A phone doesnt need a sim to connect to the network towers. Sim is only for billing. The network can see your imei and triangulate your location as long as your radio is on.
I have read this on https://discuss.grapheneos.org/d/7710-changing-imei/8, but I’m bit confused. Does this mean our IMEI is identified by cell tower as long as the airplane mode is off? What exact setting is “Radio” referring to?
Short answer: Yes The network can see your IMEI and triangulate your location even though no SIM is installed. and theoretically with airplane mode off no radio is on
Put simply the radio broadcasts a sort of hello message to the tower so the tower knows where to listen (this is about signal direction or beam shaping, but imagine the eye of Sauron swiveling to see Frodo). This includes the identifier of the handset, the IMEI number, so that the tower can keep track of who is who. The second step of getting connected to the network is done with the details inside the SIM card, specifically the IMSI number.
If your phone has no SIM card you can still make an emergency call. You can also have an eSIM which is a software version of the SIM card. In both cases you can bypass the SIM and get connected.
If you turn airplane mode on the radio is powered off in theory, but this is not absolutely guaranteed. It should be off, the system will report it is off, but there are fringe cases where it may still be very slightly active, usually from malware or similar things.
So no SIM means no IMSI, but the radio itself has the IMEI and that handset is hard coded to that identifier. If the radio powers on it will broadcast the IMEI to negotiate connection with or without the SIM and IMSI.
Yeah, it is a fairly large dataset depending on the tower location. For example, in an inner city locale you may have hundreds of devices on a single passenger train going past a local tower. These transient handsets used to cause a massive issue with drop outs and loss of signal as they would acquire and then drop service from a given tower. Nowadays we have solutions for this which centre around shaped beams along the direction of travel with communication between towers to ignore handsets which are moving along a travel corridor.
To make that clearer, imagine the overhead train line has passengers moving along and under the train line people are walking on the street. The various towers which are along the train line will pass information about which handsets are moving and which are local so the local towers can handle local handsets and specific towers above can handle the train customers. This keeps the lower towers from changing their directionality and dropping calls and data confections, but also allows the train handsets to have reasonable connection to the network.
Another interesting case is what used to happen at the edge of the range for a tower. The whole tower could modulate its power so it could reach a far off handset if nobody else was around, extending the effective range. This unfortunately meant that if someone came closer to the tower it would have to lower its power to not harm the handset and the person far away would lose signal.
Nowadays the power level can be handled per handset. Each handset gets a small portion of a second, actually a small handful of parts of a second, and the power of the tower is adjusted to reach them at their required level for their time slots. If someone comes online close to the tower you may have competition for the time of the tower and thus lower speeds but the power will still match your handset independently of the rest. Very cool technology, way better than what it was with GSM, and also much more secure.
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Put simply the radio broadcasts a sort of hello message to the tower so the tower knows where to listen (this is about signal direction or beam shaping, but imagine the eye of Sauron swiveling to see Frodo). This includes the identifier of the handset, the IMEI number, so that the tower can keep track of who is who. The second step of getting connected to the network is done with the details inside the SIM card, specifically the IMSI number.
If your phone has no SIM card you can still make an emergency call. You can also have an eSIM which is a software version of the SIM card. In both cases you can bypass the SIM and get connected.
If you turn airplane mode on the radio is powered off in theory, but this is not absolutely guaranteed. It should be off, the system will report it is off, but there are fringe cases where it may still be very slightly active, usually from malware or similar things.
So no SIM means no IMSI, but the radio itself has the IMEI and that handset is hard coded to that identifier. If the radio powers on it will broadcast the IMEI to negotiate connection with or without the SIM and IMSI.
thanks a lot that is a very clear explanation
That seems like the tower must have to keep track of a hell of a lot of beam direction info. Damn
Yeah, it is a fairly large dataset depending on the tower location. For example, in an inner city locale you may have hundreds of devices on a single passenger train going past a local tower. These transient handsets used to cause a massive issue with drop outs and loss of signal as they would acquire and then drop service from a given tower. Nowadays we have solutions for this which centre around shaped beams along the direction of travel with communication between towers to ignore handsets which are moving along a travel corridor.
To make that clearer, imagine the overhead train line has passengers moving along and under the train line people are walking on the street. The various towers which are along the train line will pass information about which handsets are moving and which are local so the local towers can handle local handsets and specific towers above can handle the train customers. This keeps the lower towers from changing their directionality and dropping calls and data confections, but also allows the train handsets to have reasonable connection to the network.
Another interesting case is what used to happen at the edge of the range for a tower. The whole tower could modulate its power so it could reach a far off handset if nobody else was around, extending the effective range. This unfortunately meant that if someone came closer to the tower it would have to lower its power to not harm the handset and the person far away would lose signal.
Nowadays the power level can be handled per handset. Each handset gets a small portion of a second, actually a small handful of parts of a second, and the power of the tower is adjusted to reach them at their required level for their time slots. If someone comes online close to the tower you may have competition for the time of the tower and thus lower speeds but the power will still match your handset independently of the rest. Very cool technology, way better than what it was with GSM, and also much more secure.
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