4. xPON Activation Processes
The activation process describes the steps in which an inactive ONU connects or reconnects to a PON [26
]. The activation process includes three phases, specifically: parameter learning, serial number acquisition and ranging. During the learning parameter phase, the ONU acquires the operational parameters that are needed in the upstream transmission. During the serial number acquisition phase, OLT discovers a new ONU (by serial number) and assigns an ONU identifier (ONU-ID) to it.
The ONU round trip delay (RTD) is the time interval between the downstream frame transmission and the corresponding upstream transmission burst from the given ONU. The RTD consists of a propagation delay that is directly proportional to the length of the fibers from the ONU and the response of the ONU. To ensure that transmission bursts from different ONUs are ordered at the interface of the same upstream GPON transmission convergence layer (GTC) frame, the delay time is assigned to each ONU to postpone the transfer of the upstream burst to the time not used for a common response time. This response time is called the equalization delay (EqD), and for each given ONU, the OLT is calculated based on the RTD measurement and consequently transmitted during the ranging state.
To avoid collisions with the upstream bursts transmitted during acquisition of the serial number and the range of the newly-connected ONU, the OLT must temporarily suppress the upstream transmission of the active ONU for the time that the arrival of upstream bursts from the new ONU is assumed. This time interval is referred to as the quiet window.
4.1. 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 Huntstate, 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 M1is 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 s 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 s. 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 s 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.
State O5, Operationstate: In this state, the ONU can now send data, physical layer operations and administration and maintenance (PLOAM) messages according to the OLT instructions. Once the network is equalized and all the ONUs are working with the correct equalization delay, all upstream bursts will be synchronized among all ONUs.
State O6, POPUP state: An ONU enters this state when any of the LoS or LoF alarms (if the signal is lost or the frame is poorly assembled) is detected. Therefore, if this condition occurs, the ONUs immediately stop sending data. After the POPUP status occurs, the ONU first attempts to retrieve the optical signal, recover the synchronization of the GTC frame and remove the LoS/LoF alarm. The ONU goes either to the Operationstate or to the Rangingstate according to the particularly targeted POPUP messages. If the ONU receives the targeted POPUP message, it returns to the Ranging State. If the ONU cannot restore the optical signal or reset itself to recover the GTC frame synchronization, it does not receive a targeted POPUP message and is moved to the Initialstate. This is where the TO2 timer is used (the recommended time for the timer is 100 ms).
State O7, Emergencystop state: An ONU that receives a Disable Serial Numberwith the “deactivate” option goes to the emergency stop state and shuts off the laser. During this state, the ONU is not allowed to send any data. If a failure on the deactivated ONU is resolved, the OLT can activate the ONU to return it to its functional state. The activation is accomplished by sending the Disable Serial Number message with the “enable” option. Subsequently, the ONU returns to the Standbystate, and all parameters are discarded and retrieved.
4.2. XG-PON Activation Process
As mentioned in Section 4.1
, the principles of the activation process for XG-PON are basically identical to those for GPON and are defined by the recommendations [26
State O1, initial state: The ONU is in this state immediately upon switching on or after switching from other states when there is an error requiring a return to the initialization state. The transmission is switched off at this time, and all the previously set TC layer parameters (e.g., ONU-ID) are cleared. Synchronization in the downstream direction is provided by the synchronization machine. The ONU starts in the Hunt State, where it uses the downstream signal to search for the PSync pattern stored in the physical synchronization block downstream (PSBd). If it is found, the ONU verifies that a 64-bsuperframe counter (SFC) structure, which is also found in PSBd and secured by a self-repairing hybrid error correction (HEC), is valid. If the SFC is valid, the ONU stores its values and moves to the Pre-Syncstate. With the next successful validation (at this point, only 62 bits out of the total number of received 64 bits are sufficient), the ONU moves to the Sync state. However, if any of these validations fail, the ONU returns to the Hunt state. The unit remains in the Sync state (the unit has already been successfully synchronized) as long as the PSync and SFC authentication are successful. If the authentication fails, the ONU moves to the Re-Sync state. It moves to the Sync state only after successful validation. The recommended value for the M parameter is three. However, if M-1consecutive physical interface (PHY) frames validating the PSync or SFC fail, the ONU declares a loss of synchronization with the downstream frame, discards the saved SFC copy and returns to the Hunt state. This process is illustrated in Figure 1
. Once synchronized with the downstream PHY frame, the ONU moves to the next state.
