The article discusses the general sequence of PON design, the principles of choosing a topology, provides calculation formulas and reference data on the parameters of optical splitters, gives examples of calculating the insertion loss of splitters and the budget of optical losses in the network.
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The task of PON design, after choosing the active equipment, in general, comes down to the sequence of the following operations: determining the locations of installation of subscriber terminals (ONUs), choosing a network topology, choosing cable routes and places for installing splitters, calculating the loss budget for each branch and determining the optimal division factors of all splitters.
If the installation sites of subscriber terminals are easily selected based on the real location of users, then the choice of topology requires the selection of one of several possible options.
In addition to the most common tree topology, in practice there may be options converted to star or bus topologies. The " star " scheme can be used when the subscribers are densely located near the main station. In this case, the splitter is located next to the station room next to the optical line terminal (OLT), which is convenient for maintenance. This scheme is simple and convenient for field measurements and line fault location. However, by analogy with point-to-point, there is no fiber savings. With a sufficiently spaced and uneven location of subscribers, such a scheme may turn out to be ineffective.
The bus topology can be used if the subscribers' homes are on the same line along the optical backbone. The scheme is quite economical, but it assumes a very large difference in the output powers of optical splitters (such as 1/99, 3/97, etc.), which is technologically difficult to implement with good accuracy. It can really be used only with a "linear" arrangement of users along the backbone and only with a small number of stages, otherwise the losses in the splitters will severely limit the transmission range.
The traditional tree topology remains the most popular. The optimal distribution of power between the various branches is solved by a successful selection of the division ratios of the optical splitters. A tree topology is very flexible in terms of potential growth and expansion of the subscriber base. Potential problems can be associated with the complexity of optical measurements, especially from the station side. In general, such a scheme can be recommended for local concentration (clusters) of subscribers in the service area.
The choice of cable routes will be determined by various local factors: the presence of cable ducts, the availability of permission to lay cables in it, the presence of supports (lighting, overhead, etc.) along the cable routes and other issues.
It is recommended to install optical splitters in places convenient for their placement and maintenance: in couplings, distribution cabinets, boxes, optical distribution frames. The easiest to install are open-frame splitters, the dimensions of which allow them to fit into the seat of a protective sleeve in a splice cassette, and bare fibers are spliced with fibers of linear optical cables. Losses in the welded joints of the splitters are much lower than in the detachable joints, and their reliability is higher. Box-type splitters with fiber-terminated connectors are more convenient for further operational measurements. In order to save optical fibers, it is advisable to install them as close to the subscribers as possible, however, the final installation location is determined by the real conditions of the project.
The most important design task is to calculate the loss budget and determine the optimal division ratios for all splitters.
The calculation procedure is as follows:calculation of total losses for each branch without taking into account the losses in the splitters;sequential determination of the division ratios of each splitter, starting with the most distant ones;calculation of the budget of losses for each subscriber terminal, taking into account losses in all elements of the circuit, comparing it with the dynamic range of the system.Since subscribers are usually located at different distances from the headend, then, if the power is equally divided in each splitter, the power at the input of each ONU will be different. The selection of the parameters of the splitters is associated with the need to obtain approximately the same optical power level at the input of each subscriber terminal of the network, i.e. build a so-called balanced network. This is fundamentally important for two reasons. First, for the further development of the network, it is important to have an approximately uniform attenuation margin in each branch of the PON “tree”. Secondly, if the network is not balanced, then signals that are very different in level will arrive at the OLT from different ONUs in the general flow. The detection system is not able to work out significant drops (more than 8-12 dB) of received signals.
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