1. Overview

The Optical Link Budget is a critical parameter for evaluating whether an optical signal in a fiber communication system can be successfully received along its transmission path. It covers all optical component losses and the transmit/receive capabilities between the OLT (Optical Line Terminal) and ONU (Optical Network Unit), ensuring stable operation within the design range.

This introduction applies to ISP scenarios and Omada optical solutions, covering two mainstream technologies: GPON and XGSPON.

2. Comparison of GPON and XGSPON

3. SFP Module Categories and Optical Factors

Different SFP module classes determine system transmit power and receiver sensitivity:

(1) GPON Common Classes: Class B+, Class C+, Class C++
- Wavelength: Downstream 1490 nm, Upstream 1310 nm
- Rate: Downstream 2.488 Gbps, Upstream 1.244 Gbps

(2) XGSPON Common Classes: Class N1, Class N2, E1
- Wavelength: Downstream 1577 nm, Upstream 1270 nm
- Rate: Both directions 10 Gbps

4. Optical Link Loss Factors Analysis

(1) Splitter Loss
Splitters are essential in PON networks, and their loss increases with the split ratio:

(2) Fiber Loss:
- ITU-T G.652 standard (G.652D single-mode fiber):
- ~0.35 dB/km (1310 nm)
- ~0.25 dB/km (1490/1550 nm)
- 1490 nm: ~0.30 dB/km
- 1550 nm: ~0.25 dB/km
- XGSPON downstream 1577 nm: ~0.25 dB/km
Fiber loss (dB) = Fiber length (km) × Attenuation coefficient (dB/km)
Example: 10 km fiber at 1490 nm ≈ 2.5 dB loss.
GPON/XGSPON is typically designed for connecting distances within 20 km.

(3) Fiber Optic Connector Loss

Each connector (e.g., SC/APC, LC/UPC) introduces the loss approximately:

• 0.2 ~ 0.5 dB

It is recommended to reserve 0.75 dB per connector in the link budget.

(4) Connectors Loss:
Each connector (SC/APC, LC/UPC) introduces ~0.2 ~ 0.5 dB loss. Reserve 0.5 dB per connector in the budget.

(4) Polish Types Loss:
- PC: ~0.5 dB insertion loss, -30 ~ -40 dB return loss
- UPC: ~0.3 dB insertion loss, -50 dB return loss
- APC: ~0.2 ~ 0.3 dB insertion loss, -60 dB or lower return loss (best for PON)

(5) Bend Loss:

Too small a bending radius in optical fiber will cause micro-bending loss:
- Small bend radius causes micro-bend loss (0.1 ~ 3 dB).
- Recommended radius ≥30 mm (standard fiber) or ≥10 mm (bend-insensitive fiber).
- Longer wavelengths (1550 nm, 1577 nm) suffer more bend loss.
- Different fiber types exhibit significant differences in bending resistance (G.652D vs G.657A).

- An additional 0.5 to 1 dB can be reserved in the budget for complex cabling scenarios.

5. Example of Link Budget Calculation (GPON C+, 1:16 Splitting)

Total Loss = Fiber attenuation + Splitter loss + Connector loss + Splice loss + Safety margin
(Safety margin: typically 3 dB for aging, temperature changes, etc.)

Example Calculation:
- Fiber length: 10 km (≈2.5 dB)
- Splitter: 1:16 (≈13.5 dB)
- Connectors: 4 (≈2 dB)
- Bend loss: 0.5 dB
- Splices: 4 (0.1 dB each)
- Safety margin: 3 dB
Total ≈ 22.9 dB (within GPON C+ max budget of 32 dB).
Conclusion: The maximum link budget for GPON C+ is 32 dB. If 22.9 dB < 32 dB, the link can function normally.

6. Design Recommendations:

- Choose appropriate SFP module (B+/C+/C++ or N1/N2) based on split ratio and distance.
- Avoid high split ratios (e.g., 1:64) to reduce loss.
- Optimize cabling: minimize connectors, avoid sharp bends, use low-loss fiber.
- Reserve 3–5 dB margin.

7. Commercial vs ISP Scenarios:

(1) Brief Comparison

(2) Typical Case Studies in Two Scenarios:

A. Commercial Scenarios (Point-to-Point Optical Links in Enterprise Buildings)

Scenario Description:

In an office building, a mainframe room connects to 10 offices, each with an independent optical link. Each link is approximately 100 meters long, using single-mode fiber, LC connectors, point-to-point connection, and without a splitter.

Components used (per link):

Fiber optic cable: 100 meters

Connectors: 2 (LC)

Fusion splices: 2 (intermediate connections)

Optical modules: 2 (SFP, 1G)

Optical link budget calculation (per link):

Fiber optic attenuation: 0.1 km × 0.35 dB/km = 0.035 dB

Connector loss: 2 × 0.5 dB = 1.0 dB

Fusion splice loss: 2 × 0.1 dB = 0.2 dB

Total link loss ≈ 1.235 dB

B. ISP scenario (FTTH, 1:32 splitter)

Scenario description:

An FTTH network is deployed in a residential area. The OLT is located in the equipment room, covering 300 users.GPON technology is used, employing a 1:32 splitter. The average fiber optic length per user is approximately 500 meters.

Components Used (per household):

Fiber Optic: 500 meters

Connectors: 2 (SC)

Fusion Splices: 4 (backbone + branch)

Splitter: 1 (1:32, shared)

Optical Modules: OLT end GPON SFP, ONU end built-in module

Optical Link Budget Calculation (per household):

Fiber Attenuation: 0.5 km × 0.35 dB/km = 0.175 dB

Connector Loss: 2 × 0.5 dB = 1.0 dB

Fusion Splice Loss: 4 × 0.1 dB = 0.4 dB

Splitter Loss (1:32): 17 dB

Total Link Loss ≈ 18.575 d