What's the GPON Frame Structure

GPON frame structure

Upstream GPON Frame

An upstream GPON frame has a fixed length of 125 µs. Each upstream frame contains the content carried by one or more T-CONTs. All ONUs connected to a GPON port share the upstream bandwidth

• All ONUs connected to a GPON port send their data upstream at their own timeslots according to bandwidth map (BWmap) requirements.
• Each ONU reports the status of data to be sent to the OLT using upstream frames. Then, the OLT uses DBA to allocate upstream timeslots to ONUs and sends updates in each frame.

In Figure an upstream GPON frame consists of the physical layer overhead upstream (PLOu), PLOAM upstream (PLOAMu), power level sequence upstream (PLSu), dynamic bandwidth report upstream (DBRu), and payload fields, as described below,

Field	Description	Function
PLOu	Upstream physical layer overhead	Used for frame alignment, synchronization, and identification for an ONU.
PLOAMu	PLOAM messages of upstream data	Used for reporting ONU management messages, including maintenance and management status. This field may not be contained in a frame but must be negotiated.
PLSu	Upstream power level sequence	Used by ONUs for adjusting optical port power. This field may not be contained in a frame but must be negotiated.
DBRu	Upstream dynamic bandwidth report	Used for reporting the T-CONT status to apply for bandwidth next time and for allocating dynamic bandwidths. This field may not be contained in a frame but must be negotiated.
Payload	Payload user data	Can be a DBA status report or data frame. If this field is a data frame, this field consists of a GEM header and frames.

 

Downstream GPON Frame

A downstream GPON frame has a fixed length of 125 µs and comprises physical control block downstream (PCBd) and payload. PCBd mainly consists of the GTC header and BWmap. The OLT broadcasts PCBd to all ONUs. Then, the ONUs receive the PCBd and perform operations based on the information contained in PCBd.

• The GTC header is used for frame delimitation, synchronization, and forward error correction (FEC).
• The BWMap field notifies every ONU of upstream bandwidth allocation. It specifies the start and end upstream timeslots for the T-CONTs of each ONU, ensuring that all ONUs send data using the timeslots specified by the OLT to prevent data conflict.

PCBd contains PSync, Ident, PLOAMd, BIP, PLend, and US BW Map fields, where US BW Map is the upstream bandwidth mapping sent by the OLT for each T-CONT.

Field	Description	Function
PSync	Physical synchronization domain, frame synchronization information	Used by ONUs to specify the start of each frame.
Ident	Identification domain	Used for sorting a frame in the frames of the same type in length sequence.
Downstream PLOAM (PLOAMd)	PLOAM messages of downstream data	Used for reporting ONU management messages, including maintenance and management status. This field may not be contained in a frame but must be negotiated.
BIP	Bit interleaved parity	Used for performing a parity check for all bytes between two BIP fields (excluding the preamble and delimit) to monitor error codes.
PLend	Length of downstream payloads	Used for specifying the length of the BWmap field.
Upstream bandwidth map (US BW Map)	Upstream bandwidth mapping	Used by the OLT for sending the upstream bandwidth mapping to each T-CONT. The BWmap specifies the start and end times for each T-CONT in transmitting data.

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How to deal with GPON Stacks

GPON Protocol Stacks

ITU-T Recommendation G.984.3 defines a new set of frame structures, which consider traditional voice, video, and Ethernet packets as payloads of GPON (GPFD) frames. 

GPON protocol stacks involve the physical medium dependent (PMD) layer and GPON transmission convergence (GTC) layer.

PMD Layer
The GPON PMD layer corresponds to the GPON interfaces between OLTs and ONUs. Parameter values of the GPON interfaces specify the maximum reach and split ratio for a GPON system.

GTC Layer
The GTA layer is used to encapsulate payloads using ATM cells or GEM frames, and GEM frames are commonly used in GPON systems. GEM frames can carry Ethernet, POTS, E1, and T1 cells.
GTC is the core GPON layer, where media access is controlled for upstream service flows and ONUs are registered. Ethernet frame payloads are encapsulated into GEM frames and then packetized as GTC frames. These GTC frames are converted to binary codes for transmission based on interface parameters configured at the physical layer. The process is reversal on the receive end. Specifically, the receive end decapsulates the data to obtain GTC frames, GEM frames, and then payloads for data transmission.

The GTC layer is classified as TC adaptation sub-layer and GTC framing sub-layer by structure.

