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.

The 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. 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

the upstream direction using the associated wavelength in the pre-defined pair.

Operators can use different mechanisms to reassign wavelengths to ONUs. In each case, the OLT sends instructions embedded in dedicated PLOAM messages defined by the standard. The ONU listens for, receives and processes messages on its current operating wavelength.

The OLT allows the ONU to activate on one of four (or eight) wavelength pairs. The OLT is responsible for redirecting the ONU to the right channel and sending down the configuration to activate the service. This can be done in two ways: • Round-robin or uniform assignment: The first ONU is assigned to the first wavelength, the second ONU to the second wavelength and so on. This assignment cycle is repeated, with every fifth ONU assigned again to the first wavelength. • Bandwidth balancing-based assignment: New ONUs are assigned to the wavelength that currently has the lowest aggregate traffic.

More advanced assignment and reassignment policies can be controlled by a mobility manager function that determines which channel the ONUs should use for operation.

Tuning strategies and considerations

The development of tunable optics is one of the most significant challenges related to TWDM-PON. The most challenging requirements include: • Reliably and predictably tuning the laser and receiver to a pre-determined wavelength • Mitigating wavelength drift induced by the rapidly changing heat effects of bursty upstream traffic • Enabling fast switching speed for both the laser and receiver • Transmitting at high output power levels with minimum crosstalk on neighboring channels and receiving the downstream signal with sufficient sensitivity and isolation from adjacent channels

Several technologies have been considered to address these challenges, both on the transmit and receiver side.

On the transmit side, the simplest approach is to use a thermally controlled distributed feedback (DFB) laser. With a DFB laser, the wavelength is set by a thermoelectric cooler that precisely controls the temperature. However, the DFB laser is directly modulated and the temperature can change when there are bursts of traffic. This causes the wavelength to drift outside the allowed wavelength window. The stabilization process involves careful temperature control techniques.

Another effective approach is to use an externally modulated laser (EML). An EML consists of a DFB that is always on, followed by an electro-absorption modulator (EAM) that sits in front of the laser to pass light for zeros and block light for ones. As with a DFB, the wavelength is tuned by temperature but drift is reduced because the laser is not turned on and off. However, the EAM adds extra loss, which makes achieving high power more challenging.

On the receiver side, two basic approaches are considered. One is a thermally controlled etalon filter that selects the desired wavelength before passing it on to the receiver. This approach suffers from relatively high attenuation and slow switching time. The alternative is a selectable device (e.g. a MEMS) that tunes or selects the appropriate filter for the desired receive wavelength.

Wavelength switches can be seamless for users. However, they happen at the speed at which the laser can tune the wavelength, which varies depending on the application and the service-level agreements that the operator has with customers. With slower lasers, tuning has a greater effect on users.

Looking ahead

TWDM-PON is the telecommunications industry’s chosen solution for implementing NG-PON2 fibre access technology. Wavelength Mobility is helping TWDM-PON gain traction with operators. Nokia is helping to lead the development of the Wavelength Mobility standard. The company is also working to improve mobility techniques and help operators use mobility to boost operational efficiency and get maximum value from their investments in TWDM-PON and fibre. n The full report can be found on Nokia Insight: www.insight.nokia.com/