Tuesday, April 26, 2016

GPON Burst Optical/Electrical Technology

In 10G GPON upstream direction, Time Division Multiple Access (TDMA) is used. An optical network unit (ONU) transmits data only within the allocated timeslots. In the timeslots that are not allocated to it, the ONU disables the transmission of its optical transceiver to prevent other ONUs from being affected. The optical line terminal (OLT) then receives the upstream data from each ONU in a burst manner based on timeslots. Therefore, to ensure normal running of the 10G GPON system.
  • Figure 1 shows the burst transmit function supported by ONU-side optical modules.
  • Figure 2 shows the burst receive function supported by OLT-side optical modules.
Figure 1 Burst transmit function supported by ONU-side 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-side optical modules do not support the burst transmit function, the transmitted signals overlap and distortion occurs.
Figure 2 Burst receive function supported by OLT-side optical modules 

  • 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-side optical modules do not support the burst receive function, the OLT may restore incorrect signals because only the level greater than the threshold is considered valid and the signals with the level lower than the threshold cannot be restored.
NOTE:
In the 10G GPON system, all data is broadcast downstream to ONUs. The transmission requires OLT-side optical modules to transmit optical signals continuously and ONU-side optical modules to receive optical signals continuously. Therefore, these optical modules are not required to support the burst receive and transmit function.


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