Telstra outage: The night a network decided the year was 2006 | Netnod
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The opposite is actually the case. If we do not have a common understanding of what “now” is, a lot of things we take for granted will stop working. This summer, Australia realized this the hard way. Let me take this opportunity to give an example of why time matters to a modern society, what happened in this particular case, and what our key takeaways are. On 8 July 2026, a large part of the mobile network run by Australia's largest cell phone operator Telstra stopped working with voice calls not getting through or text messages that didn't arrive. There were even calls to Australia's emergency number that did not get through. But the outage affected systems even further apart, such as trains, payment terminals, ticketing systems and EV chargers being disrupted. No networks were attacked. Nobody accidentally cut the fiber. All systems had electrical power. The culprit, you may ask? A single GPS receiver in a single chassis in Melbourne coming back from scheduled maintenance believing the year was 2006, and the rest of the network was persuaded to believe it. Telstra commissioned an independent review from the company Technology Audit Partners (TAP) and the report is a very interesting read, because the same type of failure could appear in many critical services, including some we depend on to keep people alive. In order for many of these systems to function, time being correct, or at least the same everywhere, is crucial. And as everyone in the business knows, “correct” is a relative term. There is no exact time, only time held within a certain margin of a reference. How wide that margin may be depends entirely on what you are doing or in which business you operate in. Running a mobile network, like Telstra, is about as time-dependent as a business gets. Modern mobile communication explains why time matters Modern cell phone protocols will not work without precision time. Mobile networks separate uplink data from downlink by either FDD (Frequency Division Duplex) or TDD (Time Division Duplex).
FDD gives each direction its own slice of spectrum, so both can run continuously without colliding. TDD instead uses an entire single block of spectrum for both directions, alternating between transmitting and receiving in very short intervals. Since far more data usually flows down than up, FDD's fixed ratios leave much of the uplink spectrum idle, while TDD can shift the ratio to match the actual traffic. That is why most modern 5G spectrum, including Sweden's main 5G band at 3.5 GHz, is TDD. It is also why TDD depends on accurate time: every cell on the same frequency has to switch direction in step with every other. A cell that lets its clock drift will transmit data into its neighbour's receive window, with the result that the network will start jamming itself.
Rather than allocating spectrum on keeping the two directions apart, the industry chose to rely on time accuracy, and accepted a hard dependency on every cell agreeing about when "now" is. Thus, being dependent on time is a design choice. However, given how much we all depend on the systems being able to agree on “now”, it is somewhat puzzling that time is not given as much consideration as it deserves. And that is a lesson that is very clear from the published report. So, what really happened? Architecture of time distribution. To begin with, it is vital to understand the architecture of time distribution. Time distribution protocols all build hierarchies; Network Time Protocol (NTP), which Telstra has deployed according to the report, expresses its hierarchy in strata. Stratum 0 is the reference itself, for instance a GPS receiver, or Netnod’s atomic clocks. Stratum 1 is a machine synchronised directly to a stratum 0 reference, for example the NTP servers that Netnod provides. Stratum 2 synchronises from a stratum 1 server, stratum 3 from a stratum 2, and so on. In Telstra's case, that hierarchy had a specific shape, at least to begin with. This design from 2010 had at the top stratum 1 sources at Australia's National Measurement Institute (NMI), which maintains the country's national time scale, much as the Research Institute of Sweden does in Sweden.
Telstra drew time from those external references into two stratum 2 servers of its own, in Sydney and Melbourne, which in turn fed three stratum 3 servers, in Sydney, Melbourne and Perth. Below them sat the clients. In this context that does not mean laptops or phones, but the entire mobile network infrastructure, for instance nodes handling handovers between cell sites.
There were thousands of nodes all over a vast geography and every one of them needed to have the same idea of what “now” is, to within a few millionths of a second. The TAP report describes this setup as “fit for purpose” and that it gave Telstra “a highly reliable and authoritative reference time source from NMI”. Stratum in itself does not say if the time is accurate, only the number of steps from a server to its reference. A stratum 1 server with a bad time reference is still a stratum 1 server. Protection against bad time sources NTP will therefore need defence against bad time sources. In fact, it has two different ones, and they do different things, both of which assume they are independent from each other.
Among otherwise comparable candidates, the lower stratum carries more weight. This is the mechanism that determines which source a client settles on. NTP compares several sources and discards those that disagree with the rest. A single source claiming an implausible time is outvoted and dropped, regardless of how authoritative it claims to be. Neither defence is specific to any particular disruption; together they protect against a broken receiver, a misconfigured server, or an external attack.