PON (Passive optical network) 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 32-128 premises. FTTH network exploits the low attenuation and high bandwidth of single-mode fiber to provide many times more bandwidth than currently available with existing broadband technologies. The components and architecture of GPON FTTH access network will be explained in this paper.
Components of GPON FTTH Access Network
Optical line terminal, optical splitters and optical network terminal are the three components of GPON FTTH access network.
OLT (Optical Line Terminal)
The optical line terminal is the main element of the network and it is usually placed in the local exchange. It is the engine that drives FTTH system. Traffic scheduling, buffer control and bandwidth allocation are the most important functions of optical line terminal. Typically, OLT operates using redundant DC power and has at least 1 Line Card for incoming internet, 1 System Card for on-board configuration, and 1 to many GPON cards. Each GPON card consists of a number of GPON ports.
The optical splitter splits the power of the signal. That is to say, each fiber link entering the splitter may be split into a given number of fibers leaving the splitter. Usually, three or more levels of fibers correspond to two or more levels of splitters. This enables sharing of each fiber by many users. The passive optical splitter has the characteristics of broad operating wavelength range, low insertion loss and uniformity, minimal dimensions, high reliability, and supporting network survivability and protection policy.
ONT (Optical Network Terminal)
ONT is deployed at customer’s premises. It is connected to the OLT by means of optical fiber and no active elements are present in the link. In GPON, the transceiver in the ONT is the physical connection between the customer premises and the central office OLT.
Architecture of GPON FTTH Access Network
With a tree topology, GPON maximizes the coverage with minimum network splits, thus reducing optical power. An FTTH access network comprises five areas, which are a core network area, a central office area, a feeder area, a distribution area and a user area (see the picture below).
The core network includes the internet service provider ISP 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 (optical distribution frames) and provide the necessary powering. Sometimes it might even include some of the components of the core network.
The feeder area extends from ODF in the central office to the distribution points. These points, usually street cabinets, called Fiber Disruption Frames FDT where level-1 splitters usually reside. The feeder cable is usually connected as ring topology starting from a GPON port and terminated into another GPON port as shown in the picture above to provide type B protection.
Distribution cable connects level-1 splitter (inside the FDT) with level-2 splitter. Level-2 splitter is usually hosted in a pole mounted box called Fiber Access Terminal FAT usually placed at the entrance of the neighborhood.
In the user area, drop cables are used to connect the level-2 splitter inside the FAT to the subscriber premises. For ease of maintenance, usually an aerial drop cable is terminated at the entrance of the subscriber home with a Terminal Box TB, then an indoor drop cable connects the TB to an Access Terminal Box ATB reside inside the home. Finally a patch cord connects the ONT to the ATB.
The basic components of the GPON FTTH access network are presented and the contribution of each component to the architecture of the FTTH network is addressed. 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. FTTH is future proof solution for providing broadband services.