System : Passive Optical Network (PON)

Passive Optical Network (PON)
A passive optical network (PON) combines, routes, and separates optical signals through the use of passive optical filters that separate and combine channels of different optical wavelengths (different colors). The PON distributes and routes signals without the need to convert them to electrical signals for routing through switches.

PON networks are constructed of optical line termination (OLT), optical splitters and optical network units (ONUs). OLTs interface the telephone network to allow multiple channels to be combined to different optical wavelengths for distribution through the PON. Optical splitters are passive devices that redirect optical signals to different locations. ONU’s terminate or sample optical signals so they can be converted to electrical signals in a format suitable for distribution to a customer’s equipment. When used for residential use, a single ONU can server 128 to 500 dwellings. In 2001, most PON’s use ATM cell architecture for their transport between the provider EO or point of presence (POP) and the ONU (in some case even to the user workstation). When ATM protocol is combined with PON system, it is called ATM passive optical network (APON).

Figure 1 shows an APON that locates ONUs near residential and business locations. This network uses ATM protocol to coordinate the PON. ONU interfaces are connected via fiber to an OLT located at the provider’s EO or POP. Each ONU multiplexes user channels (between 12 and 40) into an optical frequency spectrum allocated to that ONU. Up 32 ONU’s can share access to a single PON using the features of dense wave division multiplexing (DWDM). Some newer PON’s use high density wave division multiplexing (HDWDM). Use of HDWDM increases the number of ONU’s per PON from 32 to 64. This diagrams shows that a PON that uses HDWDM can support approximately 2500 residential customers.

Figure 1: Passive Optical Network (PON)

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