The logic reaches from ONUs to an OLT vary. Therefore, the time
required for transmitting optical signals over optical fibers is different and
the times when the ONUs receive optical signals is different. In addition, 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.
The OLT must precisely measure the distances between itself and each ONU to
provide a proper timeslot for converged upstream data from all ONUs to prevent
data conflict. In this way, the OLT controls the time for each ONU to send data
upstream.
Ranging process is as follows:
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NOTE:
- The OLT starts ranging for an ONU when the ONU registers with the OLT for the first time and obtains the round trip delay (RTD) of the ONU. Based on the RTD, the OLT calculates the physical reach of this ONU.
- The OLT specifies a proper equalization delay (EqD) for the ONU based on the physical reach.
The OLT requires a quiet zone during ranging to pause the upstream
transmission channel of the ONUs connected to it. The quiet zone is implemented
by emptying BWmap so that no timeslot is allocated for data
transmission.
Ranging Results
RTD and EqD synchronize data frames sent by all ONUs, preventing data conflict on optical splitters. In this way, all ONUs locate at the same logic reach and they send data at specified timeslots, thereby preventing upstream cell conflict.
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