Norayr Chilingarian <nor...@arnet.am> wrote: > Does anyone know what will happen in a cellular network where there is > more than one device has the same IMEI. In other words, if we all > could change our IMEI numbers, and use one imaginary number, are there > technical reasons for network to not work.
joerg Reisenweber <jo...@openmoko.org> responded: : no technical but organizational. Usually that IMEI gets an instant ban, and : a fat bold red alarm logline in carrier's network logs. Yup, if all of us were to use the same IMEI number, it would be far too easy for our enemies to ban that one single number. > I mean, MAC address is used on a physical layer, so if two network > cards connected to the same switch have same MAC adresses, network > won't work. I guess switch will down both ports connected to those > devices. The analogy between IMEIs and Ethernet MAC addresses is a good one from a manufacturing/management perspective, but not in terms of network protocol usage. Unlike MAC addresses, IMEIs are not used for any kind of addressing or routing anywhere in the network, only as a "management" identifier that is unnecessary in the strict technical sense. But from the perspective of a device manufacturer (which I will become soon, hopefully), IMEIs are just like Ethernet MAC addresses: the nominal requirement is that each be world-unique for all time (a rule that gets broken in reality with both MAC addresses and IMEIs), a manufacturer has to buy a range (supposedly "fresh" and unused) from a central registry, and then number individual produced units out of that range. > But I don't know how IMEI's work. Are they technically necessary so > that 3G/gsm network can be operational, or they are only used to > identify (and track) customers by devices? The latter. Before everyone starts changing their IMEIs just for the heck of it, let's analyze *rationally* how tracking works - or rather, what is the total set of data elements available to carriers (and their gov't partners etc) for tracking users, and how these data elements inter- relate. If you like maintaining a long-term-constant phone number at which your family and friends can reach you (i.e., the whole purpose for having a cellphone, at least for me), and you have a long-term-stable SIM card associated with that long-term-constant phone number, then it doesn't really matter if your IMEI is also constant or if you send the output of a PRNG (or even a TRNG) to the network as your IMEISV every time your phone/modem fw does the "register" operation. The constant SIM card with its IMSI, as well as the associated MSISDN (phone number for your family and friends to call you at), is what tells the network that "you" are still the same "you", no matter what device you use or what IMEISV it transmits. Yes, you can deregister from the network, then re-register with a different IMEI, making it look like you turned your phone off, moved your SIM card to another phone, then came back online with the latter - but what would be the point? Instead, there are only two scenarios I can think of in which it would make sense to change the IMEI of a GSM device: 1. If you really want to "disappear w/o trace", such that you discard your old SIM, get a new SIM (prepaid, presumably) with a different phone number (and deliberately make yourself unreachable at your old one), and you want to make it look like the user of the new SIM is a different person from the user of the old SIM - in this case the same IMEI would indeed give you away, so you might want to change it in this case. If the above applies to you (and it does *not* apply to me, as changing phone numbers constantly would defeat the whole purpose of a cellphone for me), then you need to be careful to change your IMEI *at exactly the same time* when you change your SIM - if there is any time skew between these two changes, such that a network sees {old IMEI, new SIM} or {new IMEI, old SIM} at any time, even just once, your anonymity effort will be instantly brought to naught! If you want to do this, I would recommend pulling your old SIM out first, throwing it away, then doing the IMEI changing operation on the SIM-less modem, and then finally inserting your new SIM. 2. Changing one's IMEI may be necessary if your "legitimate" IMEI from the manufacturer of your GSM device has been wrongfully banned or blocked by some GSM network you wish to use, and you need to use some non-blocked IMEI in order to get on the network. The wrongful ban scenario is particularly frightening when applied to whole classes of devices, rather than individual units. The first 8 digits of the IMEI comprise the Type Allocation Code (TAC), which is supposed to be allocated per each device type. Hence if all manufacturers involved played by the rules (of which I have no knowledge), then every IMEI beginning with 35278901 is supposed to be a Pirelli DP-L10, every IMEI beginning with 35465101 is supposed to be an Openmoko GTA02, and so on. What if some repressive network operator decides to block all IMEIs belonging to easy-to-hack Calypso devices, e.g., block all IMEIs beginning with 35278901 or 35465101, on the reasoning that "only a criminal would want to use one of these phones"? In that case we will need to lie to that network and pretend to be some Apple/Samsung/etc device in order to get GSM service, i.e., use an IMEI from one of those "sheeple device" ranges. VLR, SF _______________________________________________ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community