In fiber optic networks, especially in FTTx deployments, the number of Optical Network Units (ONUs) that a single PON port on an Optical Line Terminal (OLT) can support directly affects network planning, cost-efficiency, and service scalability. In this article, we’ll explain the concept of split ratio, the limits for EPON and GPON, and how to choose the right configuration for your scenario.
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What is a PON Split Ratio
The split ratio refers to the number of ONUs connected to a single PON port on the OLT through optical splitters. It’s written in the form of 1:N, where N is the number of ONUs (or end-user terminals) a PON port can serve.
- In a 1:64 split ratio, one PON port connects to 64 ONUs.
- The split is achieved using passive optical splitters, which divide the optical signal from the OLT to multiple ONUs and vice versa.
Why it matters: A higher split ratio allows you to connect more users per port, reducing hardware cost per subscriber. However, this comes with trade-offs in bandwidth allocation and optical power.
Maximum Split Ratios for EPON and GPON
| Standard | Max Split Ratio | Typical Use Case | PON Port Bandwidth |
| EPON | 1:64 | Residential, SME | 1.25 Gbps up/down |
| GPON | 1:128 (standard-defined) 1:64 (common in practice) | Mixed-use, enterprise | 2.5 Gbps down / 1.25 Gbps up |
EPON (Ethernet Passive Optical Network) supports a maximum split ratio of 1:64, meaning one PON port can serve up to 64 ONUs. In real-world deployments, network planners often use a more conservative ratio like 1:32 to ensure stable bandwidth and quality of service, especially for high-traffic scenarios like IPTV or business users.
GPON (Gigabit Passive Optical Network) supports a theoretical maximum split ratio of 1:128. However, many operators still choose 1:64 as a practical standard, balancing performance and cost.
Compared with EPON, GPON allows more ONUs per port while maintaining better downstream bandwidth, making it more suitable for densely populated areas or demanding applications.
What Limits the Split Ratio?
Several technical factors determine the practical split ratio you can use:
1. Optical Power Budget
Splitters introduce insertion loss. The more branches (e.g., 1:64 or 1:128), the higher the optical loss. Both OLT and ONU must remain within the optical budget to maintain signal integrity
2. Bandwidth Sharing
PON is a shared medium. Increasing the number of ONUs per port reduces the bandwidth available per user. For example:
- In GPON, 2.5 Gbps downstream shared among 64 users = -39 Mbps average per ONU (without over-subscription).
- Actual performance depends on usage patterns and QoS settings.
3. Service Quality (QoS)
High split ratios may impact latency and jitter, especially for video streaming, voice services, or business SLAs.
4. Deployment Technology and Distance
Split ratio must be balanced with fiber distance and environmental conditions. Higher ratios are easier to deploy in densely populated areas.
Split Ratio Planning: Real-World Scenarios and Best Practices
Choosing the right split ratio is a critical part of designing a PON network. It impacts not only the number of ONUs each OLT PON port can support, but also signal quality and transmission stability. Let’s explore real-world examples and key principles for effective split ratio planning.
Real-World Deployment Examples
| Scenario | Recommended Split Ratio | Reasoning |
| Urban FTTH (High Density) | 1:64 or 1:128 | Short distance, low attenuation, cost-efficient |
| Suburban Residential Areas | 1:32 or 1:64 | Balanced split and reach, scalable architecture |
| Rural Deployment | 1:16 or 1:32 | Long fiber runs, maintain signal quality |
| Enterprise/Private Networks | 1:8 or 1:16 | Lower contention, stable bandwidth demand |
Best Practices
- Match split ratio with power budget and distance: High split ratio like 1:128 require shorter fiber lengths and stronger optical budget (Class C+ optics).
- Avoid excessive multi-level splitting: Try to use single-level splitters to reduce insertion loss and simplify maintenance.
- Leave optical margin: Always reserve 2-3 dB for aging, repairs, and future capacity upgrades.
- Plan based on bandwidth forecast: For applications with high concurrent bandwidth demand (e.g., enterprises or IPTV), use smaller split ratios to avoid congestion.
Conclusion
An OLT PON port can theoretically support up to 64 ONUs in EPON and up to 128 ONUs in GPON. However, the ideal split ratio depends on multiple real-world factors including bandwidth demand, service type, fiber distance, and optical power loss. Network operators and ISPs must balance cost-efficiency with service quality when designing PON networks.
If you’re planning a PON deployment or unsure how many ONUs your current infrastructure should support, VSOL’s team can help you tailor a solution based on your scenario.
