Understand mobile phone technologies. (Advisor[R] Answer[TM]).

Q: Why can't I use any mobile phone with any carrier network?

A: Whenever a technology comes on the scene, several approaches often compete until one dominates the market. Wireless phones are no different. Most wireless phone companies support one of three incompatible technologies for transmitting and decoding phone calls. Wireless data transmission technologies are even more splintered. Each technology has its pros and cons. The most important consideration for consumers is ensuring the device and service provider "speak the same language."

CDMA 

Code Division Multiple Access (CDMA) assigns a unique code to each call in the system. This code lets calls hop from available frequency to frequency, without getting lost and without interference. CDMA is efficient because it uses whatever bandwidth is available. CDMA-based digital networks carry between 10-15 times more calls per frequency than analog (about 10,000 calls at a time, versus about 100).

TDMA 

Time Division Multiple Access (TDMA) is a time-sharing technology that lets three calls use the same frequency. Because each frequency can juggle multiple calls, TDMA-based digital networks have a call capacity of about 1,000. The network assigns each call a tiny time slot, during which it has full use of the channel. Calls take turns using the channel in a round-robin fashion. During a call, voice or data is momentarily stored and sent in bursts when the next time slot comes up. A different time slot comes around and the next user's voice or data goes through. Because the time slots are narrowly sliced and highly synchronized, users don't detect the delays.

GSM

Global Standard for Mobile Communications (GSM) borrows from both CDMA and TDMA. Like CDMA, GSM encodes calls. Like TDMA, GSM also separates calls into multiple time slots, but in a way that supports between eight and ten time slots per channel. GSM was originally developed in Europe where it operates at 900MHz and 1800MHz. North American GSM operates at 1900MHz.

Bad news

Since European and North American GSM operate at different frequency bands, you can't use most North American GSM phones while traveling through Europe.

Good news

Most GSM phones use a removable Subscriber Identity Modules (SIM), more commonly known as smart cards, containing a user's account information, phone book, and security parameters. By inserting the smart card from your North American GSM phone into a European GSM phone, you can successfully roam throughout most of Europe if your provider has roaming agreements with European carriers. For example, AT&T Wireless has launched two-way roaming capabilities with 41 GSM mobile phone carriers in 35 countries. For a list of available countries and carriers, go to: http://www.attws.com/mobileinternet/international/int_carrier.jhtml

Even better news

Manufacturers have developed multi-band GSM phones that let users from either side of the Atlantic roam on both continents (again, as long as the service provider has roaming agreements with carriers abroad). The Motorola A388 is one such phone; you can find a review of this device on page 21 of this issue.

GPRS 

General Packet Radio Services (GPRS) is a packet-based wireless data communication service that promises rates from 56Kbps up to 114Kbps and continuous connection to the Internet for mobile phone and computer users. The higher data rates let users run video conferencing and other multimedia applications on mobile handheld devices and notebook computers. GPRS is based on GSM and complements existing services, such as circuit-switched cellular phone connections and the Short Message Service (SMS).

In theory, GPRS packet-based service should cost users less than circuit-switched services because communication channels aren't dedicated to only one user at a time. It should also be easier to make applications available to mobile users because the faster data rate means the middleware currently needed to adapt applications to the slower speed of wireless systems won't be necessary. As GPRS becomes avail able, mobile users of virtual private networks (VPNs) will be able to access the private network continuously, rather than through a dial-up connection.

GPRS will also complement Bluetooth, a standard for replacing wired connections between devices with wireless radio connections. In addition to the Internet Protocol (IP), GPRS supports X.25, a packet-based protocol used primarily in Europe.

2G

Second Generation (2G) refers to digital cellular and PCS wireless systems oriented to integrated voice and low-speed data services. GSM, for example, is a 2G technology. It uses TDMA technology, proving data speeds of 9.6Kbps/14.4Kbps.

2.5G

The packet radio upgrade to GSM, called GPRS, can have speeds of up to 114Kbps. GPRS an interim technology towards 3G, and hence is known as 2.5G.

3G

Third Generation (3G), refers to the next generation of wireless systems: digital with high-speed data being standardized by the 3rd Generation Partnership Project for W-CDMA (3GPP) and the 3rd Generation Partnership Project for cdma2000 (3GPP2). 3G networks will offer dramatically improved data transfer rates, enabling new wireless applications such as streaming media.

In all, these technologies fall under the International Telecommunications Union's (ITU's) generic name of International Mobile Telecommunications 2000 (IMT-2000). But, when the ITU tried to unify and standardize 3G technologies, no consensus was reached. There were thus five terrestrial standards developed as part of the IMT-2000 program. Depending on where in the world 3G is implemented, the 3G standard will be based on CDMA variants cdma2000 or W-CDMA.

You can find more details on CDMA 3G variants in the IMT 2000 family at http://Advisor.com/doc/11141.

--John R. Vacca, IT Consultant