Cellphones that Work Everywhere?

spoonist asks: "The vast array of available wireless protocols in the US is quite bewildering: CDMA, TDMA, GSM, AMPS. I spend most of my time in urban areas. Major providers appear to be rolling out GSM, so that seems like the way to go. I also spend a lot of time hiking and climbing in rural areas and like having my mobile phone in case of an emergency. My ancient analog phone gets a signal in all but the most remote of areas LONG after friends' digital phones loose their signals. Are there any dual mode mobile phones that can talk both GSM and AMPS? Also, I occassionally travel to Europe. From what I understand, GSM in the US uses a different frequency from the rest of the world (WHY!?!). Are there any phones that talk European GSM, US GSM, and AMPS?"

Comments

Re:CDMA and AMPS

[jquirke]

Firstly, "3G" is not a standard on its own. There is not going to be a shift away from GSM here in Australia, or in many parts of the world.

The term "3G" is nothing more than a stupid marketing term, it explains very little and just confuses people about the technology. Hopefully I can clear up some of this confusing mess of acronyms.

The term GSM however does not refer exclusively to the current "2.xG" digital technology. GSM is constantly evolving, and yes, will become "3G".

The first stage in the GSM "2.5G" evolution towards 3G was GPRS (General Packet Radio Service). This extended GSM to support packet switched data over multiple timeslots (bandwidth per timeslot = 14400bps). It required little change to the GSM network except for software upgrades on the base station and other minor changes to the backbone. Hence GPRS could be called "2.75G". GPRS is implemented by all 3 Australian carriers Optus, Telstra and Vodafone. It is also implemented in many other countries.

The next phase will be EDGE (Enhanced Data rates for GSM Evolution). Also called "E-GPRS" this is much like GPRS however it packs more data onto a single timeslot (upto 3 times as much due to improved coding (8 Phase shift keying as opposed to Gaussian Minimal Shift keying) and error correction transferring up to around 3 times as much data - 3 bits per symbol with a raw symbol rate of 270K). This only generally requires hardware changes to the radio interface of the base station. No official term for this yet, call it "almost 3G - 2.9G??".

Unfortunately Optus scrapped its plans to implement this for now, however I've been told some of the newer GSM base stations do support EDGE.

It is important to note that GPRS & EDGE operate in the normal GSM spectrum (thats 900MHz and 1.8GHz here in Aus and most of the world, 1900 and 800 in the US/Canada).

The next part however is to _supplement_ EDGE in other parts of the spectrum. Branded "3GSM", I am not too familiar with the technology however I believe most GSM countries will use a Wideband-CDMA technology (W-CDMA) in the 2.0-2.2 GHz spectrum. Hence, a 3G GSM phone operates on several frequencies using several coding techniques transparently to the user. The different codings are suited to different environments so I guess you have a more reliable service. EDGE can supposedly transfer around as much as 384Kbps whilst W-CDMA can do around 2megabit or so.

EDGE can coexist peacefully with older GPRS and "plain old GSM" transmissions on the same timeslot, so it's backward compatibility will make it look promising.

In Australia GSM will be the way of the future here, however CDMA is still useful for the rural hicks :-) where GSM coverage is lacking. But most Australians live in cities.

Anyway correct me if I've made errors

--JQuirke