Digital Phone Technologies: TDMA, CDMA, and GSM

Whenever a new technology is introduced, several different versions often compete for attention until one comes to dominate the market. Digital wireless is no different; most service providers support one of three incompatible technologies for transmitting and decoding phone calls. Not surprisingly each technology has its proponents and detractors. Practically speaking, the most important consideration for consumers is ensuring that your phone and service provider both "speak the same language." For instance, you can use a CDMA-compatible phone only on a CDMA-compatible network.

Let's take a look at the different technologies:

TDMA (Time Division Multiple Access)
TDMA is a time-sharing technology that allows three calls to use a single frequency at the same time. Since each frequency can juggle multiple calls at a time, TDMA based digital networks have a call capacity of around 1,000, whereas analog networks can handle around 100 calls at once. Each call is assigned a tiny time slot during which it has use of the full channel. Calls take turns in a circular fashion to use the channel. During a call, voice or data is momentarily stored then sent in bursts when the designated time slot comes up. A different time slot comes around and the next user's voice or data is sent through. Since the time slots are very narrowly "sliced" and the calls are highly synchronized, users detect no delay whatsoever.

CDMA (Code Division Multiple Access)
CDMA assigns a unique code to each call in the system. Think of this code like a FedEx tracking number. Your package may be sent via plane, through several distribution centers, and on numerous trucks before reaching its final destination, but through the tracking number, FedEx can tell you where your shipment is at a given moment. Similarly a CDMA code allows calls to hop from available frequency to frequency without getting lost and without interference. Since CDMA calls use whatever bandwidth is available, this efficiency enables CDMA based digitial networks to carry between 10-15 times more calls per frequency than with analog, or around 10,000 calls total at a time.

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 8-10 time slots per channel. GSM was originally developed in Europe where it operates at 900 MHz and 1800 MHz. North American GSM which operates at 1900 MHz is only used by PCS providers.

• The bad news. Since European and North American GSM operate at different frequency bands, you can't use your North American GSM phone while traveling through Europe.

• The good news. Most GSM phones use a removable Subscriber Identity Module (SIM), more commonly known as a smart card, 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. (The reverse is also true for European users in North America.)

• The better news. Manufacturers are currently developing multi-band GSM that, once made available to the public, will allow GSM phone owners from either side of the Atlantic to roam on both continents with a single phone (again, as long as your service provider has roaming agreements with carriers abroad).