State of O2-3, Serial Number State: In this state, the ONU activates its transmitter in a burst mode and waits for messages from OLT. The ONU analyzes the PLOAM section of the downstream XG-PON transmission convergence layer (XGTC) frame and begins to learn the burst profile specified in the Profilesection of the message. Upon receiving a serial number grant, it reports with the XGTC frame carrying the Serial Number ONU PLOAM message to send its serial number. As soon as it receives an AssignONU-ID PLOAM message with its serial number, it sets the allocated ONU-ID together with the other assigned parameters and moves to the next state. After receiving the Disable Serial Number PLOAM message (for its serial number or for all ONUs), it moves to the EmergencySTOPState. If OLT already knows the ONU that is returning to the network (e.g., during recovery, power failure, etc.), a problem with the Assign ONU-ID PLOAM message could occur. Therefore, the ONU can go directly to the state called the Ranging State when activated without responding to the serial number grant request.
State O4, Ranging state: In this state, the ONU receives a ranging grant with a known burst profile. Consequently, the XGTC frame containing the RegistrationPLOAM message is transmitted as a response. The ONU analyzes the PLOAM section of the downstream XGTC frame and responds only to the following messages: Profile, Ranging Time, Deactivate ONU-ID and Disable Serial Number. If the ONU receives the Ranging Time message with the absolute equalization delay, it moves to the next state. In this state, the TO1 timer is used to cancel unsuccessful attempts by limiting the time that the ONU can stay in that state. The recommended value for the TO1 timer is 10 s. If it expires, the unit discards the associated ONU-ID, as well as all other parameters, and returns to the Serial Number state.
State O5, Operationstate: The ONU already transmits data and PLOAM messages in the upstream direction as instructed by the OLT. At this point, the OLT can create additional connections with the ONU if they are required. Once the network is in operation and all ONUs are working with their assigned equalization delay, all upstream bursts are synchronized with all ONUs.
State O6, IntermittentLODS state: The ONU will move to this state from the Operation state when it does not synchronize with the downstream signal. Upon entering this state, the ONU will start the TO2 timer (the recommended value for this timer is 100 ms). After the timer expires, the ONU returns to the Initial state.
State O7, Emergency Stop State: The ONU moves to this state if it receives the Disable Serial Number message with the “Disable” option. In this state, it switches off the laser and rejects all TC settings (ONU-ID, equalization delay, burst profiles, etc.). The ONU keeps the downstream synchronization machine running and analyzes the XGTC frames in the downstream direction (at this point, however, it is forbidden to pass any downstream data or send any upstream data). If the problem is resolved, the OLT can re-enable the ONU and bring it back to normal operation by sending the Disable Serial Number with the “enable” option. As a result, the ONU returns to the Initial state.
4.3. 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. While the ONU completes upstream transmission of SDU units whose fragmentation already began in the previous subset, it performs further fragmentation if necessary and transfers any unfragmented SDU units.
State O6, Intermittent LODS state: The ONU can reach this state from the O5 state in the case of downstream synchronization loss. Upon entering this state, the unit turns on the timer. When wavelength channel protection (WLCP) is enabled, the TO3 timer is turned on. If the WLCP is turned off, the TO2 timer is turned on. If the downstream signal is restored before any of the two timers expire, the ONU returns to the O5 state. However, once the TO2 timer expires, the ONU moves to the initial state O1. If, on the other hand, the TO3 timer expires, the ONU moves to the O8 state (to be described; see below).
State O7, Emergency Stop State: The ONU moves to this state if it receives the Disable Serial Number message with the “Disable” option on. In this case, it deactivates the laser. However, it keeps the downstream synchronization machine running and analyzes the PLOAM section of the downstream FS frames (however, at this point it is forbidden to pass any downstream data or send any upstream data). If the ONU receives the Disable Serial Number message with the “enable” option on, it returns to the O1 state.
