By calculating the link budget, we can find the optimal parameters of the transmitting and receiving devices to ensure proper signal transmission. A link budget assessment enables us to design the system so that it meets its requirements without being overdesigned at extra cost.

## Optical time-domain reflectometer measures the state of the fiber

To begin the link budget assessment, we need to determine the condition of the fiber. To do this, we can obtain the optical time-domain reflectometer (OTDR) readings for the fiber, which are usually available from the fiber provider if the fiber is being leased.

An OTDR is an instrument used to characterize the state of the fiber. It injects a series of optical pulses into the fiber to establish any losses or reflections. A loss means that light leaks out of the path and the signal strength becomes degraded. Reflection occurs when light is reflected back in the same direction it came from. Connectors, tight-bend radiuses and splice joints can all contribute to a fiber with higher losses than desired. These return pulses are measured and plotted as a function of fiber length.

The attenuation of the fiber is measured in decibels (dB) per kilometer, indicating how much the signal attenuates and degrades per distance it travels. For estimation purposes, a typical standard value used to estimate the losses through a fiber is a loss of 0.25dB per km. The OTDR results will give the exact measurements based on the current state of the fiber, and the resulting graph shows the attenuation of the fiber as a function of distance.

## Examples of optical link budget estimates

Once the state of the fiber is known from the OTDR results, we can complete our link budget assessment using the formula above.

**Example 1:**

Let’s say that we want to transmit 10G traffic, using a SFP+ ZR transceiver. The power budget is 23dB. Now, let’s assume that we also require a 16 channel mux/demux with a loss of 4.5dB. (The loss of network components is specified for the equipment.) The loss in a 60 km long fiber at is 0.25dB/km. The total loss in the fiber will be 0.25dB/km x 60 = 15dB.

**Link budget calculation**

**Transceiver power budget: 23dB**

Fiber losses, 60km at 0.25dB/km: 15dB

Mux/Demux losses: 4.5dB

Various patch/splicing losses (margin): 1.0dB

*Total system link loss: 20.5dB*

**RESULT: 2.5 dB **

23dB -20.5dB in losses leaves 2.5dB of additional system margin, meaning the design is within spec and will work.

**Example 2:**

Now let’s assume the loss in the fiber is 0.4dB/km (instead of 0.25dB) and make the calculation again.

**Link budget calculation**

**Transceiver power budget: 23dB**

Fiber losses, 60km at 0.4dB/km: 24dB

Mux/Demux losses: 4.5dB

Various patch/splicing losses (margin): 1.0dB

*Total system link loss: 29.5dB*** **

**RESULT: -6.5 dB**

If the same ZR transceiver would have been chosen, then the system would not work.