COMMUNICATION TECHNOLOGIES IN SECOND GENERATION CELL-PHONES

There are four common technologies used by 2G cell-phone networks for transmitting
information:

1. Frequency Division Multiple Access (FDMA) : FDMA separates the spectrum into
distinct voice channels by splitting it into uniform chunks of bandwidth. Each call
sends its signal at a different frequency within the available band. FDMA is used mainly
for analog transmission.

 

2. Time Division Multiple Access (TDMA) : TDMA is the access method used by the
Electronics Industry Alliance and the Telecommunications Industry Association for
Interim Standard 54 (IS-54) and Interim Standard 136 (IS-136). Using TDMA, a
narrow band that is 30 kHz wide and 6.7 milliseconds long is split time-wise into three
time slots. Each conversation gets the signal for one-third of the time. This is possible
because voice data that has been converted to digital information is compressed so that it
takes up significantly less transmission space. Therefore, TDMA has three times the
capacity of an analog system using the same number of channels. TDMA systems
operate in either the 800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands.

3. Code Division Multiple Access (CDMA) : CDMA takes an entirely different approach
from TDMA. CDMA, after digitizing data, spreads it out over the entire available
bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned
a unique sequence code. CDMA is a form of spread spectrum, which simply means that
data is sent in small pieces over a number of the discrete frequencies available for use at
any time in the specified range.

4. Global System for Mobile Communication (GSM) : GSM implements TDMA in a
somewhat different and incompatible way from IS-136. GSM systems use encryption to
make phone calls more secure. GSM operates in the 900-MHz and 1800-MHz bands in
Europe and Asia and in the 850-MHz and 1900-MHz band in the United States. It is
used in digital cellular and PCS (Personal Communication Services)-based systems.
GSM is also the basis for Integrated Digital Enhanced Network (IDEN), a popular
system introduced by Motorola and used by Nextel.
PCS was designed from the ground up for greater user mobility. PCS has smaller cells and
therefore requires a larger number of antennas to cover a geographic area. PCS phones
use frequencies between 1.85 and 1.99 GHz (1850 MHz to 1990 MHz).
Technically, cellular systems in the United States operate in the 824-MHz to 894-MHz
frequency bands; PCS operates in the 1850-MHz to 1990-MHz bands. And while it is
based on TDMA, PCS has 200-kHz channel spacing and eight time slots instead of the
typical 30-kHz channel spacing and three time slots found in digital cellular.
GSM is the international standard in Europe, Australia and much of Asia and Africa. In
covered areas, cell-phone users can buy one phone that will work anywhere where the
standard is supported. To connect to the specific service providers in these different
countries, GSM users simply switch subscriber identification module (SIM) cards. SIM
cards are small removable disks that slip in and out of GSM cell phones. They store all the
connection data and identification numbers you need to access a particular wireless service
provider.

The 850MHz/1900-MHz GSM phones used in the United States are not compatible with
the international system. If you live in the United States and need to have cell-phone
access when you're overseas, you can either buy a tri-band or quad-band GSM phone and
use it both at home and when traveling or just buy a GSM 900MHz/1800MHz cell phone
for traveling.