it's known to be a long outside plant run (tens of km) with a measured signal loss (using OTDR fiber test kit) that would cause such loss. In those cases, we would be using the more expensive long-haul transceivers that are more capable of sending and receiving usable signals under those more extreme situations.

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 HTF. 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+.

Network building

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.