1-6Figure 1-4 Uplink data transmission in an EPON systemThe Time Division Multiple Access (TDMA) technology is used to transmit uplink data. This ensures thatone optical fiber between the OLT and the POS can transmit data signals from multiple ONUs to theOLT without signal interference.EPON System SecurityDownlink data through the OLT is broadcast to each ONU. To prevent illegal interception of userinformation, each LLID in an EPON system is assigned a unique key, which is updated periodically:In a key update process, the OLT sends a new key request message to an ONU. Upon receiving thenew key request message, the ONU sends a new key notification message back to the OLT.In a key update process, an OLT uses the key update timer and encryption response timer:1) OLT key update timerThis timer is used to control the key update cycle. When the key update timer expires, the OLT sendsanother key request message to start another key update process.2) OLT encryption response timerThis timer is used to start another key update process when the OLT receives no new key notificationmessage, thus making the key update more reliable.Upon sending a key update request message, the OLT starts the encryption response timer:z If the OLT receives a correct new key notification message from an ONU before the timer expires,the OLT enables the new key and cancels the timer.z If the OLT receives no new key notification message before the timer expires, the OLT considersthe key update process has failed, resets the timer, and sends another key update requestmessage. Before the key update succeeds, an ONU keeps using the original key and the OLTreports the key update failure information to the network management system.z If the OLT receives no new key notification message within the encryption response timer durationafter sending three new key request messages consecutively, the OLT sends an alarm to thenetwork management system and the old key continues to be used for downlink data. In this case,you are recommended to adjust the value of the encryption response timer.EPON System ReliabilityTo ensure high reliability for the trunk fibers and OLTs in an EPON system, you can add two OLT portson one EPON card or on two different EPON cards to a fiber backup group. When a system fault occurs,
