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.
Figure 1 Cell transmission without ranging
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.
Figure 2 Cell transmission with ranging
In the ranging process, the OLT must open a window and pause upstream transmission channels of other ONUs.
