10 I ICT TODAY
The most prevalent form of PON is currently the gigabyte passive optical network (GPON). This point-
to-multipoint access network has a downstream capacity of 2.4 Gb/s and an upstream capacity of 1.2 Gb/s that
is shared among users. GPON utilizes both upstream and downstream data by means of wavelength division multi-
plexing (WDM), using one wavelength for downstream traffic (1490 nm) and another for upstream traffic (1310 nm).
Downstream signals are broadcasted to all premises sharing multiple fibers. Upstream signals are combined using
a multiple access protocol, usually time division multiple access (TDMA). For downstream, the OLT sends frames
of data to the splitter continuously. The splitter divides power into separate paths and sends the same set of frames
to each ONT. The ONT filters out only frames specific to the user and discards all other frames. For upstream, each user
is given a time slot on which data can be transmitted. Upstream traffic is often not continuous but composed of bursts
(Figure 2). Note that encryption is used to keep each user's data secure and private from other users.
FIGURE 2: The GPON principle. Broadcast for downstream (A) and TDMA for upstream (B).
A.
B.
