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Huawei GPON Features,Basic Concepts(GEM Frame) and Service Multiplexing Principles

Gigabit passive optical network (GPON) is a PON technology that is standardized by the ITU-T Recommendations G.984.x. A GPON device supports high-bandwidth transmission. GPON effectively solves the bandwidth bottleneck problem in the twisted-pair access and meets users demands on high-bandwidth services.

Introduction:

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. Gigabit passive optical network (GPON) resolves the problem by providing:

A longer transmission reach: The optical fiber is used in transmission and the coverage radius of the access layer is 20 km.

A higher bandwidth: The maximum downstream bandwidth is 2.5 Gbit/s and the maximum upstream bandwidth is 1.25 Gbit/s for each subscriber.

Quality of service (QoS) guaranteed for all services: The GPON can carry GPON encapsulation mode (GEM) frames, ensuring better QoS.

Optical split feature: Multiple optical fibers are routed to users after optical splitting from a single optical fiber in the central office (CO). This feature saves optical fiber resources, reduces the number of optical and electrical devices in the CO, and reduces the OPEX.

PON is a type of point to multi-point (P2MP) passive optical network (PON). A PON network consists of three parts, as shown below:

The Optical line terminal (OLT) is an aggregation device located at the central office (CO) for terminating the PON protocol.

Optical network units (ONUs)/Optical network terminal (ONTs) are located on the user side, providing various types of ports for connecting to user terminals. The OLT and ONUs are connected through a passive ODN for communication.

The Optical distribution network (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.

Reference Standards and Protocols:

ITU-T G.984.1: General Characteristics. This protocol mainly describes the basic features and major protection modes of GPON.

ITU-T G.984.2: Physical Media Dependent (PMD) Layer Specification. This protocol mainly describes the PMD layer parameters, including physical parameters (such as the transmit optical power, receiver sensitivity, and overload optical power) of optical transceivers, and also defines optical budget of different levels, for example, the most common Class B+.

ITU-T G.984.3: Transmission Convergence Layer Specification. This protocol mainly describes the TC layer specifications, including the upstream and downstream frame structures and GPON principle.

ITU-T G.984.4: ONT Management And Control Interface Specification. This protocol mainly describes the GPON management and maintenance protocols, such as OAM, PLOAM, and OMCI.

ITU-T G.984.5: Enhancement Band. This protocol mainly describes the GPON wavelength planning, including reserving bands for next-generation PON.

ITU-T G.984.6: Reach Extension. This protocol mainly describes several long reach PON schemes for extending GPON transmission distance.

ITU-T G.988: ONU management and control interface (OMCI) specification.

TR-156: Using GPON Access in the context of TR-101.

Overview of 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.

Working principle of the GPON network:

In the GPON network, 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. The upstream wavelength is 1310 nm and downstream wavelength is 1490 nm.

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:

Downstream communication principle of GPON 

Main features:

1,Supports point-to-multipoint (P2MP) multicast transmission.

2,Broadcasts the same data to all ONUs and differentiates ONU data by ONU ID.

3,Allows an ONU to receive the desired data by ONU ID.

GPON Upstream Transmission:

 Upstream communication principle of GPON 

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.

Basic Concepts

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.

GEM frame structure

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

1,PLI: indicates the length of data payload.

2,Port ID: uniquely identifies a GEM port.

3,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.

4,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 below:

1,The GPON system parses Ethernet frames and maps data into GEM payloads for transmission.

2,Header information is automatically encapsulated into GEM frames.

The mapping format is clear and has good compatibility.

Service Multiplexing Principles

GPON encapsulation mode (GEM) ports and transmission containers (T-CONTs) divide a passive optical network (PON) network into virtual connections for service multiplexing, as shown in Figure 1.

Figure 1 Principle of service multiplexing in the GPON system:

1,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 optical network unit (ONU) supports multiple T-CONTs that can have different service types.

2,A T-CONT can be bound to one or more GEM ports, depending on user's configuration. On the optical line terminal (OLT), GEM ports are demodulated from the T-CONT and then service streams are demodulated from the GEM port payload for further processing.

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