Optical Access Networks
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13.Optical Access Networks The selection of the correct process during construction of the access network is decisive for the subsequent capability of the operator to quickly, cheaply and reliably provide such services which clients will be willing to pay for. Various technologies and configurations of access networks with all available types of transmission environments (i.e. by means of metallic and optical cables as well as by means of radio connection) have been discussed recently. The definitely most perspective for building the access networks are the multi-point PON/AON type (Passive Optical Network/ Active Optical Network) optical access networks (OAN). They feature a high flexibility in connection with the possibility of subsequent introduction of the new services into the access networks. The used optical fibres have a transmission capacity in the order of tens of Gbit/s, which, however, today is limited, by technical capacities of the used transmission equipment. Introduction of the optical fibre into the access section of FITL (Fibre In The Loop) is one of the basic presuppositions for development of the broadband services. There are the followingadvantageswhy to use the optical technology in the access networks: huge available band width, electrical isolation, high immunity to interference, cross-talks and jamming, high degree of security against tapping, low transmission attenuation, bigger distance that can be bridged over, low temperature dependency on attenuation (no correction required), cables with lower weight and smaller dimensions, easier installation, smaller space needed for example for cable caps or splices, savings of strategic materials (copper, aluminium). Naturally, there also aredisadvantages: problems with long-distance power supply of the remote equipment, fragility of fibres, miniature dimensions of fibres, problems with splicing and interconnecting the cables, necessity of complex testing procedures, OAN Optical Access Networks The Optical Access Networks OAN must enable not only the transmission of signals of the basic narrow-band telecommunication services but also the broadband distribution and interactive services. The basic configuration of OAN consists of the following functional units: one Optical Line Terminal (OLT), this provides the function of the network interface between the access network and networks of the telecommunication services, at least one Optical Distribution Network (ODN), this includes all optical transmission facilities between the OLT terminal and ONU unit, at least one Optical Network Unit (ONU), this provides functions of the user interface between the user terminal devices and the access network – several Adaptation Units (AU). PON Passive Access Networks The PON passive access network usually is a simple or multistage star for connection of the user terminal devices, private branch exchanges, LAN computer networks or other types of devices of telecommunication or information services to their network elements (exchanges, distribution centres, databases, etc.) or to the Add and Drop Muldex (ADM) devices of the Synchronous Digital Hierarchy (SDH) in the higher level of the access or transport network. 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.