Optical Access Networks

When designing the PON, it is necessary to take into account the maximum permissible distance between the OLT and the most distant ONU. This distance is determined by the attenuation of optical interfaces between OLT and ONU units that can be bridged over and is based on the properties and length of the used optical fibre, type and number of splitters, splices and connectors. The limiting factors can also be attenuation of reflection of the optical path and the maximum admissible difference of distances between the nearest and furthermost ONU with respect to OLT in which case it is necessary to take into account the signal delay from single from single ONUs.

AON Active Access Network

If we use active elements in the optical distribution network ODN instead of passive branching elements, we obtain an active access network AON (Active Optical Network). The active elements in this case need not necessarily be only the optical branching elements but these can also be muldexes of the plesiochronous digital hierarchy PDH that demultiplex and in the opposite direction they multiplex the signals between single sections of the ODN network. The primary section of the AON active access network can also be formed by the synchronous ring network designated also as an active ring or connection ring with the ADM extraction devices of the synchronous digital hierarchy ADH and with transmission of the STM-N group signal between them.

Compared with the PON passive access networks, the main advantage of the AON active access networks is the bigger distance between the optical line terminal OLT and terminating units ONU that can be bridged over and their simpler extensibility. The main disadvantage is the necessity to provide power supply for all active elements used in the optical distribution network ODN and their more complicated construction. The optical access network provides an efficient and flexible solution for FITL systems in the access networks. It enables a fluent change over from the currently provided services to the future broadband services. It also provides investments into installations of new cable networks, as no changes in their structure are necessary when changing over to the B-ISDN services. It also protects the investments into the customer installations as these devices can be utilised when the new broadband services are introduced. With respect to the modular structure of the terminal user devices, these devices can be fitted just only with the appropriate plug-in module that enables utilisation of the required services.