In order to avoid upstream transmission collision between many ONUs, ranging is necessary. Since ONUs are located at different distances from OLT, data should arrive in the correct time slots at OLT, so no errors happen. Ranging mechanism creates transmission delay of the necessary length.
Ranging is initiated by sending ranging request message from the OLT to a particular ONU. Based on the reply and RTD (Round-Trip-Delay) the equalization delay is calculated. Then this information is forwarded towards ONU. Now the ONU is located on the same virtual distance from OLT as the other ONUs. Ranging process is shown in Figure 1. Due to ranging, data would have no transmission conflict.
Figure 1.Compensating delays in upstream transmission determined by the ranging process.
Since in the downstream direction all the data is broadcasted to all ONUs/ONTs, it is necessary to implement encryption for the privacy purpose.
Encryption begins from OLT that sends PLOAM message to the ONU. Onu generates key and sends it back to OLT. Since key is sent upstream, it cannot be seen by other ONUs. Downstream data is encrypted using provided key and AES (Advanced Encryption Standard) mechanism. Only ONU that has provided a key can decipher and read data. In order to make system more secure, keys are frequently changed.
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