G-PON Encapsulation Method (GEM) defined as part of ITU-T G.984.3 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.
Frame structure of GEM frame LayerPayload length indicator (PLI)Port IDPayload type indicator (PTI)Header Error Control (HEC)Fragment payload Length (bits)12 (0-4095 x Bytes)12 (0-4095 )3 13Varries
+PTI codeMeaning000User data fragment, not the end of a frame001User data fragment, end of a frame010Reserved011Reserved100GEM OAM, not the end of a frame 101GEM OAM, end of a frame 110Reserved110Reserved
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.
+GPON layering, OSI model OSI-Layer Examples 5+ApplicationVarious serviceT1/E1DATADATA4Transport❚❚TCP/UDP/QUIC3NetworkIP2Data LinkMPLS❚EthernetATM cellGEM frameGTC frame1PhysicalPON-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). +MPLS over GEMGEM HeaderMPLSLableEXPSTTLPayload
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. + IP over GEMGEM
HeaderIP packetVHLTOSLengthIdFLFrag. offTTLPORTCRCSource AddressDestination AddressPayload
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.
+Ethernet over GEMGEM HeaderEthernet PacketDestination AddressSource AddressLength/typePayloadFrame check sequence
References