GPON Fibre - ONU (Subscriber 'Modem') Activation

By Michael Spalter
April 2021

About the author

Michael Spalter

Michael Spalter

Michael Spalter has been a networking technician for over 30 years and has been the CEO DrayTek in the UK since the company’s formation in 1997. He has written and lectured extensively on networking topics. If you’ve an idea for a blog or a topic you’d like explored, please get in touch with us.

GPON - ONU Activation

In a previous article on GPON fibre, I focussed on the hardware components which make up a GPON installation: The OLT (active box at the end of the street), the passive splitters (along the street), the ONU - the box in your home or office that terminates the fibre and converts to Ethernet or Wi-Fi and the various cables, plugs and sockets.   If you haven't read that article, I'd recommend reading this one first. This article gets more technical for those of you who want to understand what's happening beyond the physical medium.

Once everything is physically installed and switched on, all of the components that we learnt about previously have to set up a working data path to actually connect you to the Internet. Once active, the connection then has to be maintained for best quality, apply or demarcate any additional services and reconnect the service in the event of interruptions.  Diagnostic facilities also need to be available to the subscriber and the telco (ISP/line provider).

First, a quick reminder of the terms as they are difficult to remember and not at all intuitive.  Refer back here if it gets confusing:

Optical Line Terminal (OLT) - The big head-end equipment at the end of your street or someone in your village - it can be some distance away if you are in a rural area.  One fibre can be split and split again to serve many subscribers from just one port on the OLT.

Optical Network Unit (ONU) - think of that as your "fibre modem" in your home or office. It can also be called an ONT but I'll try to be consistent and stick to ONU.

Activation of an ONU

When you connect an ONU to your fibre or any time that you reboot it, a registration process occurs, just as with any other CPE media (DSL, DOCSIS etc.). The ONU can be in one of seven states, numbered O1-O7. The ONU starts with a Loss of Signal (Los) flag/status set true and then goes through several phases to set up an operational connection:

  1. Init (O1). In the 'hunt state', the ONU continuously searches for a physical synchronisation (PSync) field in the downstream signal.  If detected, it moves to the 'Pre-sync State' and a further PSync field must be detected within 125μs (microseconds) to ensure correct timing. If they are detected without error, then the ONU will move to the sync state.

  2. Standby (O2). In this phase, the ONU acquires the information about the upstream channel in order that it configures itself to comply with the ONTs configuration. Failure to do this could not only cause problems for your own connectivity but interfere with other users on the same ONT port.

  3. Serial number registration (O3). Each ONU has a unique serial number to identify it. The ONU may also submit a password to the ONT. If the ONU is recognised, the ONT will allocate a unique ONU-ID to the ONU which lasts as long as it stays connected. The ONT will also allocate one or more ALLOC_IDs in order to identify different containers (T-CONTs) for the same ONU, each of which may have different service levels or contain different service types (IPTV, internet, VoIP etc.).  Services sensitive to delay (Video, VoIP) will be sent in high priority T-CONTs but this means services provided by your fibre provider in parallel to your Internet connection, not those you're electively receiving over your regular Internet connection.

  4. Ranging (O4).  Every ONU on a shared fibre feed is a different distance from the ONT and whilst the speed of light is fast (300m Km/second) there is still a propagation delay, and the furthest ONUs will have a higher latency than those nearer the ONT. This won't matter to the quality or speed of your service but as GPON frames are measured in μs (microseconds) it's absolutely vital that timing is synchronised. If the timing was off, ONUs would transmit when it wasn't their turn. The ONT therefore measures the Round Trip Delay (RTD) to each ONU and assigns an Equalisation Delay (EqD) to that ONU to apply to all upstream frames according to its distance from the ONT.

  5. The ONU now enters Operation State (O5) and you're online.

During the activation of an ONU, the parameters are exchanged via PLOAM (Physical Layer Operations, Administration and Maintenance) messages. PLOAM is deliberately simple in order that it can be supported by self-contained simple low power processors or microcontrollers in autonomous modules regardless of the status of the connected CPE/host.

A fibre provider/telco will normally only authorise recognised ONUs in order to enforce the use of mandated equipment. Or require you to register a new ONU, if you have replaced your device, to ensure compatibility. Only one ONU can be registered to each customer account (otherwise you could add your own optical splitter and get a 'second connection').

Quiet Windows

The ranging (see state 4, earlier) is vital before an ONU can transmit anything upstream, however, in order to measure the RTD and register its serial number the ONU has to transmit, and without the EqD, it would likely collide with another ONU's uplink frames. Chicken and egg.  To avoid this, the ONT operates periodic 'quiet windows' in which upstream frames are suspended to allow the registration of a registering ONU.  It is during this quiet window that the ONT sends out the serial number registration invitation, which the ONU can respond to. That is addressed to Alloc-ID 0xFE which is an all-station broadcast ID used before an ONU has its own ID allocated.  The ONU responds after a random delay; that is added to avoid a collision with another ONU which tries to register at the same time.

