The logic reaches from optical network units (ONUs) to an optical
line terminal (OLT) vary. The round trip delays (RTDs) between an OLT and ONUs
also vary depending on time and environment. Therefore, collisions may occur
when ONU sends data in TDMA mode (in this mode, only one of the ONUs connecting
to a PON port sends data at a moment), as shown in Figure 1.
To prevent the collisions, ranging is enabled when an ONU registers
for the first time. The OLT measures the RTD of each ONU in the ranging process
and calculates the equalization delay (EqD) of each ONU to ensure that the
values of Teqd, which is equal to RTD plus EqD, of all ONUs connected to the
same PON port are the same. Therefore, the logic reaches from ONUs to an OLT are
the same, preventing collisions during upstream transmission.
In the ranging process, the OLT must open a window and pause upstream
transmission channels of other ONUs.
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