While GAIT (both TDMA/Analog technology and GSM technology in the same phone)
will give you the "best possible coverage," actual coverage is governed by the
programming of the SIM, and it won't necessarily give you the best possible
signal in any event. From prior posts by me:

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Which network the phone will select depends on how the phone is
programmed (on the SIM) by the carrier. There are four different modes
of GAIT operation:

* GSM Native Mode: This is where the mobile is homed to a GSM
network and is operating on a GSM network. In this mode, the mobile
is able to perform standard GSM type functions including SMS and Data
services (circuit switched at 9600 or 14400 bps and packet switched
service if it is offered).

* ANSI-136 Native Mode: This is where the mobile is homed to an
ANSI-136 network and is operating on an ANSI-136 network. In this
mode, the mobile is able to perform standard ANSI-136 functions
including text messaging using GHOST SMS and Data services (circuit
switched at 9600 bps).

* GSM Foreign Mode: This is where the mobile is homed to an
ANSI-136 network but is operating on a GSM network. At this time, the
GSM portion of the phone is operational and the network's
Interworking and Interoperability Function (IIF) handles the call
delivery and SMS routing from the ANSI-136 network to the serving GSM
network.

* ANSI-136 Foreign Mode: This is where a GSM native subscriber is
accessing an ANSI-136 network. Again, routing of calls and SMS is
handled by the network's Interworking and Interoperability Function.

In short, a GAIT phone will be "homed" to either GSM or ANSI-136 (TDMA).
Network selection will be based on standard mechanisms in both
technologies.

Suppose that the GAIT phone scans for service and determines that there
are two networks available for selection, an ANSI-136 Neutral system and
a GSM Neutral system. If the PPI (Protocol Priority Indicator) is set to
ANSI-136 Preferred, then the mobile selects the ANSI-136 Neutral system.
Conversely, if the PPI is set to GSM Preferred, then the mobile selects
the GSM Neutral system.

Suppose that the GAIT phone scans for service and determines that there
are two network available for selection, an ANSI-136 Group A (Home)
system and a GSM Group B (Preferred) system. The mobile selects the
ANSI-136 system even if the signal is better on the GSM system.

Network selection is a function of preferences configured by the
carrier, not signal strength. It doesn't matter that the signal might
be better on the other technology.

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------------------------------------------------------------------------
Selection of network is controlled by GAIT SIM programming by the
carrier. The phone will be "homed" to either GSM or TDMA (ANSI-136),
and selection of network will be done by standard mechanisms in those
technologies (Home, Preferred, etc.).

* If a GAIT phone finds both a usable GSM Group B (Preferred) network
and a usable TDMA Group A (Home) network, then the phone selects the
TDMA network, regardless of relative signal strengths, and vice versa.
Home takes precedence over Preferred.

* If a GAIT phone finds both a usable GSM network and a usable TDMA
network of the same priority (e.g., Preferred), and the phone is homed
to TDMA, then the phone selects the TDMA network, regardless of relative
signal strengths. Conversely, if the phone is homed to GSM, then it
selects the GSM network, regardless of relative signal strengths.
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If you are in an area where GSM is not usable and non-Cingular ANSI-136
is usable, whether or not a Cingular GAIT phone would work on ANSI-136
would depend on the type of available ANSI-136 network. The phone would
select a Partner, Favored, or Neutral network, in descending order, if
there were any such network(s) with usable signal(s). It would not
select a Forbidden network, except for emergency calls.

So whether or not *any* (GSM-homed or TDMA-homed) Cingular GAIT phone
would work on TDMA in a GSM-only area (e.g., California) would depend on
(1) TDMA roaming agreement(s) by Cingular, and (2) programming by
Cingular to reflect those roaming agreements. I doubt that anyone here
knows for sure the answers to those questions, and I suspect it might be
hard to get that information from Cingular, leaving us only with
speculation; i.e., we can't say for sure one way or the other.

========================================================================

--
Best regards, HELP FOR CINGULAR GSM & SONY ERICSSON PHONES:
John Navas <http://navasgrp.home.att.net/#Cingular>


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