Observer nTAPs : Observer nTAPs : nTAP Models : Optical nTAPs : Attenuation : Determining the best split ratio for you
Determining the best split ratio for you
To ensure that all of the devices receive enough light power to establish and maintain a connection, you must understand where light can be “lost” as it travels between the network devices connected to the TAP and from the TAP to the analyzer.
Fiber optic data travels on light power. A fiber optic TAP makes a copy of the data for the analyzer by splitting the light power.
After the send strength and receive sensitivities of the ports and cable distances are known, a “power loss budget” can be calculated. The power loss budget can be helpful in determining if there is enough signal strength left at the analyzer receive port for a desired split ratio.
The primary factors that need to be collected to determine loss budget are the:
Transmit power from the network devices
Cable distance from the network device to the TAP
Maximum insertion loss from the TAP
Cable distance from the TAP to the analyzer
Analyzer port receive sensitivity
Other less crucial items that may also affect you include:
Number or quality of any connectors or patch panels in the path to and from the TAP
Age of the fiber cables
Amount of heat in the environment where the fiber runs
Table 3. Maximum Insertion Losses
Maximum Insertion Losses in Decibels
Multimode 62.5 micrometer
Multimode 50 micrometer
Single-Mode 9 micrometer
Split Ratio1
1300nm
850nm
1300nm
850nm
1310-1550
50/50
3.9/3.9
4.7/4.7
4.5/4.5
3.9/3.9
3.6/3.6
60/40
3.0/5.0
3.8/5.7
3.7/5.6
3.0/5.0
2.8/4.8
70/30
2.3/6.3
3.0/7.0
2.9/7.0
2.2/6.2
2.0/6.1
Fiber Loss/km2
1
3
1
3
0.4/0.3
Connector Loss
.5
.5
.5
.5
.2

1 The ratio is network/analyzer. So, a 70/30 connection has 70% of the light power for the network and 30% for the analyzer.

2 Fiber loss is per kilometer of fiber.

In each split ratio, what you are dividing is the light power from the incoming network link. The larger percentage of the light power is used for the connection to the other network device and the smaller portion is the light power for the analyzer. As long as there is sufficient light power, all data is still sent to the analyzer regardless of the split ratio chosen.
Determining your power loss budget is a several step process that requires you to know the send power and receive sensitivities of the devices connected to the TAP, and requires that you do some basic math. Use these equations to determine the light available in decibels at the analyzer.
1. Determine your power loss budget by subtracting the receive sensitivity of the device connected to Link B from the send power of the device connected to Link A. Get these values from the device manufacturers. The amount of loss that you can have through attenuation and connector loss must be less than this power loss budget.
Example: (Send Device Power) – (Receive Device Sensitivity)=Power Loss Budget
These values will be negative numbers, so you will be subtracting a negative number from a negative number and its product will be a positive number.
2. Determine the loss caused by attenuation. Use Table 3 for values to assist you. If your cables are less than one kilometer, convert your cable length for the equation.
Example: (Number of Connectors * Connector Loss) + (Fiber Length of Link A * Fiber Loss) + (Fiber Length of Link B * Fiber Loss)=Attenuation
3. Subtract the output from step 2 from step 1.
Example: Power Loss Budget - Attenuation=Actual Loss
If the actual loss is less than the power loss budget, then your budget is feasible with your chosen split ratio; however, you must also calculate the power loss budget for the analyzer from Link A and from Link B. Only if both power loss budgets are feasible is the chosen split ratio usable.
4. Determine your maximum insertion loss by subtracting the receive sensitivity of the analyzer from the send power from the device connected to Link A. Get these values from the device manufacturers. This is the amount of loss that you can have through attenuation and connector loss.
Example: (Send Device Power) – (Analyzer Sensitivity)=Power Loss Budget
5. Determine the loss caused by attenuation. See Maximum Insertion Losses for values to assist you.
Example: (Number of Connectors * Connector Loss) + (Fiber Length of Link A * Fiber Loss) + (Fiber Length of Analyzer * Fiber Loss)=Attenuation
6. Subtract the output from step 5 from step 4.
Example: Power Loss Budget - Attenuation=Actual Loss
7. Repeat step 4 through step 6 for Link B to the analyzer.
 
 
 
For example, Figure 19 shows cable lengths to the TAP from the network devices and from the TAP to the analyzer. Using these cable lengths and some information from the device manufacturers, you can determine the power loss.
 
Figure 19: Cable lengths to/from the TAP
NI nTAPs 'optical link loss' PNG [hdot24]NI nTAPs 'optical link loss' PNG [hdot24]
 
The equations here are examples of how to calculate a power loss budget with actual values.
This shows the power loss budget for Link A to Link B.
Link A ↔ Link B
Send Device Power
-9.000
Receive Device Sensitivity
--19.5
Power Loss Budget
10.500
Number of Connectors
4.0
Connector Loss 1
x 0.5
Connector Loss
2.0
Fiber Length Link A (8 meters)
0.008
Fiber Loss Link A 2
x 3.0
Fiber Loss Link A total
+0.024
Fiber Length Link B (40 meters)
0.04
Fiber Loss Link B
x 3.0
Fiber Loss Link B total
+0.120
Attenuation
-2.144
Power Loss Budget - Attenuation 3
8.356

1 Multimode.

2 850nm multimode.

3 Light power available for network. Any network split ratio smaller than this number is feasible so long as the analyzer side is also feasible.

The budget for the network side is 8.356 dB. Any split ratio is valid because 8.356 dB is greater than any of the insertion losses from Maximum Insertion Losses.
Before we can say that any split ratio will work though, we must also check the light power to the analyzer.
This shows the power loss budget for Link A to the analyzer.
Link A > Analyzer
Send Device Power
-9.000
Receive Device Sensitivity
--17.5
Power Loss Budget
9.000
Number of Connectors
4.0
Connector Loss 1
x 0.5
Connector Loss
2.0
Fiber Length Link A (8 meters)
0.008
Fiber Loss Link A 2
x 3.0
Fiber Loss Link A total
+0.024
Fiber Length to Analyzer (75 meters)
0.075
Fiber Loss Analyzer
x 3.0
Fiber Loss Link B total
+0.225
Attenuation
-2.249
Power Loss Budget - Attenuation 3
6.751

1 Multimode.

2 850nm multimode.

3 Light power available for the analyzer. Any split ratio smaller than this number is feasible so long as the network side is also feasible.

The budget for the analyzer side is 6.751 dB. The network side allowed us to choose any split ratio, but the analyzer side presents some limitations. Our budget was 9.0 dB, which is greater than our 6.751 dB availability. Since we only have 6.751 dB available, the split ratios we can use are 50/50 and 60/40 after looking at Maximum Insertion Losses. All others do not provide enough light power to the analyzer.
 
After completing this task:
Use this page to create your own power loss budget from Link A to Link B if you are considering an Optical TAP with a split ratio other than 50/50. Then use it for your Link A or the analyzer, whichever link has the longer fiber length. Use Maximum Insertion Losses to assist you.
Network > Analyzer
Send Device Power
Receive Device Sensitivity
-
Power Loss Budget
Number of Connectors
Connector Loss
x
Connector Loss
Fiber Length Link A (or Link B)
Fiber Loss Link A (or Link B)
x
Fiber Loss Link A (or Link B) total
+
Fiber Length to Analyzer
Fiber Loss Analyzer
x
Fiber Loss Analyzer total
+
Attenuation
-
Power Loss Budget - Attenuation 1

1 Light power available for analyzer. Any split ratio smaller than this number is feasible.