# Activation Process of ONU in EPON/GPON/XG-PON/NG-PON2 Networks
## 4 xPON Activation Processes
### GPON Activation States  
The following information is based on the recommendation [26]. State O1, Initialstate: In this state, the ONU switches on, waits for the downstream signal and synchronizes with it afterwards. Initially, a loss of signal (LoS) is set up to indicate the loss of a signal or a frame. It is also important for the synchronization machine of the ONU and OLT in the downstream direction to perform correct synchronization. In the synchronization state, ONU starts in the so-called Hunstate, in which it searches for the physical synchronization (PSync) field. When an error-free PSync array is found, the ONU moves to the next state, called the Pre-sync state, and sets the counter Nto one. The ONU then searches for the next PSync array that follows the previous one. For each error-free PSync array, the counter is incremented by one. If the ONU receives a corrupted PSync, it returns to the Hunt state. If the N counter in the Pre-sync state is equal to M1 (the recommended value for M1 is two), the ONU moves to the Syncstate and begins processing the information from the physical control block downstream (PCBd) header. If the ONU in the Sync state receives M2 (the recommended value for M1 is five) consecutive frames with a corrupted PSync, it can declare the loss of the downstream signal and return to the Hunt state. The ONU then deletes all transmission convergence (TC) layer-based parameters known from the previous session such as: ONU-ID, default allocation identifier (Alloc-ID), delay compensation and Burst Headerparameters. Once the downstream transmission is received, the LoS and loss of frame (LoF) are cleared, and the ONU moves to the O2 state. State O2, Standbystate: After State O1, the synchronization in the downstream direction is provided, yet the upstream direction synchronization is required and essential. Downstream transmission is received by the ONU and waits for global network parameters. Once the upstream overhead message is received, the ONU sets up the assigned parameters and moves to the O3 state. State O3, Serialnumber state: In this state, the OLT requests broadcast ONUs to send their serial number. To prevent collisions with the normal traffic, the OLT creates as mentioned above a quiet window with a duration of 250 us by sending a frame with an empty bandwidth map (BWmap) field. Subsequently, the previously mentioned SN request is sent (i.e., a request to send a serial number) with a random delay set between 0 and 48 us. As a reply to the SN request, the ONU uses the serial number ONU message to enable the OLT to examine and detect the serial number. In addition, the OLT uses the AssignONU-ID message to assign the ONU-ID. Once the number is assigned, the ONU moves to the next state. The OLT can also send an Extended Burst Lengthmessage to all connected ONUs and hand over the extended overhead parameters. However, if the ONU receives this message before the request to send the serial number, it ignores such a message. In this state, the TO1 timer is used to cancel any unsuccessful activation attempt by setting the time during which the ONU can remain in this particular state. The recommended TO1 value is 10 s. After such a time, the ONU moves to the O2 state. State O4, Rangingstate: Transmission in the upstream direction from different ONUs must be synchronized with the boundaries of the upstream GTC frame. To ensure the appearance of the ONUs, they are set at the same distance from the OLT, and the equalization delay for each ONU is required. The equalization delay is measured when the ONU is in this state. During this particular state, a quiet window with a duration of 202 us is created. The OLT sends a ranging request, and the ONU replies with a Serial Numbermessage. Furthermore, the OLT sends the Ranging Timemessage, in which the allocated equalization delay is transmitted. Once this message is received by the ONU, it moves to its working state. In such a state, the TO1 timer is used.