GPON Tutorial in 200 Words

Gigabit Passive Optical Network (GPON) is standardized by the ITU in its G.984 series. It is used around the world for FTTH and in the US is thought to be the successor to BPON. 10G GPON is covered in another article.

The ITU GPON standard allows up to 2.488Gbps symmetric transmission, but almost all GPON systems are 2.488Gbps down/1.244Gbps up. Downstream from the OLT to the ONTs is broadcast with an ONT grabbing only traffic addressed to it. Upstream is Time Division Multiple Access (TDMA) with each ONT transmitting in turn.

The GPON wavelengths are 1490nm downstream and 1310nm upstream with 1550nm reserved for downstream RF video. The following diagram shows the main elements and outlines upstream and downstream wavelength utilization  in a GPON network.

GPON RF Overlay Network Diagram

GPON is specified for 60km maximum reach with 20km differential reach, but GPON systems typically provide only 0-20km reach owing to the cost of the optics. A single GPON segment can serve up to 128 subscribers, but many carriers use a maximum of 32 subscribers on a single GPON segment. B+ optics provide for 32x split with 20km reach. C+ optics, newly available and expensive, provide for 64x split with the same 20km reach.

Interoperability between GPON OLTs and ONTs has not really materialized. Basic data transmission is possible, but managing a multi-vendor GPON solution is an operational challenge. OMCI is the management protocol used between an OLT and its ONTs (or ONUs).

Finally, here is a video on the GPON Doctor 8000, a GPON protocol analyzer from TELNET-RI. The video has an interesting animation of a GPON network.

[youtube C_tMJj_X-1U]

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