GPON

Gigabit Passive Optical Network (GPON) is a high bandwidth shared fiber access technology that is used around the world for Fiber to the Home (FTTH) and, at least in North America, is thought by many to be the successor to BPON. GPON technology is especially popular with large US-based telcos, especially Verizon, though it is used by the MSOs as well (often for serving business customers as a complement to their Hybrid Fiber Coax (HFC) networks). 10G GPON is a higher speed version of GPON that has yet to be standardized by the ITU-T. EPON, 10G EPON, WDM PON, and BPON are Passive Optical Network (PON) technologies that are also vying for carrier attention.

GPON is standardized by the ITU-T in its G.984 series (see end of this article for details), but widespread interoperability between different vendors’ equipment has not materialized. Basic data transmission is readily achievable. Managing a multi-vendor GPON solution is quite an operational challenge, however.

GPON Architectures

There are three main components in a GPON access network (other than the fiber itself). The GPON Optical Line Terminal (OLT) is the network concentrator, usually installed in a Central Office (CO). The splitter (or splitters) allows a single fiber from the CO to be shared among a number of subscribers. The Optical Network Terminal (ONT) serves a single residence, converting optical signals to electrical signals that can be used within the home. Note that the ITU standards call the subscriber device an Optical Network Unit (ONU), and many use ONU to mean an ONT serving several subscribers, which would be common in an installation serving a number of apartments in the same building.

GPON is specified to be a single or dual fiber system, but almost all GPON systems are single fiber like virtually all popular FTTH technologies. There is little reason to use dual fibers, although this option is indeed allowed in the standard.

G.984 allows for 60km maximum reach with 20km differential reach and up to 128 subscribers on a single GPON network. However, GPON systems typically provide only 0-20km reach owing to the cost of the optics. G.984.6 is a new ITU-T specification that provides for a Mid-Span Extender that can increase the reach of GPON beyond 20km to as much as 60km.

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. GPON wavelengths are 1490 nm down and 1310 nm up. RF Overlay is carried downstream on 1550nm. Forward Error Correction (FEC) potentially allows for cheaper optical transceivers, though this cost advantage in the optics comes at the cost of extra complexity and overhead (almost 10% extra overhead) to support FEC.

A GPON network can have two, three, or four wavelengths in use. Two and three wavelength systems are covered below. See my article on RFoG for a description and diagram of a four wavelength system combining GPON and RFoG.

Two Wavelength System

The following diagram shows the architecture of a basic two wavelength GPON network, which is probably the most common implementation. The downstream wavelength is 1490nm and transmits data at 2.488 Gbps. The upstream wavelength is 1310nm and transmits data at 1.244 Gbps.

GPON Network Diagram (Two Wavelengths)

The GPON Optical Line Terminal (OLT) is typically installed in a Central Office (CO), though it could be installed elsewhere. The optical splitter is installed somewhere between the CO and the subscribers. And a GPON Optical Network Terminal (ONT) is installed at each subscriber’s home. Voice, video, and data traffic must all be delivered across the single GPON downstream wavelength. A nice facet of GPON for IP video support is that its downstream is naturally a broadcast medium, and it is very efficient for delivering multicast traffic.

The optics in the GPON ONT for a two wavelength implementation is called a diplexer. See the diplexer diagram below. Diplexers can be implemented with a three dimension bulk optic design (discrete components aligned and welded together manually) or with a Planar Lightwave Circuit (PLC) design (link to a good article by Enablence explaining bulk optics and PLCs). A PLC puts all its optical components on a silicon substrate for a two dimension design, eliminates all the complexity of dealing with a third dimension, and allows for low-cost automated manufacturing.

GPON Diplexer

Three Wavelength System

The architecture of a GPON three wavelength system is identical to that of a two wavelength system with the addition of a third downstream video wavelength on the fiber and the equipment to insert this signal into the fiber. The following diagram shows the architecture of a three wavelength RF Overlay GPON network.

