| Scenario | Tech | Split | Efficiency | Utilization | Avg down/up | Concurrency | Subs/Port | Ports for 2,000 subs |
|---|---|---|---|---|---|---|---|---|
| Balanced build | GPON | 1:32 | 92% | 75% | 8/2 Mbps | 25% | 32 | 72 |
| Higher splits | GPON | 1:64 | 92% | 75% | 8/2 Mbps | 25% | 64 | 36 |
| 10G upgrade | XGS-PON | 1:64 | 92% | 75% | 20/5 Mbps | 30% | 64 | 36 |
Usable_Down = Line_Down × Efficiency × Utilization
Usable_Up = Line_Up × Efficiency × Utilization
Demand_Down = Avg_Down × Concurrency
Demand_Up = Avg_Up × Concurrency
MaxSubs_Split = SplitRatio × SubsPerONT
MaxSubs_Down = ⌊Usable_Down / Demand_Down⌋, MaxSubs_Up = ⌊Usable_Up / Demand_Up⌋
SubsPerPort = min(MaxSubs_Split, MaxSubs_Down, MaxSubs_Up)
PlannedSubs = CurrentSubs × (1 + Growth%), Ports = ⌈PlannedSubs × (1 + Spare%) / SubsPerPort⌉ + ReservedPorts
- Select the access profile your build will use, or choose custom.
- Enter split ratio and any shared-ONT factor for the design.
- Set busy-hour averages and concurrency to match service analytics.
- Adjust efficiency and utilization to reflect engineering policy.
- Add growth and spare ports for realistic construction planning.
- Review ports, cards, fibers, and slot constraints in results.
Port capacity drivers in access builds
OLT port planning is governed by three checks: physical split, downstream budget, and upstream budget. Split ratio sets the ceiling, but busy-hour demand often becomes the practical limiter. This calculator highlights the limiting factor so construction teams can correct the design before deployment for crews.
Aligning service tiers with busy-hour demand
Use measured busy-hour averages rather than advertised tiers. If homes average 8 Mbps down and 2 Mbps up, and 25% are active, effective demand is 2.0/0.5 Mbps per subscriber. As upstream use rises (video calls, backups), upstream can constrain capacity even with moderate splits.
Engineering headroom and efficiency assumptions
Payload efficiency accounts for overhead; utilization enforces headroom for bursts and latency control. Many designs use 90–95% efficiency and 70–80% utilization. Tightening utilization from 75% to 65% reduces usable bandwidth and can increase required ports, cards, and fibers.
Translating forecasts into build quantities
After subscribers-per-port is set, the calculator sizes ports from forecasted subscribers plus growth and spare percentages. Those ports map directly to feeder fibers, splitter counts, and line cards. Reserved ports can be added for protection schemes or special projects to avoid later rework. In MDU builds, increase subscribers per ONT to represent shared terminals and reduce underestimation of port loading.
Quality checks before issuing construction BOQ
Confirm installed ports and used ports match live allocation, then check that expansion stays within chassis slot limits. Compare the recommended subscribers-per-port against operational constraints such as optical budget and ODN reach. Export results to document assumptions for approvals and change control.
| Tech | Split | Avg down/up | Concurrency | Eff./Util. | Subs/port | Planned subs | Ports |
|---|---|---|---|---|---|---|---|
| XGS-PON | 1:64 | 20/5 Mbps | 30% | 92% / 75% | 64 | 2,400 | 45 |
1) What does “recommended subscribers per port” represent?
It is the smallest value from split, downstream budget, and upstream budget checks. It reflects a planning limit, not a hard operational maximum, and depends on your busy-hour traffic assumptions.
2) How should I pick a concurrency percentage?
Use busy-hour telemetry if available. If not, start with 20–35% for residential and adjust by service mix. Higher streaming or WFH adoption usually increases concurrency and upstream demand.
3) Why can upstream become the limiting factor?
Uploads are bursty and increasingly common (cloud sync, video calls). If average upstream and concurrency are high, the upstream budget per port can fall below what the split ratio would otherwise allow.
4) What values are reasonable for payload efficiency?
Many teams model 90–95% to account for protocol and framing overhead. Use a conservative value if you carry additional encapsulation, heavy multicast, or expect higher control-plane activity.
5) How does engineering utilization differ from efficiency?
Efficiency is overhead loss; utilization is an intentional cap to preserve headroom. Lower utilization reduces usable bandwidth, which increases port counts but typically improves user experience under peak load.
6) When should I use subscribers per ONT greater than 1?
Use it for shared ONT scenarios such as multi-dwelling units or small enterprises where one ONT serves multiple subscribers. This increases the split-based maximum and can change the limiting factor.
7) What does “protection/reserved ports” do?
It adds fixed ports to the final requirement to cover protection designs, planned migrations, or special customers. It helps prevent underbuilding when ports must be held back from general allocation.