State O8, Downstream tuning state: In this state, the ONU tries to restore the transmission using the new TWDM channel while maintaining the configuration of the TC layer except for its burst profiles. In this state, the TO4 timer is used. When it expires, the ONU returns to the initial O1 state and discards the TC layer configuration. In the O8.1 substate, also known as Off-Sync, the ONU tunes its receiver and tries to synchronize with the downstream signal. As soon as it is synchronized, it moves to the O8.2 substate, known as Profile Learning. In this state, it analyzes the downstream framing sublayer (FS) frame and starts collecting information about the system, channel and burst profile. When enough information has been gathered, the ONU will evaluate the downstream wavelength of the channel. If this channel is suitable for activation, the ONU continues the activation process and moves to the next state. However, if it is not suitable, it searches for an alternative channel and returns to the O8.1 substate, retaining the system and channel information, but discarding the burst profile information.
State O9, Upstream tuning state: As long as the ONU is in this state, it waits for a feedback from the OLT and performs a fine-tuning of its transmitter. Subsequently, it moves to the O5 state. In this state, the TO5 timer is started. If this timer expires, the ONU returns to the initial state.
4.4. EPON Activation Steps
The following subsection evaluates the EPON activation process according to [24
]. The MPCP defines the autodiscovery mechanism used to detect the newly-connected ONUs, a circular delay and a MAC address [29
]. This process is controlled by the OLT unit, which periodically creates an available discovery window, during which time it gives inactive units the ability to log in to the OLT. This periodicity is not specified by the standard and therefore depends on individual implementation. Autodiscovery uses the following four messages: GATE, REGISTER REQ, REGISTER and REGISTER ACK. These messages are transmitted in the MPCP frame. The autodiscovery process consists of four steps, which are illustrated in Figure 3
Step 1: The Discoveryagent decides to initiate the discovery process and assigns a discovery window (the time interval when none of the initialized ONUs can send data). The Discovery Agentinitiates the discovery process using the Discovery GATE message, which includes the starting time and the length of the slot. During the forwarding of the GATE discovery message, the MPCP stores the OLT time.
Step 2: Only previously uninitialized ONUs respond to the GATE message. After receiving such a message, the ONU sets the local time according to it. If the OLT clock reaches the starting time that is also included in the GATE discovery report, the ONU waits for a randomly selected time and then forwards the REGISTER REQ message. Accidental delays can lead to collisions when initiating multiple ONUs. The REGISTER REQ message contains the ONU source address and the time used to send the message from the ONU. When the OLT receives a REGISTER REQ message, it detects the MAC address and the circular delay.
Step 3: After analyzing and verifying the REGISTER REQ message, the OLT sends the message REGISTER directly to the given ONU using the MAC address obtained during the previous step. The REGISTER message contains a unique logical link identifier (LLID) that is assigned to all ONUs. Next, the OLT sends the GATE message to the same ONU.
Step 4: After the REGISTER and GATE ON messages are received, the REGISTER ACK confirms that an acceptance of the previous messages has been sent. The REGISTER ACK should be sent in the time interval granted by the GATE message.
In this paper, we introduced an activation process for IEEE and ITU PONs. Although both networks are standards for passive optical networks, they use completely different encapsulation methods, frame durations and frame structures. Our results demonstrate that EPONs have a faster activation process (with a maximum split ratio 1:32) and that GPONs operate with a split ratio of up to 1:128. We chose the most commonly-used split ratio, 1:32 or 1:64, with 20 km, because a higher distance does not follow either standard. However, GPON supports a higher split ratio, and there are some issues with the timing and managing of the time slots for the upstream direction. The main reason for the shorter activation time is the different frame durations of 16.384 s and 750 s for EPONs and GPONs, respectively.
XG-PON and NG-PON2 use approximately the same activation process and provide almost identical results as do GPONs. However, NG-PON2 supports wavelength tuning during transmission, but it has to be initialized by the OLT. Due to this fact, it is not a part of the activation process. An activation time of 256 end units requires approximately 820 ms, but the OLT can operate with eight or more NG-PON2 ports, which leads to a multiplication of this time. During a blackout scenario, the last end user will have to wait multiples of minutes.
Our optimization accelerates the activation process for 64 ONUs from 220 ms to 145 ms. In a real network with 512 ONUs, the operator can save approximately 40 ms. Note that our solution does not require a new PLOAM message or TC layer changes.
In future work, we would like to implement a collision scheme into our simulation models.