# Activation Process of ONU in EPON/GPON/XG-PON/NG-PON2 Networks
## 4 xPON Activation Processes
### NG-PON2 Activation Process  
The activation process is provided by time and wavelength division multiplexing transmission convergence (TWDM-TC) and is defined by the recommendation [28]. In the NG-PON2 standard, there are two options for the PLOAM channel. The in-band option is a classical PLOAM message transmission, and the auxiliary management and control channel (AMCC) option is mandatory for ONUs that do not meet the specified calibration limits for a given upstream wavelength channel (see Figure 2). State O1, Initial state: The ONU is in this state when it is turned on. At this point, scanning and downstream channel calibration occur. The unit can also move to this state when deactivated, or when the emergency stop is on. The transmitter is off and the previously set parameters, such as the ONU-ID, burst profiles and equalization delay, should be deleted. Next, the synchronization machine (see Figure 2) is started. The substate, O1.1, is called Off-Sync. In this state, the ONU searches for downstream synchronization attempts. As soon as the synchronization is finished, the ONU moves to the next substate, O1.2, known as the Profile Learning. When enough information has been gathered, the ONU evaluates the downstream wavelength of the channel. If the channel is suitable for activation, the ONU continues the process and moves to the next state. However, if it is not suitable, it searches for an alternative channel and returns to the O1.1 substate, retaining system and channel information, but discarding information about the burst profile. State of O2-3, Serial number state: In this state, ONU activates its transmitter and tries to tune the upstream wavelength channel in line with the downstream wavelength channel. Once the ONU meets the minimum requirements for calibration accuracy for the required upstream wavelength channel, it receives a request known as an SN in-band grant to send the serial number. The message Serial Number ONU is sent as a response to this request. However, if the ONU does not meet the minimum calibration accuracy, it receives a request to send the AMCC type number. In this case, the AMCC Serial Number ONU PLOAM message is sent as a response to this request. Next, the ONU waits for an OLT response, which may be in the form of an Assign ONU-ID message, a Calibration Requestor an Adjust Tx Wave-length PLOAM message. Depending on the received message or request, the ONU either stays in this state and tunes the transmitter, returns to the initial state O1 so that another TWDM channel can be calibrated or moves to the next state and continues with the activation process. In this state, the ONU starts a discovery timer called TOZ. If this timer expires without the ONU receiving a response from the OLT, it returns to the O1 state. In this case, the unit discards all the accumulated system, channel and burst profile information. State O4, Ranging state: In this state, the ONU responds to the ranging grant. If it receives a burst profile ranging grant from the previous Burst Profile PLOAM message, the FS burst carrying the Registration PLOAM message is transmitted. As soon as the ONU receives the Ranging Time message with the equalization delay, it moves to the next state. In this state, it starts the T01 timer with the recommended duration of 10 seconds. If the timer expires, the ONU deletes the allocated ONU-ID along with all the previously set parameters and returns to the O2-3 state, while retaining the collected profile information. State O5, Operation state: In this state, the ONU is already processing frames in the downstream direction and transmits bursts in the upstream direction as instructed by the OLT. This particular state is divided into two substates. The entry point of this state is O5.1, which is called Associated. The ONU is associated with a specific TWDM channel, and the no Tuning ControlPLOAM message awaits processing. Another substate, O5.2, is called Pending.