gem encapsulation

8/12/2019 GEM Encapsulation

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GEM Encapsulation GPON Encapsulation Method (GEM): A data frame transport scheme used in GPONsystems that is connection-oriented and that supports fragmentation of thenetwork administrator data frames into variable sized transmission fragments.

GPON Encapsulation Application

GEM Related Definitions: T-CONT: A traffic bearing object within an ONT that represents a group of logicalconnections, and is treated as a single entity for the purpose of upstreambandwidth assignment on the PON. In the upstream direction, it is used to bearthe service traffic. Each T-CONT corresponds to service traffic of one bandwidthtype. Each bandwidth type has its own QoS feature. ALLOC_ID: Each T-CONT is identified by the ALLOC_ID uniquely. The ALLOC_ID ranges from0 to 4095. It is allocated by OLT i.e. a T-CONT, and can only be used by one ONT

per PON interface on the OLT. GEM Port: A GPON Encapsulation Method (GEM) port is a virtual port for performing GEMencapsulation for transmitting frames between the OLT and the ONT. Eachdifferent traffic-class (TC) per UNI is assigned a different GEM Port. Each T-CONT consists of one or more GEM Ports. Each GEM port bears one kind of service
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traffic i.e. a T-CONT type. GEM Port ID: Each GEM Port is identified by a port ID uniquely. The Port ID ranges from 0 to4095. It is allocated by the OLT, i.e. a GEM port, and can only be used by a single

ONT per PON interface on the OLT.

GPON Technology

Generic Framing Procedure For history reason(it is a long story ^-^), GPON looked to the new ITU-T G.7041Generic Framing Procedure (GFP) as the model for its encapsulation method. GFPallows direct encapsulation of variable length packets without repeating theencapsulation overhead on a regular basis. Detail: 1. The GFP frame overhead, however, was optimized for point-to-point NE-to-NE

links rather type of encapsulated payload frame, whether a frame checksequence (FCS) is used over the payload, and what type of Extension header (ifany) is used.

2. Applications using GFP were typically expected to rely on higher layerprotocols for packet/frame multiplexing, so G.7041defines a second, optionalpayload header (i.e., the Extension header) for communicating channel/portnumber information in applications where its needed.

3. In contrast, since the ONTs and OLT of a PON system can be logically

considered as a single NE with the PON fiber functioning as a backplaneextension, much of this information is already known through provisioning.

4. Since the PON uplink multiplexes multiple client streams, it typically needs aport identifier for each GEM frame. Due to provisioning, the port identifiergives the OLT and ONT adequate knowledge about the type of encapsulatedframe.
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5. Consequently, the GPON standard re-used the basic concepts of GFP butredefined the overhead fields so that their information and bandwidth areoptimized for the PON application.

GEM Frame Header

GEM Frame Header detail

PLI The Payload Length Indicator (PLI) is the primary mechanism for delineating GEMframes and is functionally equivalent to the GFP PLI. The value of the PLI, L, is thenumber of bytes in the fragment payload area of the GEM frame. The PLI of thefirst GEM frame in the downstream GTC frame or upstream burst is located at thebeginning of the payload area. The next GEM frame will begin L bytes after theheader of the current GEM frame. This GEM frame also contains a PLI, and itsvalue can likewise be used to find the beginning of the next GEM frame, and soforth through the remainder of the GPON payload area. GEM Payload Note that since each GEM partition or payload begins with a GEM header, GEMframes can be delineated immediately at the start of each partition or payload.This feature is especially valuable for the OLT, since the OLT does not need to

keep track of the delineation alignment between upstream frames of each ONTseparately. PTI In contrast to GFP, the GEM payload type indicator (PTI) does not identify thetype of client data frame encapsulated into the fragment payload area. It onlyindicates whether this fragment contains the end of the client data frame or if
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this GEM frame is carrying a GEM OAM message. HEC: The header error check (HEC) comprises a 12-bit BCH-2 code followed by a paritybit that allows detection and correction of transmission errors within the GEM

header. The BCH code is a double error correcting BCH (39, 12, 2) code thatcovers all the header bits except the parity bit. The BCH generator polynomial isx12+x10+x8+x5+x4+x3+1. The parity bit is set to provide an even number of 1swithin the header. ORed In order to provide good transition density to correctly delineate a series of idleframes, the 40-bit header is exclusive ORed with the pattern 0x0xB6AB31E055prior to transmitting the frame. - See more at: http://www.fiberoptictel.com/key-technologies-of-gpon-gem/#sthash.bzs6E3Fs.dpuf

gem encapsulation

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