TDMA is used in GPON upstream direction. An ONU transmits data only within
the allocated timeslots. In the timeslots that are not allocated to it, the ONU
immediately disables the transmission of its optical transceiver to prevent
other ONUs from being affected. The OLT then receives the upstream data from
each ONU in a burst manner based on timeslots. Therefore, both OLT and ONU
optical modules must support burst receive and transmit function to ensure
normal running of the GPON system. shows the burst transmit
function supported by ONU optical modules, and shows the burst receive
function supported by OLT optical modules.
Ranging can be implemented to prevent cells transmitted by different ONUs from conflicting with each other on the OLT. However, the ranging accuracy is ± 1 bit and the cells transmitted by different ONUs have a protection time of several bits (not a multiple of 1 bit). If the ONU optical modules do not support the burst receive and transmit function, the transmitted signals overlap and distortion occurs.
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NOTE:
Ranging can be implemented to prevent cells transmitted by different ONUs from conflicting with each other on the OLT. However, the ranging accuracy is ± 1 bit and the cells transmitted by different ONUs have a protection time of several bits (not a multiple of 1 bit). If the ONU optical modules do not support the burst receive and transmit function, the transmitted signals overlap and distortion occurs.
In the GPON system, all data is broadcast downstream to ONUs. The
transmission requires OLT optical modules to transmit optical signals
continuously and ONU optical modules to receive optical signals continuously.
Therefore, these optical modules are not required to support the burst receive
and transmit function.
- The distance from each ONU to the OLT varies and therefore the optical signal attenuation varies for each ONU. As a result, the power and level of packets received by an OLT at different timeslots various.
- If the OLT optical modules do not support the burst receive and transmit function, an error occurs when the optical signals sent by the ONU with a long transmission distance and large optical attenuation are recovered on the OLT because the optical power level is less than the threshold (only the signals with the optical power level greater than the threshold can be recovered). Dynamic threshold adjustment enables the OLT to dynamically adjust the threshold for optical power levels based on the strengths of signals received by the OLT. This ensures that all ONU signals can be recovered.
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