Mobile Forensics..Understanding the Technology of GSM Vs CDMA



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 access).

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 frequency.

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 sequence.

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 bands.

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 signal.

Since 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 technologies.:

Cellular technology Generation Data transmission capacity
GSM 2G 56 Kps
CDMA (IS-95B) 2.5G 64 Kps - 140 Kps
CDMA 2000 3G 2 MBps

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

Related Articles:

Mobile Forensics..A

"Forensics and GSM Mobile Telephone System", by Svein Y Williassen, Senior Investigator, Cyber Forensics, Ibas, AS.

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