CDMA & GSM Cellular Technology

From:  http://www.protocols.com/pbook/cellular.htm

GSM

In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and phase I of the GSM specifications were published in 1990. Commercial service was started in mid­1991, and by 1993 there were 36 GSM networks in 22 countries, with 25 additional countries having already selected or considering GSM In addition to Europe, South Africa, Australia, and many Middle and Far East countries have chosen to adopt GSM. By the beginning of 1994, there were 1.3 million subscribers worldwide. The acronym GSM now (aptly) stands for Global System for Mobile telecommunications.

From the beginning, the planners of GSM wanted ISDN compatibility in services offered and control signaling used. The radio link imposed some limitations, however, since the standard ISDN bit rate of 64 Kbps could not be practically achieved.

The digital nature of GSM allows data, both synchronous and asynchronous data, to be transported as a bearer service to or from an ISDN terminal. The data rates supported by GSM are 300 bps, 600 bps, 1200 bps, 2400 bps, and 9600 bps.

The most basic teleservice supported by GSM is telephony. A unique feature of GSM compared to older analog systems is the Short Message Service (SMS). Supplementary services are provided on top of teleservices or bearer services, and include features such as international roaming, caller identification, call forwarding, call waiting, multi­party conversations, and barring of outgoing (international) calls, among others.

CDMA

Code Division Multiple Access (CDMA) is a digital air interface standard, claiming eight to fifteen times the capacity of traditional analog cellular systems. It employs a commercial adaptation of a military spread-spectrum technology. Based on spread spectrum theory, it gives essentially the same services and qualities as wireline service. The primary difference is that access to the local exchange carrier (LEC) is provided via a wireless phone.

Though CDMA’s application in cellular telephony is relatively new, it is not a new technology. CDMA has been used in many military applications, such as:

• Anti-jamming (because of the spread signal, it is difficult to jam or interfere with a CDMA signal).
• Ranging (measuring the distance of the transmission to know when it will be received).
• Secure communications (the spread spectrum signal is very hard to detect).

CDMA is a spread spectrum technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal. With CDMA, unique digital codes, rather than separate RF frequencies or channels, are used to differentiate subscribers. The codes are shared by both the mobile station (cellular phone) and the base station, and are called pseudo-random code sequences. Since each user is separated by a unique code, all users can share the same frequency band (range of radio spectrum). This gives many unique advantages to the CDMA technique over other RF techniques in cellular communication.

CDMA is a digital multiple access technique and this cellular aspect of the protocol is specified by the Telecommunications Industry Association (TIA) as IS-95. In CDMA, the BSSAP is divided into the DTAP and BSMAP (which corresponds to BSSMAP in GSM).