Out of range

In this article I would like to consider a small “lifehack”, which can be understood with a little knowledge of the functioning of the GSM network.
The meaning of lifehack is that when you call someone and in response you hear “The subscriber is temporarily unavailable”, you can approximately understand for what reason the person is now unavailable.


In my previous article on the structure of GSM / UMTS networks, I mentioned such a node as VLR. That's how he will be the key link here.

For a start, it is worthwhile to consider such a question as the state of the mobile station.

The state of the mobile station in the GSM network

In a GSM network, a mobile station may be in the following states:

• IDLE - or in Russian standby mode, this is a state when your phone is just in your pocket / on the table, no calls are happening, no sms are sent. In this state, the mobile phone transmits almost nothing to the network, its main task is to listen to the signals sent by the base station, if it hears that it is being called, then only it will respond and start sending something to the network.
• BUSY / ACTIVE - or active state, this is the mode in which the mobile phone is involved in voice / data transmission.
• DETACHED - or disconnected from the network. This is the state when your phone is turned off.

Information about these states is recorded in the same VLR. This node stores information about the states of all subscribers that are in its coverage area.
When you turn off your mobile phone, or it has a battery, it manages to send information to the network that it is now turned off. This information is recorded in the VLR.
In the case, if you pulled the battery out or went out of the network coverage area, then the VLR will store information that you are supposedly in the IDLE state now.
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Incoming call process

Now let's see how the incoming call to the subscriber whom you call in case everything is OK and his phone is in the IDLE state.


Your call goes to the switchboard operator of your friend / acquaintance, we will be interested in exactly what happens next:

1. The switch (MSC) requests the state and location of the called subscriber in its internal database (VLR). We consider the situation when the VLR responds that the state now is IDLE.
2. The switch redirects the call to the base station controller (BSC), in which the mobile phone is located.
3. The controller sends a “paging” signal to all of its base stations (in the simplest and often occurring variant).
4. All base stations relay this paging signal to the radio. Thus, all mobile stations that are in range of a given BSC will hear this paging. This paging signal contains a unique TMSI identifier, on hearing that a mobile phone with the same TMSI responds to paging. After the mobile phone responded, and this response comes to the switch, you can hear long beeps in your handset, and your friend's phone played with your favorite tune.

Here everything is beautiful and clear.
If there was a record in the VLR that the subscriber is now DETACHED, then you will hear that the “Subscriber is temporarily unavailable” immediately after the verification step described in step 1 (approximately 5 seconds after you pressed the green call button).
And now the most interesting option for us is your friend went down to the subway, where there is no cellular network signal. In this case, you will hear the standard “Subscriber is temporarily unavailable” only after stage 4, which lasts a few seconds. Thus, it will take about 15-17 seconds from the moment you clicked on the green button to send a call to the moment when you are told that the interlocutor is Out of Service.

Conclusion:

If before the standard slogan about unavailability takes 15-17 seconds, the subscriber, most likely left the zone of action.
If you immediately (4-6 seconds) hear that the subscriber is unavailable - either he turned off the phone himself, or he had a battery, or he left the zone a long time ago and the VLR had already marked it as detached (depending on the operator, this is “pretty long "usually varies from 40 minutes to 6 hours).

You ask, what is lifehack here? Well, at least, if you tried to call a person, and the switch immediately replied to you that he is unavailable, and then the person himself tells you that he was allegedly in the subway, then this is a reason to think - most likely he turned off the phone and didn't want anyone to disturb him.

UPD: questions arise, so I’ll add something else:

Procedure Periodic Location Update

The point of this procedure is that the subscriber must periodically send a packet to the network confirming that he is still there. You can hear this when the phone has been idle for a long time already, all of a sudden the speakers start to pop up, but neither the incoming call, nor the SMS comes.
Depending on the network settings, this procedure occurs periodically with a period from 6 minutes to 6 hours. As a rule, in practice, the lower limit for operators is 40 minutes.

Thus, if you removed the battery / left the network a long time ago and at the set time your phone did not perform the Periodic Location Update procedure, then the VLR status is set to Implicit Detached (forcibly disconnected from the network).

You will be in this state until any package is received from you. In this state, all callers will immediately respond that you are unavailable.

UPD2:

This comment resulted in another way for Kyivstar network.

UPD3:

Here in this comment thread it turned out that the same situation is in the CDMA system!