Help Network locking and baseband question

[Android Question]

Hi. My question is involving cell networks, carrier locking/ unlocking, and baseband firmwares.

First of all, what makes a particular model of phone that is sold/ branded on a particular network different from its other variants? If I have a Galaxy S5 that is on T-Mobile, how is it different from the Galaxy S5 on AT&T? Aside from the differences of GSM and CDMA, what makes a T-Mobile phone, or an AT&T phone special in that it has its own radio firmware? Do each varriant of a device have a different baseband processor? Why do they have their own firmwares? What happens to the radio when you unlock a phone?

 

[Hadron]

Not a different processor (depending on what you mean by that word), but they may support different frequency bands. I know that ATT & T-Mobile use different 3G bands, and an ATT phone may not support the unusual setup that the original T-Mobile 3G used (with widely separated uplink and downlink bands). Usually when phones support different bands there are differences in antenna design, possibly transceiver hardware, so you can only rarely add support by loading different firmware (i.e. only if the hardware already supports both, which occasionally is the case but not in general).

Unlocking doesn't change any of this. That just allows the phone to accept a sim from a different carrier, has no effect on what bands or protocols the phone supports.

 

[Siriusly]

Not a different processor (depending on what you mean by that word), but they may support different frequency bands. I know that ATT & T-Mobile use different 3G bands, and an ATT phone may not support the unusual setup that the original T-Mobile 3G used (with widely separated uplink and downlink bands). Usually when phones support different bands there are differences in antenna design, possibly transceiver hardware, so you can only rarely add support by loading different firmware (i.e. only if the hardware already supports both, which occasionally is the case but not in general).

Unlocking doesn't change any of this. That just allows the phone to accept a sim from a different carrier, has no effect on what bands or protocols the phone supports.

So thats where the whole qaud band/ dual band thing comes in?

So if a phone is rated as qaud band GSM 800-1900, it can work on any network?

 

[chanchan05]

If the firmware supports it.