GPON RF Overlay Network Diagram

Note that only up to 32 GPON ONTs are indicated for a single GPON OLT port. This is because of the RF Overlay video signal and not the GPON signal. For 20km reach and 32 subscribers on a single network, the maximum amount of light that a fiber will accept (20 dBm or 100 mW) must be inserted into the fiber by the RF Overlay video equipment, and any additional optical power is just wasted. New C+ optics allow for 64x splits and 20km reach for the GPON signal, but this is no help for the RF Overlay video signal. The RF Overlay transmit signal is already at maximum for 32x split and 20km and only the receiver sensitivity can be improved. This may come in time.

The transceiver in the GPON ONT for a GPON RF Overlay video implementation is called a triplexer (see diagram below). Triplexers are more expensive than diplexers and generally are implemented with a three dimensional bulk optic design (discrete components aligned and welded together manually). PLC circuits not as common for triplexer implementations though Enablence describes a PLC triplexer.

GPON Triplexer

The OLT does nothing with the third RF Overlay wavelength other than filter it out. The ONTs merely convert the 1550nm optical signal to an electrical signal for delivery throughout the home over 75 ohm coax. The thorniest issue to solve with a three wavelength signal is how to get the upstream data for controlling the RF Overlay signal to the headend. One option is to convert the traffic into IP and send it upstream on the 1310nm upstream wavelength, though this method has some limitations. A more robust (and more expensive) option is RFoG, which is described in my article on RFoG.

Transmission

The ITU GPON standard allows up to 2.488Gbps symmetric transmission, but almost all GPON systems are 2.488Gbps down/1.244Gbps up. Both downstream and upstream bandwidth is shared although in different ways.

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 (with perhaps multiple turns per ONT). A single ONT can have multiple upstream timeslots, and each timeslot can be a different size. Additionally, Dynamic Bandwidth Assignment (DBA) allows for real-time changes in upstream timeslot sizes to accommodate varying traffic conditions. A typical implementation has one upstream timeslot for management, one for voice, and one for data traffic for each ONT. A GPON network with 32 ONTs may have about 100 upstream timeslots in use.

GPON natively supports Ethernet (GFP), ATM, and TDM, but most systems run just Ethernet (sort of like a souped up EPON). Upstream and downstream frames are transmitted 8000 per second (the downstream frame is twice the size of the upstream frame), which provides a nice 8 kHz signal to the ONTs for POTS service (required for good fax speeds).

OMCI

ONT Management and Control Interface (OMCI) is the management protocol used between the OLT and the ONTs. With OMCI, external management systems do not have to communicate directly with the ONTs. OMCI allows a single IP address to be used to manage an OLT and, through OMCI, all of its associated ONTs. This is very efficient for IP address conservation, and it reduces the load on a management system, but it does require the implementation of a technology-specific management protocol. If there is a VoIP implementation in the ONTs, it is likely they will require separate management and IP addresses anyway. Owing to the popularity of VoIP in these systems, IP address conservation with OMCI is of dubious benefit in many GPON implementations.

Standards

GPON is standardized by the ITU-T in its G.984 series. The list below provides links to the relevant ITU-T GPON standards.

• G.984.1, General characteristics, [A general overview. Easy reading.]
• G.984.2, Physical Media Dependent (PMD) layer specification, [Optics, mostly irrelevant.]
  • G.984.2 Amendment 1, [B+ optics, most common implementation.]
  • G.984.2 Amendment 2, [C+ optics and some tweaks. Allows 64x splits with 20km reach.]
• G.984.3, Transmission convergence layer specification, [The protocols. Pretty darn technical.]
  • G.Imp.984.3, Implementators’ Guide for ITU-T Rec. G.984.3,
• G.984.4, ONT management and control interface specification, [OMCI. Yawn.]
  • G.Imp.984.4, Implementor’s Guide for ITU-T Rec. G.984.4,
• G.984.5, Enhancement band, [Next generation PON compatibility.]
• G.984.6, Reach extension. [Increased range using an active mid-span extender.]