Engineering:G-PON encapsulation method
| Gigabit-capable passive optical networks (G-PON) Transmission convergence layer specification: encapsulation method | |
| Status | Active |
|---|---|
| Year started | 22 February 2004 |
| First published | 22 February 2004 |
| Latest version | 3.0 13 January 2014 |
| Organization | International Telecommunication Union Telecommunication Standardization Sector |
| Domain | Telecommunication |
| License | Freely available |
| Abbreviation | GEM |
| Website | https://www.itu.int/rec/T-REC-G.984.3 |
G-PON Encapsulation Method (GEM) defined as part of ITU-T G.984.3[1] is the method used to package and transport diverse user-plane and management traffic such as Ethernet, TDM and OMCI messages over a Gigabit-capable Passive Optical Network. The encapsulation mechanism is defined as part of the GPON transmission convergence layer and uses the GEM to map variable-length client frames into GPON payload units for transport between an optical line termination (OLT) and optical network units/terminals (ONUs).
Overview
The encapsulation objective is twofold:
- Provide an efficient, flexible framing that carries multiple service types across a shared downstream/upstream medium
- Support per-flow quality of service, fragmentation, and orderly reassembly.
GEM decouples client service formats from the underlying GPON framing so that Ethernet frames, ATM cells, or generic payloads can be carried transparently while the GPON transmission convergence (GTC) layer handles multiplexing, operation, administration and maintenance (OAM), synchronization and bandwidth allocation.[1]
Frame structure of GEM frame
| Layer | Payload length indicator (PLI) | Port ID | Payload type indicator (PTI) | Header Error Control (HEC) | Fragment payload |
|---|---|---|---|---|---|
| Length (bits) | 12 (0-4095 x Bytes) | 12 (0-4095 ) | 3 | 13 | Varries |
| PTI code | Meaning |
|---|---|
| 000 | User data fragment, not the end of a frame |
| 001 | User data fragment, end of a frame |
| 010 | Reserved |
| 011 | Reserved |
| 100 | GEM OAM, not the end of a frame |
| 101 | GEM OAM, end of a frame |
| 110 | Reserved |
| 110 | Reserved |
Transport of user traffic over GEM
Depending on Service type selected for the GEM Port-ID, both OTL and ONU know how to handle the payload, and can save bandwidth by not encapsulation Ethernet. The GEM model is placed in layer 2.
| OSI-Layer | Examples | ||||||
|---|---|---|---|---|---|---|---|
| 5+ | Application | Various service | T1/E1 | DATA | DATA | ||
| 4 | Transport | ❚ | ❚ | TCP/UDP/QUIC | |||
| 3 | Network | IP | |||||
| 2 | Data Link | MPLS | ❚ | Ethernet | |||
| ATM cell | GEM frame | ||||||
| GTC frame | |||||||
| 1 | Physical | PON-PHY | |||||
Multiprotocol Label Switching
MPLS packets are carried directly in GEM payloads. ; an MPLS packet (or fragment) maps to one or more GEM frames according to the general fragmentation rules, a GEM frame shall carry no more than one MPLS packet (or MPLS fragment).[1]
| GEM
Header |
MPLS | ||||
| Lable | EXP | S | TTL | Payload | |
IP packets
IP packets are carried directly in GEM payloads; an IP packet (or fragment) maps to one or more GEM frames according to the general fragmentation rules, a GEM frame shall carry no more than one IP packet (or IP fragment). To make Ethernet work proxy ARP is used, for upstream/downstream devices.[1]
| GEM
Header |
IP packet | ||||||||||||
| V | HL | TOS | Length | Id | FL | Frag. off | TTL | PORT | CRC | Source Address | Destination Address | Payload | |
Ethernet
Ethernet is directly mapped on GEM packet, only Ethernet layer 2 packets are used, layer 1 part (Preamble, Start frame delimiter, Interpacket gap) is not generated.[1]
| GEM
Header |
Ethernet Packet | ||||
| Destination Address | Source Address | Length/type | Payload | Frame check sequence | |
References
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