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Cellular Data Transmission Techniques
There are two main techniques for cellular data transmission, TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access).

The objective of both techniques is to support multiple simultaneous data channels. TDMA achieves this objective by dividing the radio frequency into time slots. TDMA is used by the GSM cellular system. CDMA comes from a military/defense background and is currently used by major cellular carriers in the United States. QUALCOMM has patented large parts of CDMA (www.QUALCOMM.com). CDMA uses a more complex mechanism to support simultaneous data channels, which is outside our scope.

Note: There are other techniques to distinguish data channels. For simplicity's sake, we've chosen to discuss only TDMA and CDMA since they're used in cellular systems worldwide. Other methods of cellular data transmission include FDMA (Frequency Division Multiple Access) and PDMA (Polarization Division Multiple Access). For more information on cellular data transmission techniques, see http://en.wikipedia.org/wiki/Cellular_network.

The Historical Evolution of Data Transmission Techniques

In addition to different types of data transmission techniques, cellular systems can be viewed as the more familiar generations of systems as they evolved over time (2G, 2.5G, and 3G). The main difference between generations is support for greater bandwidth. Obviously as you go towards 3G and beyond, the bandwidth increases and the applications supported become richer.

First-generation (1G) systems were analog systems. From 2G (second generation) onwards, cellular systems have been digital. We won't discuss analog systems so we'll start our discussion at 2G systems.

2G Systems
GSM (Global System for Mobile) is the most popular 2G system. GSM originated in Europe and is the dominant mobile system across the world. In some form, it's present on all continents including North America. GSM (based on TDMA) is a digital system with a relatively long history (the study group was founded in 1982) and is governed by the GSM Association (www.gsmworld.com/index.shtml). The GSM Association provides functional and interface specifications for functional entities in the system but not the actual implementation. Besides GSM, other examples of 2G systems are cdmaOne (mainly in the USA) and PDC (Personal Data Cellular) in Japan.

2G technologies are typically capable of supporting up to 14.4Kbps of data. 2G systems are circuit-switched (i.e., a circuit is first established between the sender and receiver before sending the information and is maintained for the duration of the session). The next evolutionary step (2.5G) is packet-switched (the data is broken into packets and no connection is maintained for the duration of the communication).

2.5G Systems
2.5G networks are an intermediate step undertaken by most mobile operators in their evolution from 2G to 3G. The main functional leap between 2G and 2.5G networks is the adoption of packetswitched technologies (in 2.5G networks) as opposed to circuit-switched technologies (in 2G networks). 2.5G networks are capable of theoretically supporting bandwidth up to 144Kbps but typically support 64Kbps. GPRS (General Packet Radio Service) in Europe and CDMA2000 1X in North America are examples of 2.5G networks. Applications such as sending still images are possible over 2.5G networks.

3G Systems
Most people have heard about 3G and have an opinion about it. If nothing else, they know about 3G in terms of the high prices paid by mobile operators for 3G licenses.

However, from a mobile operator's perspective there's a clear business case for investing in 3G because existing 2G networks are congested and 2.5G solutions are a halfway house and won't cope with the increasing demand (i.e., both the number of consumers and the richer application types).

From an application development perspective, 3G technologies are differentiated from 2.5G technologies by a greater bandwidth (theoretically 2Mbps but typically 384Kbps). Possible 3G applications include video streaming.

From the user point-of-view, the move from 2.5G networks to 3G networks is more evolutionary than revolutionary except for devices. 3G devices are significantly more complex because they have to support complex data types like video, provide more storage, and support multiple modes.

UMTS (Universal Mobile Telecommunications System) in Europe and CDMA2000 in the North America are examples of 3G systems. Note that 3G systems are all based on CDMA technologies.

The entire relationship between the various players in the mobile data industry can be shown by the mobile value chain.

This content is reprinted from Real-World AJAX: Secrets of the Masters published by SYS-CON Books. To order the entire book now along with companion DVDs, click here to order.