Here are few questions: Why ranging is required? What is the purpose? What happens if ranging is not performed?
PON upstream transmission uses the TDMA mode. Each OLT can be connected to multiple ONUs. The distance between an ONU and the OLT can be tens of meters at least or 20 km at most. The transmission distance differences cause that signals are transmitted from ONUs to the OLT at different time. Consequently, optical signal overlap, causing data conflicts.
As show in the figure, we expect that all ONUs are on a concentric circle to the OLT so that the signals from all ONUs can be reach the OLT at the same time.
After being sent upstream by each ONU to the public fiber, and insert with specified timeslots, the data will not collide and does not need too long timeslots. The OLT must precisely calculate its distance to each ONU to control the time for each ONU to send upstream data. Based on this idea, the GPON system uses the following method. Based on the farthest ONU, the delay of each ONU to the OLT is calculated first and then the distance of each ONU to the OLT is precisely calculated. A delay is added to an ONU nearer to the OLT to ensure that the period of sending signal from this ONU to the OLT is the same as the period for the farthest ONU. The actual purpose is to ensure that the logical distance of all ONUs to the OLT is the same and from the view of the OLT, the transmission distance of all ONUs is the same. This added delay is called equalization delay (EqD). This is the ranging process.
The OLT obtains t round trip delay (RTD) through the ranging process, and then calculates the physical between the OLT and each ONU. The OLT specifies suitable EqD. The OLT uses RTD and EqD to synchronize the data frames sent by each ONU and ensure that the data sent by each ONU does not conflict on the splitter. That is, the OLT ensures that all ONUs are at the same logical distance and send data in corresponding timeslots. In the ranging process, the OLT needs to open a window, that is, quiet zone, and pause upstream transmission channels for other ONUs. To open a window, the OLT sets BWmap to empty and does not authorized any timeslot. If ranging is not carried out, data frames sent by ONUs may collide on the splitter because of various delays caused by different distances. The OLT uses ranging to allocate different timeslots for each ONU to send upstream data, and equalization delay parameters. In this manner, no collision occurs and the gap is not large after the upstream data of all ONUs is aggregated on the fiber and inserted into the specified timeslots. Each ONU sends upstream data in the specified timeslots according to bandwidth information contained in the BWmap field inside downstream packets sent by the OLT.
