What is the main difference between GSM & CDMA?
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Answer Posted / shashi
In cellular service there are two main competing network
technologies: Global System for Mobile Communications (GSM)
and Code Division Multiple Access (CDMA). Cellular carriers
including Sprint PCS, Cingular Wireless, Verizon and T-
Mobile use one or the other. Understanding the difference
between GSM and CDMA will allow you to choose a carrier
that uses the preferable network technology for your needs.
The GSM Association is an international organization
founded in 1987, dedicated to providing, developing, and
overseeing the worldwide wireless standard of GSM. CDMA, a
proprietary standard designed by Qualcomm in the United
States, has been the dominant network standard for North
America and parts of Asia. However, GSM networks continue
to make inroads in the United States, as CDMA networks make
progress in other parts of the world. There are camps on
both sides that firmly believe either GSM or CDMA
architecture is superior to the other. That said, to the
non-invested consumer who simply wants bottom line
information to make a choice, the following considerations
may be helpful.
Coverage: The most important factor is getting service in
the areas you will be using your phone. Upon viewing
competitors' coverage maps you may discover that only GSM
or CDMA carriers offer cellular service in your area. If
so, there is no decision to be made, but most people will
find that they do have a choice.
Data Transfer Speed: With the advent of cellular phones
doing double and triple duty as streaming video devices,
podcast receivers and email devices, speed is important to
those who use the phone for more than making calls. CDMA
has been traditionally faster than GSM, though both
technologies continue to rapidly leapfrog along this path.
Both boast "3G" standards, or 3rd generation technologies.
EVDO, also known as CDMA2000, is CDMA's answer to the need
for speed with a downstream rate of about 2 megabits per
second, though some reports suggest real world speeds are
closer to 300-700 kilobits per second (kbps). This is
comparable to basic DSL. As of fall 2005, EVDO is in the
process of being deployed. It is not available everywhere
and requires a phone that is CDMA2000 ready.
GSM's answer is EDGE (Enhanced Data Rates for GSM
Evolution), which boasts data rates of up to 384 kbps with
real world speeds reported closer to 70-140 kbps. With
added technologies still in the works that include UMTS
(Universal Mobile Telephone Standard) and HSDPA (High Speed
Downlink Packet Access), speeds reportedly increase to
about 275—380 kbps. This technology is also known as W-
CDMA, but is incompatible with CDMA networks. An EDGE-ready
phone is required.
In the case of EVDO, theoretical high traffic can degrade
speed and performance, while the EDGE network is more
susceptible to interference. Both require being within
close range of a cell to get the best speeds, while
performance decreases with distance.
Subscriber Identity Module (SIM) cards: In the United
States only GSM phones use SIM cards. The removable SIM
card allows phones to be instantly activated, interchanged,
swapped out and upgraded, all without carrier intervention.
The SIM itself is tied to the network, rather than the
actual phone. Phones that are card-enabled can be used with
any GSM carrier.
The CDMA equivalent, a R-UIM card, is only available in
parts of Asia but remains on the horizon for the U.S.
market. CDMA carriers in the U.S. require proprietary
handsets that are linked to one carrier only and are not
card-enabled. To upgrade a CDMA phone, the carrier must
deactivate the old phone then activate the new one. The old
phone becomes useless.
Roaming: For the most part, both networks have fairly
concentrated coverage in major cities and along major
highways. GSM carriers, however, have roaming contracts
with other GSM carriers, allowing wider coverage of more
rural areas, generally speaking, often without roaming
charges to the customer. CDMA networks may not cover rural
areas as well as GSM carriers, and though they may contract
with GSM cells for roaming in more rural areas, the charge
to the customer will generally be significantly higher.
International Roaming: If you need to make calls to other
countries, a GSM carrier can offer international roaming,
as GSM networks dominate the world market. If you travel to
other countries you can even use your GSM cell phone
abroad, providing it is a quad-band phone
(850/900/1800/1900 MHz). By purchasing a SIM card with
minutes and a local number in the country you are visiting,
you can make calls against the card to save yourself
international roaming charges from your carrier back home.
CDMA phones that are not card-enabled do not have this
capability, however there are several countries that use
CDMA networks. Check with your CDMA provider for your
specific requirements.
According CDG.org, CDMA networks support over 270 million
subscribers worldwide, while GSM.org tallies up their score
at over 1 billion. As CDMA phones become R-UIM enabled and
roaming contracts between networks improve, integration of
the standards might eventually make differences all but
transparent to the consumer.
The chief GSM carriers in the United States are Cingular
Wireless, recently merged with AT&T Wireless, and T-Mobile
USA. Major CDMA carriers are Sprint PCS, Verizon and Virgin
Mobile. There are also several smaller cellular companies
on both networks.
Code division multiple access (CDMA) is a form of
multiplexing and a method of multiple access to a physical
medium such as a radio channel, where different users use
the medium at the same time thanks to using different code
sequences.
A number of terms are used to refer to CDMA
implementations. The original US standard defined by
QUALCOMM was known as IS-95, where IS refers to an Interim
Standard of the US Telecommunications Industry Association.
IS-95 is often referred to as the second generation (2G)
cellular, or as cdmaOne (the QUALCOMM brand name). CDMA has
been submitted for approval as a mobile air interface
standard to the International Telecommunication Union (ITU).
Whereas the Global System for Mobile Communications (GSM)
standard is a specification of an entire network
infrastructure, the CDMA interface relates only to the air
interface—the radio part of the technology. For example,
GSM specifies an infrastructure based on internationally
approved standard, while CDMA allows each operator to
provide the network features as it finds suited. On the air
interface, the signalling suite (GSM: ISDN SS7) work has
been progressing to harmonise these features.
After a couple of revisions, IS-95 was superseded by the IS-
2000 standard (CDMA2000). This standard was introduced to
meet some of the criteria laid out in the IMT-2000
specification for third generation (3G) cellular. It is
also called 1xRTT which means "1 times Radio Transmission
Technology" because IS-2000 uses the same 1.25 MHz carrier
shared channel as the original IS-95 standard. A related
scheme called 3xRTT uses three 1.25 MHz carriers for a 3.75
MHz bandwidth that would allow higher data burst rates for
an individual user, but the 3xRTT scheme has not been
commercially deployed. More recently, QUALCOMM has led the
creation of a new CDMA-based technology called Evolution-
Data Optimized (1xEV-DO, or IS-856), which provides the
higher packet data transmission rates required by IMT-2000
and desired by wireless network operators.
This CDMA system is frequently confused with a similar but
incompatible technology called Wideband Code Division
Multiple Access (W-CDMA) which is the basis of the W-CDMA
air interface. The W-CDMA air interface is used in the
global 3G standard UMTS and the Japanese 3G standard FOMA,
by NTT DoCoMo and Vodafone; however, the CDMA family of US
national standards (including cdmaOne and CDMA2000) are not
compatible with the W-CDMA family of ITU standards.
Another important application of code division
multiplexing — predating and distinct from CDMA — is the
Global Positioning System (GPS).
The QUALCOMM CDMA system includes very accurate time
signals (usually referenced to a GPS receiver in the cell
base station), so cell phone CDMA-based clocks are an
increasingly popular type of radio clock for use in
computer networks. The main advantage of using CDMA cell
phone signals for reference clock purposes is that they
work better inside buildings, thus often eliminating the
need to mount a GPS antenna outside a building.
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