GSM
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Unlocking Your GSM Phone
Any GSM phone can work with any SIM card, but some service
providers "lock" the phone so that it will only work with
their service. If your phone is locked, you can't use it
with any other service provider, whether locally or
overseas. You can unlock the phone using a special code --
but it's unlikely your service provider will give it to you.
There are Web sites that will give you the unlock code, some
for a small fee, some for free. |
TDMA is also used as the access technology for
Global System for Mobile communications
(GSM). However,
GSM
implements TDMA in a somewhat different and incompatible way from
IS-136. Think of GSM and IS-136 as two different
operating systems that work on
the same
processor, like Windows and
Linux both working on an Intel Pentium III. 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
(sometimes referred to as 1.9-GHz) band in the United States. It is used
in digital cellular and
PCS-based
systems. GSM is also the basis for
Integrated Digital Enhanced Network
(IDEN), a popular system introduced by
Motorola and used by
Nextel.
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.
Unfortunately, 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. You
can get 900MHz/1800MHz GSM phones from
Planet Omni, an online
electronics firm based in California. They offer a wide selection of
Nokia,
Motorola and
Ericsson GSM phones. They don't
sell international SIM cards, however. You can pick up prepaid SIM cards
for a wide range of countries at
Telestial.com.
To read about any of the technologies click on the blue links
2G cell-phone networks
for transmitting information (we'll discuss
3G technologies in the 3G section):
·
Frequency division multiple access
(FDMA)
·
Time division multiple access
(TDMA)
·
Code division multiple access
(CDMA)
