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Comment by coin | Hacker News Reader

'Cellular phones work by periodically scanning for and "registering" with the nearest base station (generally the one with the strongest radio signal).'

This is not true. A handset registers when it enters a new zone. A city is typically broken down into several zones. The handset does not register itself with every new cellsite ("tower") it encounters. If one's phone stays in the same area, it will never transmit while idle. This is the basis for its long battery life.

http://denbeste.nu/cdmafaq/voicemail.shtml

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There was a DEFCON talk two years ago in which the presenter assembled large antennas and ran open-source software that allowed him to mimic a base station. Everyone who was using phones on a specific network (I don't remember which -- I want to say AT&T) connected to his base station within a few minutes, and he proved the concept by hijacking outgoing calls.

>If one's phone stays in the same area, it will never transmit while idle.

Maybe phones used to do this, but nowadays I'm not so sure.

CDMA also has optional timer-based registration, with intervals between ~12 sec and ~55 hours (depending on what the base station wants; search 3gpp2 C.S0005 for REG_PRD for the gory details), or the base station can send a Registration Request Order message (which does what you might expect) to the phone of interest.

I'm not sure that's true for dense urban zones like NYC. I'm pretty sure my phone registers with many different towers frequently.

There's a tower on almost every other building in New York, and sometimes even within buildings and underground.

Considering that smart phones are particularly promiscuous, especially in the presence of poorly conceived and opportunistic apps (downloaded by my rotten teenage kids), my phone is probably a shining beacon of electromagnetic activity, constantly transmitting high-score updates, and my entire contact list and GPS coordinates to web servers hosting silly JSON APIs.

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