[Fathom22]

I have a couple questions regarding cell providers in the US. I was wondering how companies such as AT&T and T-Mobile use the same GSM frequency through different towers. Can anyone give me a run down of how the companies differentiate their subscribers even though they are on the same frequency band? Also why do companies like Verizon use CDMA? Is it purely for competitive purposes?

Thanks

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[nicknrm]

I believe the frequencies of both T-Mobile and AT&T are the same throughout the country...yes I said two...there are two primary GSM frequencies in the U.S. Don't quote me on that...but I'm pretty certain.

Verizon, Sprint, and others use CDMA not for competition. They were here first...why change? If Verizon wanted to go to GSM, it would take years, lots of money, and a lot of unhappy customers before it would be perfected. CDMA is new to other parts of the world...but very old here. It was the first complete national technology launched. iDEN, by Nextel, was launched early on also...but it lacked and still lacks a good national base for coverage.

I have a couple questions regarding cell providers in the US. I was wondering how companies such as AT&T and T-Mobile use the same GSM frequency through different towers. Can anyone give me a run down of how the companies differentiate their subscribers even though they are on the same frequency band? Also why do companies like Verizon use CDMA? Is it purely for competitive purposes?

Thanks

[cryptzog]

I was wondering how companies such as AT&T and T-Mobile use the same GSM frequency through different towers." CDMA is a newer technology than GSM. There are two frequency bands in the US for GSM and CDMA: 850 and 1900 (but they use different specific frequencies in those bands....) They are assigned specific frequencies within the bands by the FCC. Companies can purchase specific frequencies, as well.

"Can anyone give me a run down of how the companies differentiate their subscribers even though they are on the same frequency band?" The GSM phone is given a SIM chip which has an IMSI on it. The phone itself has an IMEI. When a phone registers with a tower, it sends its IMEI and IMSI. If the AT&T network recognizes the IMEI and IMSI as a valid AT&T customer, it lets it connect. In the case of "roaming agreements", if an AT&T tower recognizes the IMEI / IMSI combo as a T-Mobile, it may let it connect because of the agreement. If it does not, it rejects it "roaming not allowed".

Think of GSM as Spanish and CDMA as Japanese. AT&T and T-mobile both speak Spanish therefore, a telephone that speaks Spanish can talk to AT&T and T-Mobile towers. Verizon and Sprint speak Japanese. When a phone connects to a tower, it identifies itself and the network (whether speaking Spanish or Japanese) and then the network will either permit or deny the connection based on roaming agreements, etc... An AT&T phone, after it identifies itself speaking Spanish, may be denied access to a T-Mobile tower if T-Mobile decides not to let it connect, for example. Once a Verizon tower realizes that a Sprint phone is trying to connect, it tells it to go look for a Sprint tower.... in Japanese.

A GSM phone cannot speak CDMA and a CDMA phone cannot speak GSM (unless it's a special type of phone......)

GSM uses a technology called TDMA (Time Division Multiple Access) and users are designated a "Time Slot" on a frequency.

CDMA is CDMA (Code Division Multiple Access) Users are assigned a "code" (that can be loosely considered to be an "address"), allowing them to use the same frequency and only listen to "packets" addressed to them. It is much more complex than TDMA, but it works.

One TDMA freqency only allows 8 timeslots, while a CDMA frequency allows 60 codes... therefore more people are able to connect on one CDMA frequency than they are on one TDMA frequency.

GSM technology has been around longer, so there are more towers in place and therefore a larger network of towers, but CDMA has many advantages including higher capacity per tower, less power consumption on cell phones... multipath concerns.... in other words.... there are more GSM towers, but one CDMA tower packs a bigger punch than one GSM tower.

GSM allows many users to use the network by providing more towers...i.e. available frequencies. CDMA allows many users to connect, not by more towers, but by letting more users connect per tower. Both have a strong network throughout the U.S., and both use legal mumbo jumbo and word trickery to make you think their network is better than the other.

The big difference in the future will be the provider's ability to provide internet/data.... But with the coming of LTE, most of the big GSM and CDMA carriers are going to be going to LTE for their internet/data needs. Sprint apparently will be going to WIMAX, but many people have their doubts that it will outperform LTE.

Hope this helps!

[misterSelf]

I believe the frequencies of both T-Mobile and AT&T are the same throughout the country...yes I said two...there are two primary GSM frequencies in the U.S. Don't quote me on that...but I'm pretty certain.

Except for a very small portion which they bought from Suncom (or rather when they bought Suncom), T-Mobile's entire native GSM (2G) network uses the 1900MHz frequency/band. Their 3G network uses the 1700MHz frequency band. AT&T uses both the 850MHz and 1900MHz frequencies/bands for 2G and 3G.