WAVELENGTH MOBILITY OPENS PATHS TO EFFICIENCY TWDM-PON Wavelength Mobility technology gives network operators new flexibility to move users and services between wavelengths. It allows operators to create efficiency by supporting novel use cases for equipment protection, bandwidth rebalancing, reduced power usage, network maintenance, rogue optical network unit (ONU) mitigation and infrastructure sharing.
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he TWDM-PON standard calls for colourless ONUs that are completely tunable in the upstream and downstream directions. ONUs are typically slaves that follow wavelength tuning instructions from the optical line termination (OLT). If required, these ONUs can be re-tuned to different wavelengths during operations. Nokia has helped to define and advance the TWDM-PON Wavelength Mobility standard by providing approaches for managing mobility across different channel terminations. These approaches have enabled several key use cases.
Equipment protection If a failure affects specific wavelengths, the operator can minimise the service impact by redirecting traffic to other wavelengths, including wavelengths terminated on different line cards. This strategy is primarily used when an OLT port or the line card behind it fails. It can also be useful if power fails or a wavelength is lost – for example, if a channel attachment fibre is cut. To support the use case, the operator can statically assign a ‘protection’ channel next to the ‘operating’ channel for individual ONUs. The OLT uses Physical Layer Operations, Administration and Maintenance (PLOAM) messages to convey this assignment to each ONU. If a failure occurs, the ONU tunes to the alternative wavelength and negotiates with the new OLT port. 12 Industry Europe
Bandwidth rebalancing Operators can use bandwidth rebalancing to react to changing bandwidth demands or usage trends. When bandwidth thresholds are reached – for example, if bandwidth usage reaches 80 per cent of the threshold on a given channel – the operator can move some users to another channel.
Reduced power usage The operator can reduce power usage by temporarily redirecting subscribers to a specific set of wavelengths and turning some equipment off. For example, at the OLT, the operator can redirect all users to one designated wavelength and shut other OLT ports down during overnight or lowusage hours.
Rogue ONU mitigation A rogue ONU is an ONU that transmits on the PON when it is not supposed to. Traditional rogue ONU mitigation schemes involve shutting off the ONU’s timeslots in the hope that the ONU will listen and stop transmitting. The problem with this approach is that the rogue ONU may ignore those instructions and transmit outside of its assigned window. Wavelength Mobility offers the ability to tune the rogue ONU to a different, unused channel to determine and eventually mitigate its activity. Alternatively, ONUs disturbed by such behavior can also be re-tuned to non-affected channels.
Wavelength Mobility elements and approaches Three elements are required for Wavelength Mobility. The first is the physical aspect of tuning wavelengths on transceivers. The second is the PON protocol, which supports PLOAM interactions and state machines. The third is additional logic and higher-level decision making that can control wavelength and user assignments. Operators can choose from two different approaches to assign wavelengths to ONUs: • Non-calibrated optics on the ONU side: The OLT monitors the signal from the ONU and, based on the received signal, instructs the ONU to move up or down until it reaches the correct wavelength. Once it reaches the correct wavelength, the ONU is continuously fine-tuned through ongoing feedback from the OLT. This feedback ensures that the signal is arriving at a peak level. The ONU similarly tunes its receive side to the correct wavelength and then fine-tunes the filter setting to ensure that a peak signal is received. • Pre-calibrated lasers on the ONU side: The wavelength pairs are pre-calibrated in the ONU transceiver so that the ONU can tune directly to the wavelength pair indicated by the OLT. There is no need for fine-tuning. Because the devices are pre-calibrated in the factory, they can go to an exact wavelength without any feedback from the OLT. If an ONU receives a message on a specific downstream wavelength, it is assumed to be communicating in
