Gigabit Passive Optical Network (GPON) is a Fiber to the Home (FTTH) technology standardized by the ITU-T in its G.984 series. It is popular in North American and around the world, and it competes with EPON, BPON, and WDM PON for carrier attention. GPON provides high bandwidth broadband Internet services and natively delivers ATM, TDM, and Ethernet traffic.

Each of the ITU GPON documents in the G.984 series (G.984.1, G.984.2, G.984.3, G.984.4, G.984.5, G.984.6) is discussed and linked below.

G.984.1, General Characteristics

G.984.1 is a general overview of GPON as specified in the G.984 series of ITU-T standards. It covers network architecture, interfaces, rates, reach, split ratios, and redundancy, among other topics. It is easy reading compared to the other documents in this series.

G.984.2, Physical Media Dependent (PMD) Layer Specification

G.984.2 covers optical specifications for GPON implementations. However, B+ and C+ are the most commonly implemented optical specifications, and neither is covered in this document. It does have some good general information on optics, but available implementations follow the specifications in the two amendments to this document.

G.984.2 Amendment 1 is the specification for GPON B+ optics, which is by far the most common type of optical interface available on GPON equipment. G.984.2 Amendment 2 (a draft document) specifies GPON C+ optics, which add an extra 4 dB to the loss budget on a GPON network compared to B+. Additionally, it covers optical layer supervision and a change to the downstream extinction ratio.

G.984.3, Transmission Convergence Layer Specification

Covering framing, upstream Time Division Multiple Access (TDMA), Physical Layer OAM (PLOAM), Dynamic Bandwidth Allocation (DBA), Optical Network Unit (ONU, equivalent to ONT) activation, Forward Error Correction (FEC), and security, G.984.3 is the core of the GPON specifications. It is quite long and technical, and it is essential reading for vendors of GPON hardware and software. G.Imp.984.3, Implementators’ Guide for ITU-T Rec. G.984.3, clarifies G.984.3 and allows vendors to more easily achieve interoperability.

G.984.4, ONT Management and Control Interface Specification (OMCI)

OMCI, specified in G.984.4, “hides” the ONTs from external network management systems and allows an OLT and all its associated ONTs to be managed as a single entity with a single IP address.  G.Imp.984.4, Implementor’s Guide for ITU-T Rec. G.984.4, clarifies provisioning and management and allows for easier multi-vendor interoperability.

G.984.5, Enhancement Band

G.984.5 specifies optical wavelength usage to support RF Video overlay and next generation PON services like 10G GPON.

G.984.6, Reach Extension

G.984.6 provides specifications for Mid-Span Extenders that provide additional reach on GPON networks. These devices are either regenerators (Optical Electrical Optical or OEO) or optical amplifiers (OA) and are used between an OLT and its optical splitter. Without requiring changes to existing ONT implementations, these devices provide a maximum reach of up to 60 km on a single GPON network . The diagram below shows a G.984.6 Mid-Span Extender in a GPON access network.

G.984.6 Reach Extension

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