(Passive Optical Network) An optical point-to-multipoint access network. There are no optical repeaters or other active devices in a PON, hence the name "passive." PONs are designed for local loop transmission rather than long distance and serve to bring fiber closer to the customer in order to obtain higher speed. PONs began in 1995 when a group of telecom providers organized the Full Service Access Network group (see FSAN).

An optical line terminal (OLT) device resides in the telco central office or cable company head end. It generates or passes on SONET and DWDM signals via fiber to an optical network unit (ONU) in the field. The ONU provides the optical to electrical (O-E) and electrical to optical (E-O) conversion between the fiber and the copper wires that reach homes and offices in a "fiber to the curb" (FTTC) or "fiber to the neighborhood" (FTTN) scenario. When the optical line goes directly into the building for "fiber to the home" (FTTH), an optical network terminal (ONT) is used to terminate the fiber. Fiber to the home is also called "fiber to the premises" (FTTP).

APON, BPON, EPON and GPON
APON (ATM PON) was the first passive optical network and uses ATM for transport. BPON (Broadband PON) includes APON, Ethernet and video transports. GPON (Gigabit PON), which uses the SONET GPF frame, is designed to be efficient for packets as well as TDM. BPON and GPON are the ITU-T G.983 and G.984 standards respectively. EPON is the IEEE Ethernet standard for PONs.

               BPON       GPON       EPON
               ITU-T      ITU-T      IEEE
               G.983      G.984      802.3
  Classes
  Supported    B,C       A,B,C       PX10, PX20**

  Downstream   155 Mbps  1.25 Gbps   1.25 Gbps
  Speeds       622 Mbps  2.5  Gbps

  Upstream     155 Mbps   155 Mbps   1.25 Gbps
  Speeds       622 Mbps   622 Mbps
                          1.25 Gbps
                          2.5  Gbps


   Maximum Number of ONTs
   Based on Class and Distance

   ODN CLASS    7 km    10 km    20 km

   Class A       16      13       6
   Class B       40      32      15
   Class C      101      81      39

  ** PX10 & PX20 are similar to B and C