The mobile industry in India is divided between the two
technologies GSM and CDMA. While the earlier service providers had adopted the
GSM technology, the new players have been using CDMA technology and have notched
up a significant share of the Indian market. Hence any discussion on Mobile
Forensics need to take into account the presence of the two technologies.
It is necessary for us to understand the basic
differentiation of the two technologies as they may have an impact on the
Forensics. An attempt is made here to present the fundamental technical aspects
about the two systems.
GSM stands for Global System for Mobile
Communications and CDMA stands for Code Division Multiple Access. They represent
different systems of sharing of the radio spectrum for communication.
Normally the radio spectrum can be
shared by different users accessing the same frequency band without causing
interference. The techniques used for this are TDMA (Time division multiple access), FDMA (Frequency
division multiple access) and CDMA (Code division multiple
GSM (Global System for Mobile Communications) is a
form of multiplexing, which divides the available bandwidth among
the different channels.
GSM is a combination of Time and
Frequency-Division Multiple Access (TDMA/FDMA). The FDMA part
involves the division by frequency of the (maximum) 25 MHz bandwidth
into 124 carrier frequencies spaced 200 kHz apart. Each of these
carrier frequencies is then divided in time, using a TDMA scheme.
The fundamental unit of time in this TDMA scheme is called a burst
period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst
periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615
ms), which forms the basic unit for the definition of logical
channels. One physical channel is one burst period per TDMA frame.
Thus GSM allows eight simultaneous calls on the same radio
CDMA (Code Division Multiple Access) is a form of
multiplexing (access to the same resource will be given to more than
one user),which allows the use of a particular frequency for a
number of signals, optimizing the use of available bandwidth. It is
a cellular technology that uses spread-spectrum techniques. In CDMA
technology every channel uses the full available spectrum.
Individual conversations are encoded with a pseudo-random digital
CDMA employs analog-to-digital conversion (ADC)
in combination with spread spectrum technology. Audio input is first
digitized (ADC) into binary elements. The frequency of the
transmitted signal is then made to vary according to a defined
pattern (code), so it can be intercepted only by a receiver whose
frequency response is programmed with the same code, so it follows
exactly along with the transmitter frequency. There are trillions of
possible frequency-sequencing codes; this enhances privacy and makes
cloning difficult. The technology is used in ultra-high-frequency
(UHF) cellular telephone systems in the 800-MHz and 1.9-GHz
GSM was first introduced in 1991 and until
recently before the establishment of CDMA networks, GSM was the only
mobile communication system present in the market. CDMA was first used during World War II by the English
allies to foil German attempts at jamming transmissions. The allies
decided to transmit over several frequencies, instead of one, making
it difficult for the Germans to pick up the complete
bandwidth is the major problem in the modern times the CDMA has a
very clear advantage over the GSM in these terms. The number of
channels(users) that can be allocated in a given bandwidth is
comparatively higher for CDMA than for GSM. The cost of setting up a
CDMA network is also comparatively less than the GSM network. Due to
these advantages there is high probability that CDMA technology will
dominate the future of mobile communications.
The technologies are normally evaluated
on the following three parameters namely the data transmission capacity,
security and radiation levels.
Following table indicates the data transmission of different
||Data transmission capacity
||64 Kps - 140 Kps
The idea of technology with superior security is
not a new one. In 1935, a Russian researcher Dmitrii Vasilevich AGEEV, published
his book "The basics of linery selection theory", where he explained the concept
of coding the signals. After the WWII, Soviet and American military
communication systems started to use the concept very widely because of many
valuable advantages of the system. The origin concept of CDMA scheme was
recommended by QUALCOMM (the famous communication provider in the US and
worldwide), however Korean research institute, ETRI and companies like Hyundai,
LG, and Samsung performed its realization for the first time in the world in
1995. As of today many countries have accepted it as a national standard of
mobile communication and worldwide number of CDMA subscribers has climbed to
over 100 million.
As already explained, CDMA uses a radically
different approach to what GSM does. It assigns a unique "code" to put multiple
users on the same wideband channel at the same time. The codes so-called
"pseudo-random code sequence" is used by both the mobile station (handset) and
the base station to distinguish between conversations. This gives a greater
level of privacy and security to the communication.
As far as radiation level concerned, CDMA is the
most harmless one among all existing technologies. Of course, it transmits
microwaves while on standby mode, like other technologies do. However, CDMA
technology checks 800 times per second its transmission level. Therefore,
radiation level is 10 times less than GSM. Another important thing to
point out is that CDMA system transmits signals only when the user starts
conversation. Simply saying, when you're listening the other ends conversation,
you are not affected by microwave as the speaking person does.
It appears that CDMA would be the dominating
technology in future and Mobile Forensics has to gear itself to the requirements
of the CDMA technology.
December 4, 2004
and GSM Mobile Telephone System", by Svein Y Williassen, Senior Investigator,
Cyber Forensics, Ibas, AS.