Text 259, 184 rader
Skriven 2004-08-02 00:23:00 av Michiel van der Vlist (2:280/5555)
    Kommentar till text 257 av Robert Bashe (2:2448/44)
Ärende: Fido on the move via cell phone.
========================================
Hello Bob,

 > MV> Actuallly there is no technical need to change the SIM card
 > MV> for a change of telephone number. The telephone number is not
 > MV> on the SIM. The SIM contains the IMSI.

 > You've got it exactly backwards there, Michiel. The IMSI is
 > in the phone firmware, and the telephone number (the one
 > you call) is on the SIM card.

No, no no, you got it mixed up. There is the IMSI end the IMEI. The IMEI
(International Mobile Equipment Identifier) is in the phone. It is the
electronic equivalent of a serial number.

The IMSI (International Mobile Subscriber Identification) is on the SIM.

 > At least that's the case with all the phones I've ever had or
 > seen (Nokia 3310, 3330, Motorola T191).

All GSM phones adhere to the same standars.

 > Remember, the IMSI is there to identify the phone.

No, the IMEI is to identify the phone. The IMSI is to identify the customer.

 > Putting it on the SIM card would defeat that
 > purpose, since throwing away the SIM card would
 > automatically remove the phone ID.

See above.

 > You can change the IMSI (depending on the country either
 > illegal or frowned upon, but it's possible) without having
 > a SIM card at all.

Some phones allow you to change the IMEI. Some by flashing, some by deploying a
soldering iron. Yes and the lagality is country dependant.

 > However, I don't know of any way to program the SIM card to change
 > the number, even if that would be accepted by the provider.

It is not a matter of programming the SIM card. The phone identifies the
customer by means of the IMSI imprinted on the SIM. The network translates this
into a phone number. Conversely when someone dials that number, the network
usues a look-up table to translate the telephone number into an IMSI. The
network pages the phone by IMSI, not by phone number.

 > MV> The link between the IMSI and the phone number is in the
 > MV> provider's Home Location Registry. The phone number can
 > MV> be changed by editing the HLR. Not all providers offer
 > MV> that service though...

 > Never heard of that, and it would conflict with what I know
 > about such phones since in the above mentioned ones, you
 > change the phone number (the one you call, not the phone
 > ID) by using a different SIM card.

Think of it as addressing a particular computer in the InterNet by using a url.
A DNS lookup translates it into an IP address.

The phone number is the url. The IP address is the IMSI. The provider's HRL is
te DNS.

 > As a matter of fact, the last time I checked (in 2003),
 > most providers didn't even check the IMSI. If they would,
 > it would be easier to block stolen phones from the network.

Replace IMSI by IMEI and you got it right.

 > And altered IMSIs would be easier to catch, since they
 > would generally match an existing one and be caught when
 > two phones with the same IMSI logged into the same network.

With five networks that chance is only 20%.

That is /if/ providers woukld check on duplicate IMEI's. Which AFAIK they do
not. No percentage in it for them.

[..]

 > MV> By having two SIM's for different providers one phone?

 > OK, that's naturally possible, and I even know that you can
 > get special multi-SIM holders for some phones for the purpose,

Yes, that exists.

 > but I don't know anyone who actually takes advantage of that
 > option. Most everyone just has a phone with a provider, and that's that.

In the early days, I met about a dozen people doing just that. Probably history
by now..

 > RB>> A SIM card doesn't just "fall out" of the phone, since it's in
 > RB>> some kind of socket in the phone's interior.

 > MV> That indeed is unlikely, but some people frequently travel between
 > MV> two countries and they have a SIM card for each country.

 > Yes, Peter also explained that, and if you do that you
 > naturally have such problems. The only problem is the one I
 > mentioned, that you have to keep funneling money into the
 > unused cards to keep them active. If you don't travel very
 > frequently indeed, this kind of thing would be pretty
 > uneconomical even if you _don't_ lose a SIM card.

Granted. It is ony attractive for the frequent travellr.

 > MV> As for upgrades: I once had my SIM replaced by a so called dual
 > MV> card. A dual card is two SIM's with the same IMSI.

 > As I noted above, it's no problem (with some phones)
 > getting a holder for more than one SIM, and the IMSI is in
 > the phone firmware so naturally it stays the same for as
 > many SIM cards as you can fit into the phone.

No, no. This is the other way around. It is not one phone with multiple
subscriber identities, it is two phones (physical equipment) with the same
subscriber identity. Comparable to the phone in your study and tha phone in
your bedroom that respond to the same telephone number.

 > MV> My old phone (Sony CMD-X2000) became unstable for shirt
 > MV> pocket use but still worked fine in the car kit. I got
 > MV> myself a SIEMENS M35 for regular use.

 > I have a mobile phone, the Motorola T191, but practically
 > never use it... bought it more or less out of enthusiasm
 > about the technology. The mobile phone rates are simply too
 > high for my taste, but I'm old-fashioned.

Well, I don't use my cell phone very much either. Indeed, the rates are high.
But I do use it when I feel I need it.

 > MV> So instead of getting a new car kit, I got myself a dual card.
 > MV> One card is in the old phone that remains in the car, in
 > MV> the car kit, and the other card is in the "handy".

 > I suppose you can do that, but why not just buy a new phone?

Because a new phone would not go with the old car kit. I'd have to buy a new
car kit as well...

 > The older models that "only" let you telephone other
 > people (but don't play a symphony while you're waiting, nor
 > make coffee in the morning) are meanwhile quite inexpensive.

Oh, you mean a new phone of the same brand and model as he old one? Not an
option, you can't buy a working Sony CMD C2000 anywhere. It is a museum piece.

 > MV> So instead of having to swap the SIM card, I just switch off
 > MV> the "handy" when I take the car. The phone in th ecar
 > MV> comes on when I switch on the ignition.

 > ;-)

 > I've never felt the need to be "wired for sound" everywhere
 > I go, but maybe you're different - or maybe you too just do
 > stuff like this because of enthusiasm for the technology.
 > It certainly is attractive.

I guess that is my bad. I grew up with cars as my dad was in it. I also had in
interest in radio and I got my ham license as soon as the law permitted. So
naturally I combined the two. I had communication stuff in my car from the
first car.

Plus that I also was a phone phreak.

Whith the advent of GSM, the natural step was to have a GSM with car kit...

 > MV> I should add that car kits are compulsory when using a phone
 > MV> while driving in The Netherlands.

 > If you use the phone while driving. That's also the case in
 > Germany. But as I say, I lived some 60 years without
 > feeling the need to call or be called on the phone while
 > driving, and I suppose I'll continue that way for a while,
 > so there wouldn't be much point in this in my case.

Not in my case. I grew up wiyh having a microphone in the car.

Cheers, Michiel

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