My February column covers the reasons for power and loss budgets and how to interpret them. In this article, I’ll show you how to calculate loss budgets properly. Remember the differences: a power budget gives you the range of decibel (dB) loss in the cable plant that a communication system can tolerate, while a loss budget is an estimate of the loss of a cable plant if properly installed.
Consider a typical duplex fiber optic link like this one:
The transmitter output power is coupled into an optical fiber and transmitted over the cable plant to the receiver. The receiver needs a certain amount of power to receive data error-free. The difference between the two is the power budget. Here are two examples.
Transmit out dBm
Receiver in dBm
Power Budget dB
Multimode, premises LAN @ 850 nm
|-10 dBm||-17 dBm||7 dB|
Single-mode outside plant long haul @ 1,310 nm
|+3 dBm||-20 dBm||23 dB|
Remember the output and input powers are absolute optical power expressed in decibel-milliwatts (dBm), while the difference is relative power and is expressed in decibels.
We’ll use these two examples to calculate the loss budgets of proposed cable plants for them.
The loss budget will include estimates of the loss of all components that contribute to the cable plant loss, generally the fiber, splices and connections. If the link also includes splitters like a passive optical network, that should also be included.
Multimode, premises LAN
The multimode premises local area network (LAN) described in the table above has a power budget of 7 dB, so let’s compare the loss budget of a cable plant we are designing.
- Length of cable plant: 500 meters
- Number of connections: 4 (including the connections on the end)
- Number of splices: 0
First, we calculate the fiber loss. The typical multimode LAN operates at 850 nm, where fiber has a loss of 3 dB/km.
Loss of fiber: 500 m = 0.5 km, 0.5 km X 3 dB/km = 1.5 dB
Then the connection loss: 4 connections X 0.5 dB/connection = 2.0 dB
And since we have no splices, that’s 0 dB.
Total loss = 1.5 dB fiber loss + 2.0 dB connection loss + 0 dB splice loss = 3.5 dB
Since the communications equipment has a power budget of 7 dB, we have 3.5 dB margin with the cable plant’s loss budget.
But let’s elaborate on the component losses we chose. Fiber loss at 850 nm varies according to the standard you might use or the specs of the grade of fiber you might choose. Telecommunications Industry Association (TIA) in Arlington, Va., sets standards for fiber attenuation at 850 nm as 3.5 dB/km, which is high enough that the manufacturers who wrote that spec can always meet the standard. But they are routinely producing fiber with losses under 3 dB/km, which is the spec we use.
The TIA connection loss spec is a whopping 0.75 dB, much too high for any SC, LC or ST connector, but it is there to cover the multifiber connectors such as the MPO. We choose to use 0.5 dB as a reasonable upper limit for connection loss, although most SC, LC or ST connectors will be better than that.
Note we say “connection loss,” not connector loss, since a single fiber optic connector has no loss unless it is mated to another connector, which creates a connection.
And yes, we do count the connectors on each end of the cable plant, because they will be included in loss tests results since you will connect to them with your test reference cables.
And finally, for multimode fiber, the loss budget may not be the gating item for cable plant length. Multimode fiber networks operating at gigabit speeds are limited by the bandwidth of the fiber grade chosen, so OM3 or OM4 fiber can go significantly longer distances than OM2. Consult the distance limits for the specific network to be used to determine the length limit. That consideration, however, does not affect the cable plant loss budget used for comparison to test results.
Single-mode outside plant long haul
The single-mode OSP long haul network has a power budget of 23 dB, which means it can be much longer. Let’s do a loss budget for these specifications:
- Length of cable plant: 25 km
- Number of connections: (just the connections on the end)
- Number of splices: 8
Loss of fiber: 25 km X 0.4 dB/km @ 1310 nm = 10.0 dB
Connection loss: 2 connections X 0.5 dB/connection = 1.0 dB
For splices: 8 X 0.2 dB = 1.6 dB
Our total loss = 10 dB fiber loss + 1.0 dB connection loss + 1.6 dB splice loss = 12.6 dB
The fiber attenuation at 1,310 nm is about 0.4 dB/km, but only 0.2 dB/km at 1,550 nm if we design for longer distances using transmitters at 1550 nm. At 1,550 nm, our fiber loss would be this:
Loss of fiber: 25 km X 0.2 dB/km @ 1,550 nm = 5.0 dB
Thus, the total loss budget would be 5 dB less or 7.6 dB.
Here is a challenge for you. How long of a distance could this single-mode system go with its 23 dB loss budget, assuming that you leave a 3 dB margin, e.g., 20 dB maximum loss. Hint: you need to remember that a splice will be needed about every 5 km on a long cable plant. (Answer at the end.)
Remember that loss budgets are estimates, not hard and fast numbers. Likewise, loss tests on the cable plant have uncertainties, so exercise some judgement when using these numbers.
Here is the answer to the challenge problem:
The max loss is 20 dB, including fiber, splices and connectors, and the connector loss is fixed at 1 dB, so we have 19 dB loss budget to use for fiber and spices.
Every 5 km, we have a splice of 0.2 dB, adding a contribution of 0.2 dB per 5 km, for a contribution to loss of 0.04 dB/km.
Then, including estimated splice loss, at 1,310 nm, the fiber loss is 0.44 dB/km, and at 1,550 nm it’s 0.24 dB/km.
Then for 1,310 nm, a total of 19 dB / 0.44 dB/km = 43.2 km
For 1,550 nm, 19 dB / 0.4 dB/km = 79.2 km
This is a useful process to know if you are designing networks.