A confusing and often misunderstood topic when working with optical networks are the dB and dBm values associated with power and loss. You will sometimes hear these values used interchangeably when they are actually two different measurements. The following article was written to explain these values, how they are used, and how you should interpret them.
dBm or decibels per milliwatt, is an optical power measurement that you will typically see displayed on network equipment and test sets. The “m” in dBm stands for milliwatt, which is a reference power of 1 milliwatt. While 0 dBm equals one milliwatt, 1 is referred to as the reference power because any value greater than 0 is more than one milliwatt, and any value less than 0 is less than one milliwatt. For example, an optic transmitting at -3dBm is outputting .5mW, where as an optic transmitting at 3dBm is outputting 2.0mW. The formula P=1mW * 10x/10 (where P=power and x=dBm) can be used to determine this increase or decrease of power in milliwatts transmitted or received.
dB is a value that represents the difference between the input power and output power measured in dBm. In figure 1 below, you will see the input power (P1) 0dBm and the output power (P2) -20dBm. The optical loss is P1-P2 which equals 20db
Figure 1
Now that we have an understanding of dB and dBm, lets go over some dB values to show what optical loss looks like. The chart below shows how much power is lost between P1 and P2
Figure 2
|
Optical loss in dB |
Power out as a % of power in |
% Power lost |
|
1 |
79% |
21% |
|
3 |
50% |
50% |
|
4 |
40% |
60% |
|
7 |
20% |
80% |
|
10 |
10% |
90% |
|
13 |
5% |
95% |
|
20 |
1% |
99% |
|
40 |
.01% |
99.99% |
The first column shows the optical loss value, and the second column is showing how much power will continue down the fiber. Finally, the last column is showing the percent of power lost overall.
As you can see 3 dB loss is a lot, 50% power is being lost overall. These values are not linear, as this is based on a logarithmic scale where each dB is a ratio of 10. In order to calculate a more exact value, the formula dB = 10 x Log10 (P1/P2) is used.
Check with the manufacturer to determine what acceptable levels of loss are for the optic to operate optimally. To give you an idea, one of the most commonly used Cisco optics within a LAN for relatively short distance is GLC‐SX‐MM. If we look at the datasheet for this, the minimum receive power is negative 17.
A couple of ways to determine dB loss are with optical loss test sets (OLTS) or by the device if it supports reading these values from the optic.
OLTS determines the amount of light lost by sending a known level on one side, and measuring the output on the other. For Cisco devices, Digital Optical Monitoring (DOM) is a feature used to gather TX and RX levels from the optic if it is DOM capable. The device and platform as well as a minimum software version may be required for this feature to work.
Being that I am not a mathematician, my explanations above for the gain and loss calculations stopped at the formula. However, I put together a dB loss calculator for the windows command line, (sorry Mac users) that will allow you to enter the loss in dB and give you results similar to the table in figure 1, just more accurate. Values may be entered for the loss in dB up to a thousandth of a dB.
Click Here for dB Loss Calculator
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