Powering considerations of the ONTs in a passive optical LAN

Passive Optical LAN is the application of PON technology within the enterprise environment. It´s innovative, disruptive and in particular advantageous in multi-floor buildings and campus networks. It involves a single centralised active equipment, the Optical Line Terminal (OLT), a simple and passive fibre network down to the endpoints and thin-client edge devices, Optical Network Terminal (ONT), which are centrally managed.

In this blog, we describe the different powering options of the ONTs.

Local powering of ONT with AC power

In the main IT room, we have the OLT which is equipped with redundant power feeds (AC power, AC/DC converter & UPS). SMF and splitters make up the passive infrastructure and finally, we have the ONTs that provide connection to the end-point devices.

In this case, the ONTs are powered from a local AC power via a separate power supply unit (PSU). The physical size of the PSU is comparable with a PSU of a laptop for example.

Powering ONTs with AC power is used in indoor deployments and addresses most applications in educational institutions, enterprises, real estate, airports, warehouses, manufacturing, etc. by utilising the same electrical outlets provided at the endpoint devices e.g. a desk in an office workspace.

Local powering of ONT with battery backup

We have the same optical LAN network configuration. However, in this case, the ONTs are equipped with a battery backup unit to prevent network connectivity loss during power outages. The backup time is typically between 30 mins – 120 mins allowing maintenance staff to detect the power outage and switch over to the redundant/standby power generator system in the building/business/unit.

It is mainly used in indoor deployments and addresses applications where power service is known to be unreliable or the application is a mission-critical operation like a call centre in a hospital. The typical guidance is to provide battery backup whenever the application itself is also on a battery backup.

The pros and cons of local powering of an ONT

Pros:

• Delivers all the benefits of a passive optical LAN solution as the infrastructure is fully passive.
• No active elements between the OLT and ONTs and hence no failure points in the midspan of the network.
• Reach is unaffected; one is still able to cover distances up to 20km.
• Powering ONTs with AC power addresses most applications but in some cases a battery backup unit may be required.

Cons:

• Powering both with AC power and BBU limits the ONT location placement; in both cases the ONT needs to be in close proximity to the AC power outlet.
• The battery backup unit is a separate device that can increase the complexity of the network installation.
• Limited battery lifespan of 5 to 7 years requiring regular replacement of the BBU.

Remote powering of ONTs

Deploying Wi-Fi access points, HD cameras, or other devices, especially in outdoor environments, can be difficult and pose a challenge to find a local power source e.g. on the sides of buildings, on palls or on lamp posts.

There´s an alternative solution to power the ONTs through a remote power distribution unit. We are introducing a new element and in this case, the network configuration differs from the previous one because the infrastructure is no longer purely passive. From the power distribution unit to the ONT the cabling is a combination of fibre (data) and copper (power).

There are two remote powering options: centralised or distributed.

Centralised remote powering

In a centralised deployment, a -48VDC power plant and batteries are placed in a central location such as the main IT room and can also power e.g. the OLT. The -48VDC power connects to a power distribution unit that can be co-located with the passive splitters equipped in IT closets on each floor of a building. This configuration is simple to maintain and requires minimum space on each floor.

Distributed remote powering

In a distributed layout, a smaller -48VDC power plant with batteries is installed with each power distribution unit. However, this requires extra space in the IT closet on each floor.

Also, in this case, the power sourcing solution can be co-located with e.g. the passive splitter.