Why did some people get the UK emergency alert late or not at all?

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On Sunday, millions of phones simultaneously launched a distress signal during a UK-wide government test. However, a number of phones received the alert late or not at all.

Here we answer your questions about what happened.

Did the alert go as planned?

Phones across the UK rang and vibrated an alarm at 3pm on Sunday for 10 seconds, accompanied by a text message entitled “Severe Alert”. It stated: “This is a test of Emergency Alerts, a new UK government service that will alert you if there’s a life-threatening emergency nearby.”

The government said it had “effectively completed” the test, describing it as “the largest public speaking exercise of its kind ever”. It was aimed at reaching a substantial portion of the country’s estimated 85 million cell phones, with only those too old to connect to a 4G network or without a software update in the past two years.

However, not all users received it at the right time, or in some cases not at all. The government said a small number of phones did not receive the notice. He said he was “working with mobile network operators to review the results and lessons learned”.

Network Three appears to be the hardest hit. He said: “We are aware that a number of customers have not received the test notice. We are working closely with the government to understand why and ensure it doesn’t happen when the system is in use.”

Some users of the O2 and Vodafone networks also reported difficulties, while others reported receiving the alert a minute early or 20 minutes late.

Why did some Three users not receive the alert?

Unlike the pandemic, when SMS alerts were sent specifically to individual numbers, Sunday’s alert was transmitted via the cell phone masts that form the core of the nation’s 4G and 5G networks. It means that if you were within range of a tree, you should have received an alert.

However, what appears to have happened on the Three network is a misconfiguration preventing most users from receiving the alert. The “cellular broadcast system” that underlies the alert is a key part of the international standard for mobile phones, and mobile mast technology requires networks to send the alert at regular intervals to ensure it is picked up by all phones. Most networks did just that, repeating the pulse several times a second for up to 20 minutes, to ensure users with intermittent coverage still received it.

But according to signal analysis, Three apparently only sent the signal once, at 3pm sharp, making sure that only phones that were actively connected to the network at exactly that millisecond received it.

What can the government and mobile phone networks learn from the alert?

At the top of the list will be uncovering the chain of events that resulted in many customers on Three’s network not receiving the alert, and trying to uncover similar root causes for other alert delivery failures. A small amount of users on other networks also reported that the message was not delivered on their devices.

In addition to technical feedback, the government will want to evaluate reactions to alerts. Previous tests had found a number of difficulties to overcome, including the problem that the alert itself is disruptive enough that users scrambled to ignore it to silence the alarm, without actually reading the alert itself.

Was it disruptive?

The alert had been followed up so well that the disruption was minimal. Play was stopped at the World Snooker Championship in Sheffield to allow for the alert, while the alarm was sounded during a live BBC news segment on alert and some London Marathon runners had their warm-up cut short.

However, the test was for a system whose purpose is for local or national emergencies, so when used seriously it is likely to cause more disruption. Since the system uses the mobile phone transmission network, alerts can be limited to specific areas affected by, for example, flooding, while the rest of the country goes about its business undisturbed.

As a result, some users may choose to turn off alerts on their phones. “Extreme” and “severe” alerts can be disabled on all phones, but the system also allows for an even higher severity level, referred to as “presidential” in international standards but renamed “government” in the UK. That layer can’t be disabled except on some Android phones, but it’s (hopefully) unlikely to ever be used.

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