Growing demand for high speed internet drives the new access technologies which enable experiencing true broadband. This leads telecommunication operators to seriously consider the high volume roll-out of optical fiber based access networks. In order to allow faster connections, the optical fiber gets closer and closer to the subscriber. Then FTTH (Fiber To The Home) appears the most suitable choice for a long term objective because it will be easier to increase the bandwidth in the future if the clients are wholly served by optical fibers. FTTH is a future-proof solution for providing broadband services.
Passive optical network (PON) based FTTH access network is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 32-128. The GPON FTTH access network is highly emphasized in this article.
Taking advantages of WDM (wavelength division multiplexing), PON uses one wavelength for downstream traffic and another for upstream traffic on a single fiber. The OLT (Optical Line Terminal) is the main element of the network. Placed in the Local Exchange, OLT is the engine that drives FTTH system. OLT performs the function of traffic scheduling, buffer control and bandwidth allocation. The optical splitter splits the power of the signal and enables sharing of each fiber by many users. ONT (Optical Network Terminal) is deployed at customer’s premises and connected to the OLT through optical fiber and no active elements are present in the link.
With a tree topology, GPON is able to maximize the coverage with minimum network splits, thus reducing optical power. A FTTH access network comprises five areas, namely a core network area, a central office area, a feeder area, a distribution area and a user area as shown in the following diagram.
The core network includes the ISP (internet service provider) equipment, PSTN (packet switched or the legacy circuit switched) and cable TV provider equipment. The main function of the central office is to host the OLT and ODF and provide the necessary powering. The feeder area extends from ODF (optical distribution frames) in the CO (central office) to the distribution points. Distribution cable connects level-1 splitter with level-2 splitter. Level-2 splitter is usually hosted in a pole mounted box placed at the entrance of the neighborhood. In the user area, drop cables are used to connect the level-2 splitter to the subscriber premises.
The data is transmitted from OLT to ONT in downstream as a broadcast manner and as a time division multiplexing (TDM) in upstream. The wavelength of the downstream data is 1490 nm. Core network data services transported over the optical network reaches the OLT and then distributed to the ONTs through the FTTH network by dint of power splitting. Every home receives the packets intended to it through its ONT. The upstream represents the data transmission from the ONT to OLT and the wavelength is 1310 nm. If the signals from different ONTs arrive at the splitter input at the same time and at the wavelength 1310 nm, it will lead to superposition of different ONT signals when it reaches OLT. Thus TDMA is adopted to avoid the interference of signals from ONTs. In TDMA time slots will be provided to each user on demand for transmission of their packets. At the optical splitter packets arrive in order and they are combined and transmitted to OLT.
This paper presents the components, architecture, and traffic flow in GPON FTTH access network. The content may not be detailed, but GPON FTTH network architecture is indeed reliable, scalable, and secure. It is a passive network, so there are no active components from the CO to the end user, which dramatically minimizes the network maintenance cost and requirements. It is a future-proof architecture.