Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches. On a GPON link, the trick that makes many services share one fiber is not magic — it is a fixed 125 microsecond frame period and a clever encapsulation method called GEM.
The GPON TC (Transmission Convergence) layer runs on a 125 us frame period. That number is inherited directly from legacy TDM's 8 kHz cadence (1/8000 = 125 us, the same lineage as G.992/G.984). Every 125 us, the OLT sends one downstream frame and receives a slice of upstream burst.
The benefit of fixed frames is practical: slot positions are fixed, planning is simple, and jitter is controllable — which is friendly to timing-sensitive services like voice and TDM.
GEM (GPON Encapsulation Method) is the generic way GPON packs any service into the PON. Ethernet frames, TDM timeslots, or anything else is first wrapped into a GEM frame, then placed into the payload of the downstream TC frame. Unlike the older ATM encapsulation, GEM carries variable-length packets directly, so it is efficient.
| Field | Bits | Purpose |
|---|---|---|
| Port-ID | 12 | Identifies the GEM Port — a logical service channel |
| Length | 12 | Payload length indicator |
| PTI | 3 | Payload Type Indicator — user data vs OMCI vs management |
| HEC | several | Header error check / correction |
The Port-ID is the key: the OLT and ONU use it to deliver a GEM frame to the right service channel. PTI lets the OLT tell at a glance whether this is user data or the OMCI management channel — and OMCI itself rides a GEM channel.
These three concepts are easy to confuse; remember the division of labor:
GEM Port: the service encapsulation channel — "which service does this data belong to."
T-CONT: the upstream bandwidth scheduling unit — the OLT allocates upstream slots by it.
Alloc-ID: the identifier of the T-CONT — the OLT uses it to say "this upstream bandwidth goes to which T-CONT."
The relationship: multiple GEM Ports can map onto one T-CONT, and the T-CONT is scheduled by the OLT through its Alloc-ID. The "GEM Port + T-CONT + Alloc-ID" string in an OLT provisioning template is exactly the configuration entry point for this mapping — binding service, bandwidth, and scheduling into three layers.
Ethernet frames vary in size, so GEM uses two operations to fit the fixed-frame structure:
Fragmentation: one over-long Ethernet frame may span multiple GEM frames, with header fields marking the boundaries; the receiver reassembles.
Concatenation: several small packets can be packed into one GEM frame's payload, improving fill efficiency and reducing waste on small packets.
These two techniques are what let variable traffic lie neatly inside the 125 us fixed rhythm.
Rayin GPON OLTs follow ITU-T G.984 GEM encapsulation and T-CONT scheduling on the PON port. At provisioning, the "GEM Port + T-CONT + Alloc-ID" template binds services to upstream bandwidth, so data, voice, and management do not cross streams. Pairing the central office with industrial switches covers the whole link from the field to the equipment room. See the company profile for background.
No. A GEM Port is a service encapsulation channel; a T-CONT is an upstream bandwidth scheduling unit, linked through the Alloc-ID. One T-CONT can host multiple GEM Ports, sharing one upstream bandwidth segment for several services.
It aligns with TDM's 8 kHz for fixed slot planning and low jitter, which is friendly to voice/TDM services and simplifies OLT scheduling.
PTI roughly separates the user-data channel from the management (OMCI) channel, so the OLT routes service data and management messages to different processing paths.
If it exceeds a single GEM frame, it is fragmented and reassembled at the receiver; small packets can be concatenated into one GEM frame to improve fill efficiency.
"GEM Port + T-CONT + Alloc-ID" is the configuration entry for the encapsulation-and-scheduling mapping — not decoration, but the actual binding of service, bandwidth, and scheduling.
GPON nails the upstream/downstream rhythm to a 125 us fixed frame, then uses GEM to encapsulate services into Port-ID-identified channels, scheduling bandwidth via T-CONT/Alloc-ID.
The "GEM Port + T-CONT + Alloc-ID" template is the real configuration entry for this mapping.
Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches, and its GPON OLTs implement G.984 GEM encapsulation and T-CONT scheduling.
About Rayin → https://www.szrayin.com/Profile/
Written by Sara, Customer Manager at Rayin — over 10 years in communications, focused on helping ISPs and factories validate and maintain PON and industrial-switch networks for emerging markets.
