Telstra is keeping track of continuous mobile data sessions in 117 MB increments to alert customers when they reach 50 and 85 percent of quota as early as possible.
Telstra solved part of the mystery in a blog post last week.
As part of last year's revised Telecommunications Consumer Protection (TCP) code, all mobile telcos are required to provide spend management tools that alert customers when they reach 50 percent and 85 percent of their mobile data quota as of this coming September.
What was unclear was whether telcos would have to make configuration changes to their IT systems in order to implement data usage alerts at the set 50 percent and 85 percent intervals.
In Telstra's case, some configuration changes were required — resulting in the 'mystery' 117 MB number.
The biggest beneficiaries of mandatory data alerts are people whose data consumption occurs in long ("continuous") sessions.
Customers that run continuous sessions that chew through more than 100 MB of data will see a change in the way that usage appears on their bills. Low or intermittent mobile data uses won't see any change.
If a Telstra customer starts and ends a mobile data session without cracking 100 MB, a billing record is created once the session ends. Customers that leave a data session open all day, but don't crack the 100 MB mark, will see a billing record generated every 24 hours.
Telstra said it has changed the way it generates billing records for domestic data so a record is created and applied after a customer has used 100MB-130MB of continuous data.
The changes mean some customers "may ... begin to see blocks of data use between 100MB-130MB on their data usage meter" and billing records.
The most frequent blocks seen by Telstra users seem to be a specific 117MB in size. Telstra revealed to iTnews in the instance of continuous data usage, the data network creates and sends data usage records in 29.3 MB blocks.
"For the charge to land on the customer's account the network now holds these records until the customer hits the 100MB threshold — this equates to four records (29.3 MB x 4 = 117 MB) before it is consolidated and passes our notification engine and onto the customer's account."
It is understood that even though a single, continuous session is counted in 117 MB increments, the 117 MB doesn't act as a session cap - the session should remain open, no matter how many increments of 117 MB are used.
Despite speculation, Telstra said "the user experience should not be interrupted" as each 117MB is counted.
Why is this necessary?
Telcos have to be able to accurately warn customers when they chew through 50 percent, 85 percent and 100 percent of their data allowances.
That means being able to account for data usage faster than billing systems may currently allow.
"The change to our network service together with our new data alerts may result in customers receiving data alerts earlier than they had done previously during the month," Telstra said.
"The change to the way we generate data charging records means our data alerts can be delivered more quickly providing up-to-date information and helping them to avoid bill shock."
iTnews also sought comment from Vodafone Hutchison Australia and Optus on whether their billing systems had to be reconfigured to implement the data usage notification alerts. Their responses can be found here.