GPON's line rates are fixed by the physical layer: 2.488 Gbps downstream (OLT → ONU) and 1.244 Gbps upstream (ONU → OLT), defined in ITU-T G.984.2. Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches, and understanding where those numbers come from helps you size a PON correctly instead of guessing.
GPON physical layer (ITU-T G.984.2) defines:
Downstream (OLT → ONU): 2488.32 Mbps, commonly written 2.488 Gbps;
Upstream (ONU → OLT): 1244.16 Mbps, commonly written 1.244 Gbps.
These are not rounded — they are exact values, and downstream is exactly twice upstream. Note this is line rate, which includes frame overhead; the real user-payload "effective bandwidth" is lower after FEC, GEM encapsulation, and OAM overhead — about 2.2 Gbps down and 1.1 Gbps up in practice.
| Direction | Rate | Why |
|---|---|---|
| Downstream | 2.488 G | Point-to-multipoint broadcast; all ONUs share this one path, so total demand is high |
| Upstream | 1.244 G | Many ONUs time-share one pipe with TDMA slots, so per-port upstream is split |
Downstream is broadcast: one OLT port feeds dozens or hundreds of ONUs, so total bandwidth demand is naturally high. Upstream is time-division: all ONUs under one PON port take turns transmitting in their assigned slots, summing into the single 1.244 G pipe. The "download-heavy, upload-light" residential model also fits this asymmetry. For symmetric 10 G, you need XGS-PON (10 G both ways).
GPON frames are organized on a 125 μs cycle, matching the 8 kHz cadence (aligned with TDM voice frames). In each downstream frame, the payload is carved into GEM frames (GPON Encapsulation Method), carrying Ethernet packets, IP packets, and voice. The OLT assigns each ONU a timeslot upstream (using ranging to compute equalized delay so no ONU's light collides); slot boundaries align to the 125 μs frame, and the OLT uses burst-mode reception, re-clocking every frame.
Downstream normally enables FEC (forward error correction, RS(255,239)): 16 check bytes per 239 data bytes, coding efficiency about 93.7%. So of the 2.488 G line rate, about 6.3% becomes correction overhead, leaving about 2.33 Gbps of visible downstream throughput. Upstream usually skips FEC (trading overhead for latency, since upstream slots are already tight).
2.488 G is the total capacity of one PON port, shared by every ONU under it. How much a single subscriber actually gets is set by DBA (Dynamic Bandwidth Allocation, G.984.3), which schedules by queue, priority, and T-CONT type in real time — more when busy, reclaimed when idle. So "GPON is 2.5 G" and "my house gets a full 1 G" are two different layers: the former is port capacity, the latter is what DBA splits among, say, 128 users.
Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches. Its gpon olt runs each PON port at 2.488 G down / 1.244 G up per the physical layer, with DBA dynamically splitting bandwidth across the ONUs on that port and FEC enabled downstream by default. When planning capacity, remember the single-port net throughput is about 2.3 G down, shared across 64~96 subscribers by concurrency ratio; to raise per-subscriber speed, either reduce subscribers per port or move to XGS-PON. For the upstream side, a gpon olt at the aggregation node hands the aggregated traffic to 10GE ports on an industrial switch.
No. That is the line rate of one PON port; after FEC and GEM overhead it is shared by every ONU under that port through DBA. A single subscriber's rate depends on subscriber count, concurrency, and scheduling policy.
Downstream is broadcast to all ONUs with high total demand; upstream is many ONUs time-sharing the same 1.244 G pipe with TDMA. The download-heavy residential model also fits the asymmetry.
Downstream FEC uses RS(255,239), about 93.7% efficiency, so roughly 6.3% becomes correction overhead and net throughput is about 2.33 Gbps. Upstream usually skips FEC to save latency.
Move to XGS-PON (10 G both ways) — that is a different PON standard, not a tunable GPON parameter.
For a GPON rollout, Rayin's GPON OLT is a compact, standards-based choice, running 2.488/1.244 G per port with DBA and downstream FEC built in.
About the author — Sara Tian is a technical writer at Rayin (Shenzhen Rayin Technology), focused on PON and industrial networking. Connect with Sara on LinkedIn.
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