• The TC adaptation sub-layer involves the ATM, GEM TC, and optical network terminal management and control interface (OMCI) adapters and dynamic bandwidth assignment (DBA) control module. ATM and GEM TC adapters identify OMCI channels by virtual path identifier (VPI)/virtual channel identifier (VCI) or GEM port ID. OMCI adapters can exchange OMCI channel data with the ATM and GEM TC adapters and send the OMCI channel data to OMCI entities. The DBA control module is a common functional module, which generates ONU reports and controls DBA allocation.
• On the GTC framing sub-layer, GTC frames include GEM blocks, PLOAM blocks, and embedded OAM blocks. The GTC framing sub-layer supports the following functions:
  • Multiplexes and demultiplexes data. Specifically, the GTC framing sub-layer multiplexes PLOAM and GEM data into downstream TC frames based on the boundary information specified in the frame header. In addition, the GTC framing sub-layer demultiplexes PLOAM and GEM data from upstream TC frames based on frame header instructions.
  • Generates frame headers and decodes data. The GTC framing sub-layer generates the TC header of downstream frames in a specified format and decodes the frame header of upstream frames. In addition, the GTC framing sub-layer terminates the embedded OAM data encapsulated into the GTC header and uses the OAM data to control this sub-layer.
  • Routes data internally based on alloc-IDs. The GTC framing sub-layer routes the data sent by or to the GEM TC adapters based on internal alloc-IDs.

The GTC layer consists of plane C/M and plane U based on functions.

• The protocol stacks of plane C/M include embedded OAM, PLOAM, and OMCI. Embedded OAM and PLOAM channels are used for managing PMD and GTC sub-layer functions. OMCI provides a unified system for upper-layer sub-layer management.
  • Embedded OAM channels are defined in GTC frame headers for determining bandwidths, exchanging data, and dynamically allocating bandwidths.
  • Dedicated space is reserved in GTC frames for format-based PLOAM channels. The PLOAM channels carry the PMD and GTC management information that does not pass through the embedded OAM block.
  • OMCI channels are used for managing services.
• Service flows on plane U are identified based on service flow types (ATM or GEM) and port ID/VPI. Port IDs identify GEM service flows and VPIs identify ATM service flows. In T-CONTs, bandwidths are allocated and QoS is controlled using the timeslots that can be adjusted.

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GPON Service Multiplexing

GPON encapsulation mode (GEM) ports and transmission containers (T-CONTs) divide a PON network into virtual connections for service multiplexing.

• Each GEM port can carry one or more types of service stream. After carrying service streams, a GEM port must be mapped to a T-CONT before upstream service scheduling. Each ONU supports multiple T-CONTs that can have different service types.
• A T-CONT can be bound to one or more GEM ports, depending on customers' data plan. On the OLT, GEM ports are demodulated from the T-CONT and then service streams are demodulated from the GEM port payload for further processing.

Service Mapping Relationships

• In the upstream direction,
• An ONU sends Ethernet frames to GEM ports based on configured mapping rules between service ports and GEM ports. Then, the GEM ports encapsulate the Ethernet frames into GEM packet data units (PDUs) and add these PDUs to T-CONT queues based on mapping rules between GEM ports and T-CONT queues. Then, the T-CONT queues use timeslots for upstream transmission to send GEM PDUs to the OLT.
• The OLT receives the GEM PDUs and obtains Ethernet frames from them. Then, the OLT sends Ethernet frames from a specified uplink port based on mapping rules between service ports and uplink ports.

• In the downstream direction,
• The OLT sends Ethernet frames to the GPON service processing module based on configured mapping rules between service ports and uplink ports. The GPON service processing module then encapsulates the Ethernet frames into GEM PDUs for downstream transmission using a GPON port.
• GPON transmission convergence (GTC) frames containing GEM PDUs are broadcast to all ONUs connected to the GPON port.
• The ONU filters the received data according to the GEM port ID contained in the GEM PDU header and retains the data only belonging to the GEM ports of this ONU. Then, the ONU decapsulates the data to Ethernet frames and sends them to end users using service ports.

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Pay attention to GPON Networking Applications

GPON is a passive optical transmission technology that applies in FTTx solutions, including fiber to the building (FTTB), fiber to the curb (FTTC), fiber to the door (FTTD), fiber to the home (FTTH), fiber to the mobile base station (FTTM), fiber to the office (FTTO), and fiber to the WLAN (FTTW), for voice, data, video, private line access, and base station access services. Figure 1 shows FTTx networking applications.

The FTTx network applications in GPON access have the following in common: The data, voice, and video signals of terminal users are sent to ONUs, where the signals are converted into Ethernet packets and then transmitted over optical fibers to the OLT using the GPON uplink ports on the ONUs. Then, the Ethernet packets are forwarded to the upper-layer IP network using the uplink port on the OLT.

