With Passive Optical Networks a passive optical coupler either mixes the light at different wavelengths or conversely splits or decouples the components of the light. Some loss can be expected at the point where two or more fibres are joined together as some light spills from the fibre due to local disruption of TIR.
A PON physically comprises fairly short runs of fibre from subscribers which are joined at a local passive optical coupler to connect to the main Telecommunications Operator fibre. At the present time a coupler can take up to 32 subscriber fibres, and the light from those fibres being multiplexed onto the Telco fibre.
In the opposite direction, light from the Telecommunications operator is distributed to all connected subscribers to ensure they all get the same information at the same levels.
The aim by most Telcos in delivering data services, and indeed triple play services is to have a consistent data link layer with Ethernet being the favoured candidate. Ethernet First Mile services using a PON is becoming popular with the term EPON (Ethernet Passive Optical Network) being used.
The passive optical coupler does not directly replicate a standard Ethernet segment, because the ONU (Optical Network Unit) at the subscriber site cannot communicate with other ONUs. Traffic from an individual ONU is unicast to the Telco OLT (Optical Line Termination) and broadcast from the OLT out to ONUs. ONUs cannot detect collisions, only the OLT can do this. This restricts the system so that only one Optical Network Unit can transmit at any one time. For this reason, the OLT must enforce a means of scheduling, and a number of scheduling methods are undergoing evaluation including:
Grant / Request methods
IEEE 802.3 MAC control
In summary, a PON is a very efficient way to deliver access subscriber services, particularly when WDM (Wave Division Multiplexing) is used. WDM can provide many high bandwidth channels over a single fibre optic connection and is flexible in what type of data can be carried. A big plus is the fact that these Passive Optical Couplers do not require any power in the field which is a big cost saving for carriers and Telcos..