NBN Co has publicly defined the threshold at which it considers fixed wireless towers to be congested enough to warrant an upgrade.

In a surprise reveal earlier this week, the network builder said cells were deemed to have an unacceptable level of congestion when they provided “less than 3Mbps in the busiest hour of the day”.

NBN Co said it could take between three and 12 months to upgrade a cell, depending mostly on ownership of the infrastructure that the antennas are attached to.

“When NBN Co owns and operates the tower, the lead time to upgrade a sector can be as low as three months," it said.

The company said it was working on a “zero-based design project to optimise the capacity upgrade process and reduce lead times”, though it did not elaborate.

Zero-based design (ZBD) is a project methodology that encourages teams to abandon preconceptions and existing models when trying to find a solution to a problem. NBN Co has previously used it for internal process redesign.

The company revealed nine towers currently had unacceptable congestion and were in line for upgrades.

They included Marian North in Queensland; Howard Springs, Bees Creek and Humpty Doo in the Northern Territory; Smythesdale and Clunes North in Victoria; Woolgoolga and Millthorpe in NSW; and Worrolong in South Australia.

Upgrades of those towers were scheduled between December 2017 and July 2018.

NBN Co relied on predictive usage modelling to identify when cells may need to be upgraded; it said it “future-dated” when it believed cells or sectors would dip “below expected performance”.

“We have detailed internal systems that track and predict network traffic and capacity in the fixed wireless network,” the company said.

“Part of the internal systems that track network capacity and end user take-up is a database that shows the current state of the network sections and their capacity, the date they will require an upgrade and the date of upgrades when they are scheduled.”

However, NBN Co noted that the models were not always accurate - meaning some towers may be congested, even though they are not flagged in the company’s systems.

“In some cases, the end user demand – either in terms of take up of the service, or of usage of internet services over the fixed wireless network – exceeds our modelled timeframes,” the company said.

“In those cases, end users may experience slower speeds than nbn expects during peak times of usage.

“Where this happens, NBN Co prioritises the upgrades of those areas and accelerates the upgrade process wherever possible, while still following prescribed regulatory and legal processes.”

Common upgrades included transmission, packet gateway, aggregation network, fibre spur, microwave backhaul, radio access network, and power and battery backup.

“Each of these steps in upgrading the network must be forecast in advance and planned with NBN network construction partners, local authorities, landlords, infrastructure owners, regulatory authorities and/or local government authorities,” NBN Co said.

However, once the company has approvals and access arranged, the actual technology upgrade can usually be accomplished in between two and four hours.

Update, 16/3 at 5.10pm: NBN Co has clarified its cell site upgrade thresholds since publication of this story.

The company said it generally aims to trigger upgrades to sites so they do not drop below 6Mbps per user at peak times. The 6Mbps level is judged by measuring the average throughput of all end-users in a cell in the busiest hour averaged over a month.

On some cells, however, demand exceeds expectations and performance drops below the 6Mbps thresholds before NBN Co can upgrade the site. These sites are deemed "critical" once they dip below 3Mbps per user at peak.