• FTTB/FTTC: The OLT is connected to ONUs in corridors (FTTB) or by the curb (FTTC) using an optical distribution network (ODN). The ONUs are then connected to user terminals using xDSL. FTTB/FTTC is applicable to densely-populated residential communities or office buildings. In this scenario, FTTB/FTTC provides services of certain bandwidth for common users.
• FTTD: uses existing access media at user homes to resolve drop fiber issues in FTTH scenarios.
• FTTH: The OLT connects to ONTs at user homes using an ODN network. FTTH is applicable to new apartments or villas in loose distribution. In this scenario, FTTH provides services of higher bandwidth for high-end users.
• FTTM: The OLT is connected to ONUs using an ODN network. The ONUs are then connected to wireless base stations using E1. The OLT connects wireless base stations to the core IP bearer network using optical access technologies. This implementation mode is not only simpler than traditional SDH/ATM private line technologies, but also drives down the costs of base station backhaul. FTTM is applicable to reconstruction and capacity expansion of mobile bearer networks. In this scenario, FTTM converges the fixed network and the mobile network on the bearer plane.
• FTTO: The OLT is connected to enterprise ONUs using an ODN network. The ONUs are connected to user terminals using FE, POTS, or Wi-Fi. QinQ VLAN encapsulation is implemented on the ONUs and the OLT. In this way, transparent and secure data channels can be set up between the enterprise private networks located at different places, and therefore the service data and BPDUs between the enterprise private networks can be transparently transmitted over the public network. FTTO is applicable to enterprise networks. In this scenario, FTTO implements TDM PBX, IP PBX, and private line service in the enterprise intranets.
• FTTW: The OLT connects to ONUs using an ODN network, the ONUs connect to access points (APs) using GE for WLAN traffic backhaul. FTTW is the trend in Wi-Fi construction.

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What's GPON transmission?

Introduction to the GPON System

Mainstream PON technologies include broadband passive optical network (BPON), Ethernet passive optical network (EPON), and gigabit passive optical network (GPON). Adopting the ATM encapsulation mode, BPON is mainly used for carrying ATM services. With the obsolescence of the ATM technology, BPON also drops out. EPON is an Ethernet passive optical network technology. GPON is a gigabit passive optical network technology and is to date the most widely used mainstream optical access technology.

• In the GPON network, such as GPFD,  the OLT is connected to the optical splitter through a single optical fiber, and the optical splitter is then connected to ONUs. Different wavelengths are adopted in the upstream and downstream directions for transmitting data. Specifically, wavelengths range from 1260 nm to 1360 nm in the upstream direction and from 1480 nm to 1500 nm in the downstream direction.
• The GPON adopts WDM to transmit data of different upstream/downstream wavelengths over the same ODN. Data is broadcast in the downstream direction and transmitted in the TDMA mode (based on timeslots) in the upstream direction.

GPON Downstream Transmission

All data is broadcast to all ONUs from the OLT. The ONUs then select and receive their respective data and discard the other data.

Main features:

• Supports point-to-multipoint (P2MP) multicast transmission.
• Broadcasts the same data to all ONUs and differentiates ONU data by GEM port ID.
• Allows an ONU to receive the desired data by ONU ID.

GPON Upstream Transmission

In the upstream direction, each ONU can send data to the OLT only in the timeslot permitted and allocated by the OLT. This ensures that each ONU sends data in a given sequence, avoiding upstream data conflicts.

Main features:

• Supports time division multiple access (TDMA).
• Transits data on an exclusive timeslot.
• Couples optical signals on an optical splitter.
• Detects and prevents collisions through ranging.

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What's GEM Frame and T-CONT of GPON?

GEM Frame

In the gigabit-capable passive optical network (GPON) system, a GPON encapsulation mode (GEM) frame is the smallest service-carrying unit and the basic encapsulation structure. All service streams are encapsulated into the GEM frame and transmitted over GPON lines. The service streams are identified by GEM ports and each GEM port is identified by a unique port ID. The port ID is globally allocated by the OLT. Therefore, the ONUs connected to the same OLT cannot use GEM ports that have the same port ID. A GEM port is used to identify the virtual service channel that carries the service stream between the OLT and the ONU. It is similar to the virtual path identifier (VPI)/virtual channel identifier (VCI) of the asynchronous transfer mode (ATM) virtual connection.

A GEM header consists of PLI, Port ID, PTI, and header error check (HEC) and is used for differentiating data of different GEM ports.

