Passive Optical Network (PON), comprises a family of Physical Layer (Layer 1) access technologies based on the specifications developed by the Full-Service Access Network(FSAN) initiative for an ATM-based Passive Optical Network scheme produced by an international consortium of vendors and ratified by the ITU-T within the G.983.1 standard (October 1998). A PON is a fibre-optic local loop network without active electronics, such as repeaters, which can be both costly and troublesome. Rather, a PON uses inexpensive passive optical splitter and fibre coupler to deliver signals form the network edge to multiple customer premises. The PON splitters are placed at each fibre junction, or connection, throughout the network, providing a tremendous fan-out of fibre to a large number of end points. By eliminating the reliance on expensive active network elements and the ongoing powering and maintenance expenses associated with them, carriers can realize significant cost savings. PON technology usually used in the local loop to connect customer premises to an all-fibre network.
In fact, an efficient and reliable optical network (PON), depends on appropriate testing and measurement. During the construction phase, proper testing is the only way to guarantee that all the required transmission specifications are met, the network is ready for actual traffic, and subscribers are supplied with the expected service quality. During initial commissioning and subscriber activation, testing and diagnosis can ensure that the whole system operates within the acceptable specifications. When the network is activated and operation begins, the quality of service (QoS) must be tested and monitored to meet up with service-level agreements with subscribers. When problems are detected and diagnosed (e.g. Low signal or no signal), troubleshooting networks help to minimize network downtime, rapidly restore failed services, and efficiently manage network performance.
Figure 1 The architecture of a passive optical-access network
A PON is a point-to-multipoint, fibre-to-the-premises network architecture in which unpowered optical splitters (either splitting in optical power or wavelength) are used to enable a single optical fibre to serve multiple premises. Figure 1 shows the generic PON architecture. A PON does not use any active electronic components (devices consuming power), form the central office (CO) to the consumers’ premises. The network carries a single strand of fibre, which undergoes multiple splits to serve many consumer installations. This splitting is achieved by way of passive splitters. To the side of the local exchange there is an optical line termination (OLT), on the user side there is an optical network optical fibres and one or more splitters (in cascade), a number of ONUs are connected to an OLT in a tree topology. An ONU can be combined with a network termination unit(NT). This produces an optical network termination (ONT). The OLT has the interfaces with the backbone network that supply the services to the users. Hence a PON’s passive part consists of splitters and fibres located within the field. Reasonably complex active components are needed in the local exchange (the OLT) and on the side of users (the ONU/ONTs).
Unlike the point-to-point terrestrial and undersea amplified wavelength-division-nultiplexed (optical amplifier) fibre systems, the point-to-multipoint nature of PON has made the optical diagnosis, performance monitoring, and characterization a challenge. The key tests performed during a PON’s construction include total link loss measurement (optical power budget), optical return loss (ORL) measurement–especially when cable TV (CATV) services are provided, link characterization using an optical time-domain reflectometer (OTDR). During a PON’s opteration, network operators need to detect signal presence, measure them,and verify that they are within acceptable power ranges. Thorough performance assessment, accurate bit-error-rate (BER) measurement can help to define competitive, customer-retaining service-level agreements, and, most importantly, to make sure and sustain them.