Optical Access Networks

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 following advantages why 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 are disadvantages:

• 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.