• PLI: indicates the length of data payload.
• Port ID: uniquely identifies a GEM port.
• PTI: indicates the payload type. It is used for identifying the status and type of data that is being transmitted, for example, whether the operation, administration and maintenance (OAM) message is being transmitted and whether data transmission is complete.
• HEC: ensures the forward error correction (FEC) function and transmission quality.
• Fragment payload: indicates the frame fragment.

The following section describes the GEM frame structure based on the mapping of the Ethernet service in GPON mode, as shown in Figure 2.

• The GPON system parses Ethernet frames and maps data into GEM payloads for transmission.
• Header information is automatically encapsulated into GEM frames.
• The mapping format is clear and has good compatibility.

T-CONT

Transmission container (T-CONT) is a service carrier in the upstream direction in the GPON system. All GEM ports are mapped to T-CONTs. Then service streams are transmitted upstream by means of OLT's dynamic bandwidth allocation (DBA) scheduling. T-CONT is the basic control unit of the upstream service stream in the GPON system. Each T-CONT is identified by Alloc-ID. The Alloc-ID is allocated by the GPON port of the OLT, and the T-CONTs used by ONUs connected to the same GPON port of OLT cannot have the same Alloc-IDs.kangyu.

There are five types of T-CONT. T-CONT selection varies during the scheduling of different types of upstream service streams. Each T-CONT bandwidth type has its own quality of service (QoS) feature. QoS is mainly represented by the bandwidth guarantee, which can be classified into fixed, assured, non-assured, best-effort, and hybrid modes.

Type 1:

• The fixed bandwidth is reserved for specific ONUs or specific services on ONUs. It cannot be used by other ONUs even if no upstream service streams are carried on the specific ONUs.
• It applies to services that are sensitive to service quality. The services can be TDM or VoIP services.

Type 2:

• The assured bandwidth is available at any time required by an ONU. When the bandwidth required by the service streams on the ONU is smaller than the assured bandwidth, the system can use the DBA mechanism to allocate the remaining bandwidth to services on other ONUs.
• Because DBA is required, this type provides a less real-time performance compared with the fixed bandwidth.

Type 3:

• This type is the combination of the assured bandwidth and maximum bandwidth. The system assures some bandwidth for subscribers and allows subscribers to preempt bandwidth. However, the total used bandwidth cannot exceed the maximum configured bandwidth.
• It applies to VoIP services.

Type 4:

• This type is the maximum bandwidth that can be used by an ONU, fully providing the bandwidth required by the ONU.
• It applies to IPTV and other high-speed Internet services.

Type 5:

This type is the combination of the fixed, assured, and maximum bandwidth. It supports the following functions:

• Reserves bandwidth for subscribers and the bandwidth cannot be preempted by other subscribers.
• Provides the bandwidth to an ONU at any time when required
• Allow subscribers to preempt some bandwidth. (The total used bandwidth cannot exceed the maximum configured bandwidth.)

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What Is GPON?

PON is a point to multi-point (P2MP) passive optical network. GPON, a type of PON technology, is defined by ITU-T Recommendation G.984.x. Figure 1 shows a GPON network.

IFgpon: GPON interface
SNI: service node interface
UNI: user to network interface
CPE: customer premises equipment

• The optical line terminal (OLT:MA5683T) is an aggregation device located at the central office (CO) for terminating the PON protocol.
• Optical network units (ONUs)/Optical network terminal (ONTs:HG8245H) are located on the user side, providing various ports for connecting to user terminals. The OLT and ONUs are connected using an optical distribution network (ODN) for communication.
• The ODN is composed of passive optical components (POS), such as optical fibers, and one or more passive optical splitters. The ODN provides optical channels between the OLT and ONUs. It interconnects the OLT and ONUs and is highly reliable.

As the wide use of broadband services and fiber-in and copper-out development, carriers require a longer transmission reach, higher bandwidth, reliability, and lower operating expense (OPEX) on services. GPON supports the following functions to meet these requirements:

• Longer transmission distance: The transmission media of optical fibers covers up to 60 km coverage radius on the access layer, resolving transmission distance and bandwidth issues in twisted pair transmission.
• Higher bandwidth: Each GPON port can support a maximum transmission rate of 2.5 Gbit/s in the downstream direction and 1.25 Gbit/s in the upstream direction, meeting the usage requirements of high-bandwidth services, such as high definition television (HDTV) and outside broadcast (OB).
• Better user experience on full services: Flexible QoS measures support traffic control based on users and user services, implementing differentiated service provisioning for different users.
• Higher resource usage with lower costs: GPON supports a split ratio up to 1:128. A feeder fiber from the CO equipment room can be split to up to 128 drop fibers. This economizes on fiber resources and O&M costs.

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