GPON (Gigabit Passive Optical Network)
One way of providing fiber to the home is through a Gigabit Passive Optical Network, or GPON.
Known as a high bandwidth shared fiber access technology, GPON networks are currently the leading form of Passive Optical Networks (PONs), and offer up to a 1:64 ratio on a single optical fiber, meaning, a single fiber from the OLT can deliver video, data and voice signals to up to 64 end users (or residences).
GPON is a point-to-multipoint access mechanism. Its main characteristic is the use of passive splitters in the fiber distribution network, enabling one single feeder fiber from the provider’s central office to serve multiple homes and small businesses.
GPON has a downstream capacity of 2.488Gbps and upstream capacity of 1.244 Gbps that is shared among users. Encryption is used to keep each user’s data secured and private from other users. Although there are other technologies that could provide fiber to the home, passive optical networks (PONs) like GPON are generally considered the strongest candidate for widespread deployments.
TYPICAL GPON NETWORK
the architecture of a basic two wavelength GPON network
It 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 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.
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.
GPON technology is designed to provide the speed and bandwidth required for seamless services including voice over IP, high-speed Internet, and IPTV.
TYPICAL GPON NETWORK WITH RF OVERLAY
The RF overlay incorporates standard and high definition video into the fiber network using the 1550nm transmitter and EDFA. For cost savings, this topology is often used over the GPON IP solution.
GPON provides Triple play services over IP of video, data and voice are often cited as driving user demand for heavier usage of broadband that justifies PON investment. While RF overlay has been popular in some countries and minimizes congestion caused by usage of video services, the convergence of HDTV and IPTV, and the growth in internet cloud services could create demand for bandwidth that exceeds the capacity of gigabit services in future. Teleworking and video conferencing are other applications demanding such triple play capabilities.
Features of GPON Networks
- Provide downstream speeds of 2.5 Gbps and upstream speeds of 1.25 Gbps.
- Support long distances of up to 20 km and unlike copper does not suffer from decreasing performance over distance.
- Standards based and equipment are available from a large and growing number of vendors giving service providers the peace of mind with being locked into a single vendor.
- Inherently secure wherein wiretapping, eavesdropping and other hacking is nearly impossible.
Advantages of GPON Networks
The most obvious advantage of PON networks is that a single shared optical fiber can support multiple users through the use of inexpensive passive optical splitters. In GPON networks, up to 64 ONTs can share one fiber connection to the OLT. This makes Gigabit Passive Optical Network an attractive option for service providers wanting to replace copper networks with fiber, particularly in high-density urban areas.
- Allow service providers to deliver more capacity to carry bandwidth-intensive applications.
- Provide one of the most cost-effective ways for service providers to deploy fiber.
- Provide a future proof mode of access as the speed of the broadband connection is limited by the terminal equipment rather than the fiber itself. Future speed improvements can be achieved via equipment upgrades before any upgrades on the fiber itself.
How far does GPON reach?
The distance between the GPON OLT and the ONT is generally determined by the launch power of the optical transmitter, receive sensitivity of the far end receiver, and the optical loss associated with the optical distribution network (ODN). Optical loss comes primarily from connectors, splices, and splitter.
The total GPON distance is a trade-off between distance and split ratio. For instance, if the split ratio is very low, it allows for longer distances (as far as 40km), but if the split ratio is very high, up to 128 splits enabling more subscribers, the overall distance from the splitter to the ONT is decreased.
Think of it this way:
Say you have a garden hose with water under normal pressure, and you attach a showerhead with 128 holes. Then you plug up all but 3 of those holes, the holes that are open will squirt out water very far (as far as 40km, providing services to those 3 end users). However, if you put on that same showerhead and unplug all 128 of the holes, the water won’t go far (as far as 20km), but it will saturate the immediate area (provide services for up to 128 end users). Important to remember: The over-all water capacity of the hose is not impacted by the distance or split ratio – it will still carry all of the water from the source to the end, no matter what type of showerhead is attached.