Maintaining Connection

The ONU continuously monitors the connection and if the PSync. fields are no longer detected reliably, the LoS flag is set and the ONU will report loss of connection though obviously only on the LAN side as WAN connectivity is lost. The ONU will enter Popup State (O6) and try to recover but otherwise go back to the hunt state (Init O1).

Loss of Signal (LoS)

When the connection is lost, unlike on copper mediums, there's no electrical signal to detect and all ONUs are transmitting the same frequency of light so the ONT needs a mechanism to detect that an ONU has disconnected. When an ONU has no data to transmit, it will send empty frames during its upstream time, marked with its ONU-ID, so the ONT can detect that those are no longer being received and can raise an alarm.  If the ONU wants to deliberately disconnect or power down it can issue a Dying Gasp (DG) message to the ONT to prevent an alarm being raised.  The Dying Gasp mechanism exists on most other 'Broadband' mediums including DSL though in practice, few devices are shut down 'nicely' - people just pull the plug - so the ONT (or MSAN/DSLAM/CMTS for VDSL/ADSL/Cable respectively) don't generally cause any active response to individual LoS.

Emergency Shutdown

The ONT can send an emergency stop message to any ONU which must then immediately turn off its laser and stay in Emergency Stop State (O7) until the OLT sends a re-enable message. If the ONU is rebooted, the transmit laser will re-enable momentarily but be shut down again as soon as the ONU recognises it.  The main reason that an ONU might need to shut down an ONU is that it detects anomalous transmission - the timing is off, the signal too strong or the transmission is otherwise out of specification. Such rogue behaviour can affect all other users on the same ONU port.


ALLOC_IDs are assigned to an ONU to identify each traffic-bearing entity (a container or 'T-CONT') that has been allocated upstream bandwidth within the ONU.  The ONT may create several T-CONTs and there are five types (from 1-5) to distinguish priority or Class of service to ensure there is fixed or guaranteed bandwidth for mission critical of delay sensitive applications (Video, VoIP etc.) to best effort (type 4).  Type 5 may contain mixed data classes.

Dynamic Bandwidth Allocation (DBA)

As multiple subscribers (and their ONUs) share one fibre feed they cannot transmit whenever they want to as the traffic would collide. Instead, each user's T-CONTs are allocated time slices for their transmission. This is called time division multiplexing (TDM).  If there's little traffic from other users, an active user will get more of the bandwidth (more frequent or longer time slices), resulting in higher throughput. The process of allocating bandwidth according to demand is known as Dynamic Bandwidth Allocation (DBA).  Each upstream frame on GPON is just 125us long (0.000125 of a second) and every ONUs transmits within that time so the ONT can adapt and allocate very quickly.

DBA can use one of two methods:

  1. Non-Status Reporting DBA (NSR-DBA). Each ONU is allocated a minimal time slice. It then transmits frames with a data payload or empty frames. If the frames contain data the ONT will keep increasing the bandwidth (time) allocation to that ONU (up to its fair share of current demand), decreasing it again once the frame content reduces.

  2. Status Reporting DBA (SR-DBA), the ONU will explicitly inform the ONT of its uplink buffer status (the volume of data waiting to be sent for each T-CONT) so that the ONT can allocate bandwidth accordingly.

TDM is not used for downstream - there is no need to coordinate slices for each ONU (customer) because there is only one sending device (the ONT) and it's sending to every ONU at once. Every ONU receives all data for all of the ONUs on that fibre - there's no difference to what data is received in your premises vs. your neighbour.    Each frame starts with a header, the Physical Control Block Downstream (PCBd) before the Payload which identifies which ONU the frame is for, identified by the ALLOC_ID and your ONU can discard everyone else's data payloads.

A downstream frame has a fixed duration of 125us (0.000125 seconds) and is 38880 bytes long, which is how we get the maximum data rate of 2.48832 Gbps after overheads (headers and error correction), shared between all customers on each physical ONT interface.    Each downstream frame contains a Physical Layer Overhead upstream (PLOu) header and the payload. The downstream frame uses a PCBd (Physical Layer Overhead downstream) header which includes the ALLOC-ID so that an ONU can identify its own T-CONT (its data payload that follows) and ignore everyone else's.

Subscriber Options

GPON always runs at 2.488Gb/s downstream so every subscriber has the same physical connection speed. The telco/ISP, however will typically offer different subscriptions according to the speed you want - generally from 50Mb/s to 1Gb/s.  If no other users on the same ONT were active and the backhaul had sufficient bandwidth, you would get the full 2.488Gb/s. To accommodate different speed subscribers, the ONT will simply throttle each user according to their chosen plan - limiting the speed at which it will deliver payloads to the ONU. On a 2.488Gb/s connection with a 50Mb/s subscription, if you download at full speed, that fibre circuit is running at 2% of capacity and wasting 98% if no-one else on your fibre is active - like an almost empty truck driving down the motorway (though not as wasteful).

I hope you've found this article useful - please do share a link on social and business networks / media and do make any comments below.


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