## Thursday, December 17, 2009

### How to calculate end to end transmit power in fiber

Calculation of power budget in fiber deployent is one of the biggest challenge. If the power is more or less than the power budget, it will bring lot of problems like throughput, drops and many more. One must ensure that the end to end link is working up to the level and able to delivery according to the commitment and design. To ensure these parameters, one must know about how to calculate the end to end power budget. Power budget is nothing but it provides the margin that compensates for variations of transmitter power and receiver sensitivity levels.

Power Budget = Minimum Transmit Power - Minimum Receiver Sensitivity Level

Minimum and maximum receiver and transmit levels could be obtained from the product specifications.
With this we need to calculte the total loss caused by the equipments in end to end deployment. Click here to see the values of losses w.r.t to equipment.

End to End Loss Calculation = (Fiber attentuation*km) + (Connector attenuation *Number of connectors) + (Splices * Number of splices) + (Passive devices*Number)

The above mentioned calculations are basics but the question comes in mind that how much power should one point transmit and another point will receive. Whenever a starting point will transmit some power the same power doesn't receive by the another point; This is because of losses in the path. The another point is that the power received by the end point should not exceed its maximum receiver sensitivity. So we can use the given formula to calculate the input power to link.

Input Power = Maximum Tranmit Power - End to End Loss Calucation < Maximum Power Receiver Sensitivity example will help to understand the whole calculations w.r.t to the live deployment:-

The minimum optical transmitter launch power is –12.5 dBm, and the maximum optical transmitter launch power is –2 dBm at 1310 nm. The minimum receiver sensitivity is –30 dBm, and the maximum receiver sensitivity is –3 dBm at 1310 nm. Total fiber stretch is of 3 Km with 3*fusion splices and 2*patch panels. We can ad the connectors also but in this example I am using that.

First calculate the total power budget which means how much maximum power we can allow for the link. As mentioned above in the formula we need to add the values.

Power Budget = Minimum Transmit Power(-12.5) - Minimum Receiver Sensitivity Level(-30)
= -12.5 + 30
= 17.5 dB

Now calculate the losses

This is the MMF, assume graded index 50/125 = 3 Km * 0.7 dB/KM => 2.1dB

2* Fusion Splices = 2* 0.02dB => 0.04dB
2* Patch Panels = 2* 2dB => 4.0 dB

Total Fiber Loss in 3 Km = 2.1 + 0.04 + 4 => 6.14dB

Now we need to calculate the input power:-

Maximum Transmit Power(-2) - Total Fiber Loss(6.14) < Maximum Receiver Sensitivity -2-6.14 => -8.14dB < -3dB