Newbie alert - GSM network selection question

Gordon Brown wrote:
> "Chris Woodfield" <me@privacy.net> wrote in message
> newsdmdnRrPfM8Wgm6iRVn-gw@speakeasy.net...
>
>>I've got a question regarding GSM network selection. Let's say I
>>subscribe to provider T. There are towers belonging to provider T,
>>provider A, and provider C that serve my location. Provider A has a
>>roaming agreement with provider T, but Provider C does not.
>>
>>My question is this: what mechanism or protocol determines whose
>>providers' signals my phone can "see"? Does each tower broadcast a list
>>of networks it provides service for, or is it more complicated than that?
>
>
> On the first-time-power-up, you GSM phone will see all the GSM broadcast
> from all the towers. It will decode the Network ID Code from the broadcast
> and compare this against the Network Code on the SIM. It will always choose
> its home network against all other Networks even if the home network signal
> is weaker than the others. Only if it cannot communicate with its home
> network will it attempt to communicate with the other networks on a list
> stored in the SIM.
>
>

So, if I'm understanding this correctly, the list of my provider's
roaming partners is stored on my SIM in the phone...if that is the case,
is the SIM updated when there is a new roaming agreements with a carrier
not on the list, or when a provider terminates an existing roaming
agreement?

-C

Chris Woodfield wrote:
> Hello,
>
> I've got a question regarding GSM network selection. Let's say I
> subscribe to provider T. There are towers belonging to provider T,
> provider A, and provider C that serve my location. Provider A has a
> roaming agreement with provider T, but Provider C does not.
>
> My question is this: what mechanism or protocol determines whose
> providers' signals my phone can "see"? Does each tower broadcast a list
> of networks it provides service for, or is it more complicated than that?

No, each tower only broadcasts it's own netork credentials, and your
handset is expected to determine which network it attempts to register on.

>
> Next question: in the above example, My phone can see two towers that it
> is capable of making calls through. How does the phone decide which
> tower to use? Does it use the strongest signal, or will it prefer a
> tower belonging to my provider if available?

Inside your handset is an area of memory which stores what is called the
IRDB (Intellignet Roaming DataBase). The IRDB contains the necessary
info for determining which networks are available for you to use. Your
home carrier periodically provides automatic IRDB updates over the air,
but only when one is actually regostered on your home network, not while
roaming somewhere else. Based on your carrier's preferences downloaded
into your IRDB, your handset follows a prescribed list of carriers and
network types to be used, largely predicated on who your home carrier
has roaming contracts with. If your home carrier doesn't have a roaming
contract with the only signal your handet can 'see', then despite any
signal strength indicator, that signal cannot be used by you at that time.

I think I've got all that in the proper order - if not, someone will
have to jump in here and let us both know.



--
jer email reply - I am not a 'ten' ICQ = 35253273
"All that we do is touched with ocean, yet we remain on the shore of
what we know." -- Richard Wilbur

"Chris Woodfield" <me@privacy.net> wrote in message
news:8itbc1-45h.ln1@tino.semihuman.com...
> > On the first-time-power-up, you GSM phone will see all the GSM broadcast
> > from all the towers. It will decode the Network ID Code from the
broadcast
> > and compare this against the Network Code on the SIM. It will always
choose
> > its home network against all other Networks even if the home network
signal
> > is weaker than the others. Only if it cannot communicate with its home
> > network will it attempt to communicate with the other networks on a list
> > stored in the SIM.
> >
> >
>
> So, if I'm understanding this correctly, the list of my provider's
> roaming partners is stored on my SIM in the phone...if that is the case,
> is the SIM updated when there is a new roaming agreements with a carrier
> not on the list, or when a provider terminates an existing roaming
> agreement?


Okay, my explanation is somewhat over simplified because this is actually a
very complex matter which the GSM standards also give some latitude to the
handset manufacturer. There is also a difference between automatic selection
and manual selection, plus the subscriber is normally allowed to edit the
preferred list of Public Land Mobile Network (PLMN) stored in the SIM.

The important points are that the home PLMN is a special case which the
handset must search for on power up; If this is not found than, and only
then can it consider alternative PLMN (sometimes referred to as VPLMN). When
a GSM subscriber signs up for roaming, a SIM update is normally sent to the
subscriber's equipment. This will normally update the list of preferred PLMN
(which will also include the Home PLMN) and a list of forbidden PLMN. But as
you correctly pointed out, it will be impossible to maintain such a list as
new agreements could be constantly set-up and terminated. So the equipment
also self-learns which network it can work on by the location updates during
roaming. If a VPLMN rejects a location update from the handset terminal this
is added to a list stored in the handset (and not the SIM) so that it will
not attempt to access this VPLMN again.

But this presents a problem, what if some time later an agreement is set-up
with this VPLMN? The standards do not deal with this and this is left to the
equipment manufacturer to deal with. Anyway, I think this simple explanation
describes briefly how it all just about hangs together!