When comparing optical transceivers as part of your network planning, there are a lot of specifications and figures to review. However, there is a single number that takes paramount importance: link budget. In this article, we’ll look at how it’s calculated, and how to use it in practice.
Link Budget: What It Is and Where It Comes From
Quite simply, the link budget is the total amount of optical power available over a single fiber span between two optical transceivers, as measured in decibel-milliwatts (dBm). The basic calculation is derived from the minimum output power value from the transmitter (Tx) on one end to the sensitivity of the receiver (Rx) on the other side of a simple point-to-point connection. For example, let’s look at the values for some typical transceivers:
Let’s look at the 10GSFP+E-LR first. Its minimum output power is -8.2dBm, and its receiver sensitivity is -14.4dBm.
Therefore, its link budget is 6.2dBm (-8.2 – (-14.4) = 6.2dBm).
By contrast, the 10GSFP+E-ZR has minimum Tx and Rx values of 0.0dBm and -23.0dBm, respectively, for a corresponding link budget of 23.0dBm.
These two examples illustrate the relationship between rated distance and link budget: the higher the link budget, the longer the rated distance. Therefore, rated distance is the expression of distance a transceiver should cover after factoring in standard levels of loss over the link (e.g., fiber cable loss specifications, connector loss, etc).
Using Link Budgets
While rated distance is useful for preliminary application sizing, it is, in practice, something of a best-case scenario guideline. This is why it is incredibly important not only to know your link budget but also to validate your deployment through accurately measuring the actual loss on your fiber link. This will ensure the total loss in your system doesn’t exceed the link budget. This is especially true if your link span approaches the specified rated distance; always err on the conservative side!
In our next article, we will discuss the importance of link budgets to passive WDM architectures.
