The optical budget is the difference between the minimum power the transmitter delivers and the minimum sensitivity of the receiver. That value must cover fiber attenuation, splices, connectors and a margin. This guide calculates it step by step and shows the level table for seven SFP transceiver types taken from a multiplexer test report.
What the optical budget is
Short answer. The budget of a fiber link is the amount of loss, in decibels, that the link can have between the transmitter and the receiver and still work. It is calculated as the minimum transmit power minus the minimum receiver sensitivity. If the sum of the real losses plus a margin is less than that budget, the link is viable.
Field testing of fiber is covered in fiber network tests at a solar plant. This guide explains the calculation that precedes them.
The formula
Budget = P_tx(minimum) − P_rx(minimum sensitivity)
Link loss = α · L + n_connectors · A_c + n_splices · A_e
Condition: Link loss + Margin ≤ Budget- α is the fiber attenuation per kilometer, at the working wavelength, and L the length in kilometers.
- A_c and A_e are the loss per connector and per splice.
- Powers are in dBm and losses in dB. The subtraction of two levels in dBm gives dB.
The calculation always uses the worst case of the transmitter (the minimum level) and of the receiver (the minimum sensitivity). The values of α, A_c and A_e are taken from the datasheets of the cable and the installed connectors. As a reference, datasheets of G.652.D or G.657.A1 cables give maximum attenuations of about 0.4 dB/km at 1310 nm and 0.25 to 0.3 dB/km at 1550 nm, as indicated in fiber network tests.
Real SFP transceiver levels
The test report of a FOX615 multiplexer gives, for each SFP type, the expected transmit power range and the minimum receiver sensitivity. With those data, the worst-case budget turns out as follows:
| SFP | Wavelength | Transmit power | Minimum sensitivity | Worst-case budget |
|---|---|---|---|---|
| 1G SX | 850 nm | 0 to -9.5 dBm | -17 dBm | 7.5 dB |
| 1G LX | 1310 nm | -3 to -9 dBm | -19.5 dBm | 10.5 dB |
| 1G EX | 1310 nm | 2 to -3 dBm | -23 dBm | 20 dB |
| 1G ZX | 1550 nm | 5 to 0 dBm | -22 dBm | 22 dB |
| 10G LR | 1310 nm | 0.5 to -8.2 dBm | -12.6 dBm | 4.4 dB |
| 10G ER | 1550 nm | 4 to -4.7 dBm | -14.1 dBm | 9.4 dB |
| 10G ZR | 1550 nm | 4 to 0 dBm | -23 dBm | 23 dB |
The budget in the last column is the minimum transmit power minus the sensitivity. It is data from that manufacturer for its modules; with another module, use the values from its datasheet.
Calculation example
Values assumed only to illustrate the method:
- 1G ZX transceiver at 1550 nm: budget of 22 dB.
- 50 km link with 0.3 dB/km fiber: 15 dB.
- Four 0.5 dB connectors: 2 dB. Ten 0.1 dB splices: 1 dB.
- Total loss: 18 dB. Available margin: 22 − 18 = 4 dB.
A 4 dB margin covers aging and some repairs. If the same link used the 10.5 dB LX SFP, it would not close: the budget would not even cover the fiber.
- Link loss + margin ≤ budget?
- Yes: Can the receiver get too much power?
- Yes: Add an attenuator or choose a lower-power transceiver
- No: The link is viable
- No: Switch to a longer-reach transceiver or shorten the route
- Yes: Can the receiver get too much power?
What to measure in the field and compare
- Insertion loss with source and meter, on each fiber and at each wavelength, to compare with the calculation.
- OTDR to see where each splice, connector or bend is.
- Received power at the device port with the link in service. It must be within the receiver range and leave the planned margin.
Differences between what was calculated and what was measured point to dirty connectors, poorly made splices or routes longer than those on the drawing. These measurements are recorded at commissioning, together with those of the multiplexer (see multiplexer testing).
Direct or multiplexed fiber
In line differential protection, the budget decides whether a dedicated fiber between relays is enough or a repeater or multiplexer is needed. See direct or multiplexed fiber for 87L and single-mode and multimode fiber.
Link budget with Tiza Group
Do you need to validate that a fiber link closes before buying transceivers or before energizing? At Tiza Group we calculate the budget and measure the link within our commissioning tests and IEC 61850 and communications engineering.
Related guides
- Single-mode vs multimode fiber in substations
- Fiber network tests at a solar plant
- Multiplexer and fiber optic network
- Telecommunications multiplexer tests
- Direct or multiplexed fiber for 87L
- IEEE C37.94
References
- FOX615 multiplexer equipment test report: expected transmit power and sensitivity levels by SFP type.
- ITU-T G.652 and G.657: single-mode fiber characteristics.
Frequently asked questions
How much margin is left on a fiber link?
The designer sets it according to the risk and the expected life of the link. The margin must cover aging, dirt on connectors and future splices from repairs. A link without margin works on the day of the test and fails months later.
Why is measuring with the OTDR not enough?
The OTDR shows where each loss is, but the total loss of the link, the one compared with the budget, is measured with a light source and a power meter. The two tests complement each other.
What happens if the receiver gets too much power?
A receiver has a maximum input level as well as a minimum. On short links with long-reach transceivers it may be necessary to attenuate the signal; the exact limit is in the transceiver datasheet.