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Cell phone technology seems an obvious approach to mobile computer communication, and indeed data services based on cellular standards are commercially available. One drawback is the cost to users, due in part to cellular’s use of licensed spectrum (which has historically been sold off to cellular phone operators for astronomical sums). The frequency bands that are used for cellular telephones (and now for cellular data) vary around the world. In Europe, e.g., the main bands for cellular phones are at 900 and 1800 MHz. In North America, 850-and 1900-MHz bands are used. This global variation in spectrum usage creates problems for users who want to travel from one part of the world to another, and has created a market for phones that can operate at multiple frequencies (e.g., a tri-band phone can operate at three of the four frequency bands mentioned above). That problem, however, pales in comparison to the proliferation of incompatible standards that have plagued the cellular communication business. Only recently have some signs of convergence on a small set of standards appeared. And finally, there is the problem that most cellular technology was designed for voice communication, and is only now starting to support moderately high-bandwidth data communication.

Like 802.11 and WiMAX, cellular technology relies on the use of base stations that are part of a wired network. The geographic area served by a base station’s antenna is called a cell. A base station could serve a single cell or use multiple directional antennas to serve multiple cells. Cells do not have crisp boundaries, and they overlap. Where they overlap, a mobile phone could potentially communicate with multiple base stations. This is somewhat similar to the 802.11 picture. At any time, however, the phone is in communication with, and under the control of, just one base station. As the phone begins to leave a cell, it moves into an area of overlap with one or more other cells. The current base station senses the weakening signal from the phone, and gives control of the phone to whichever base station is receiving the strongest signal from it. If the phone is involved in a call at the time, the call must be transferred to the new base station in what is called a handoff .


As we noted above, there is not one unique standard for cellular, but rather a collection of competing technologies that support data traffi c in different ways and deliver different speeds. These technologies are loosely categorized by “ generation. ” The first generation (1G) was analog, and thus of limited interest from a data communications perspective. Most of the cell phone technology currently deployed is considered second generation (2G) or “ 2.5G ” (not quite worthy of being called 3G, but more advanced than 2G). The 2G and later technologies are digital. The most widely deployed 2G technology is referred to as GSM — the Global System for Mobile Communications — which is used in more than 200 countries. North America, however, is a late adopter of GSM, which helped prolong the proliferation of competing standards.

Most 2G technologies use one of two approaches to sharing a limited amount of spectrum between simultaneous calls. One way is a combination of frequency-division multiplexing (FDM) and time-division multiplexing (TDM). The spectrum available is divided into disjoint frequency bands, and each band is subdivided into time slots. A given call is allocated every n th slot in one of the bands. The other approach is code division multiple access (CDMA). CDMA does not divide the channel in either time or frequency, but rather uses different chipping codes to distinguish the transmissions of different cell phone users.

The 2G and later cell phone technologies use compression algorithms tailored to human speech to compress voice data to about 8 Kbps without losing quality. Since 2G technologies focus on voice communication, they provide connections with just enough bandwidth for that compressed speech — not enough for a decent data link. One of the first cellular data standards to gain widespread adoption is the General Packet Radio Service (GPRS), which is part of the GSM set of standards and is often referred to as a 2.5G technology.

GSM networks make use of a multiplexing technique called time-division multiple access (TDMA). (Confusingly, there is also a particular cellular standard that is sometimes called TDMA, but is known formally as IS-136.) You can think of TDMA as being like TDM — traditionally used for telephone services — with the additional feature that the timeslots can be dynamically allocated to users or devices that need them (and deallocated from devices that no longer need them). The number of timeslots that are available for GPRS at a given frequency depends on how many cellular voice calls are currently in progress, since voice calls also consume timeslots. As a result, GPRS data rates tend to be lower in busy cells. In practice, users often get between 30 and 70 Kbps — coincidentally, just about the same as a user of a dial-up modem on a landline. Nevertheless, GPRS has proven quite useful and popular in some parts of the world as a way to communicate wirelessly when faster connection methods (such as 802.11) are not available. Other 2.5G data standards have also become available and some manage to be quite a bit higher in bandwidth than GPRS.

The concept of a third generation (3G) was established before there was any implementation of 3G technologies, with the aim of shaping a single international standard that would provide much higher data bandwidth. Unfortunately, at the time of writing, several mutually incompatible 3G standards are emerging. Thus, the possibility that cellular standards will continue to diverge seems quite realistic. Interestingly, all the 3G standards are based on variants of CDMA. For example, the Universal Mobile Telecommunications System (UMTS) is based on wideband CDMA (W-CDMA). UMTS appears poised to be the successor to GSM, and in fact is sometimes referred to as 3GSM (i.e., the third-generation version of GSM). UMTS is intended to support data transfer rates of up to 1.92 Mbps, although real network conditions will probably result in lower rates in practice. Nevertheless, it should represent a significant performance improvement over GPRS. And like GSM, it should have quite widespread (if not actually universal) adoption around the world.

There are a number of commercial UMTS networks in operation at the time of writing with many more announced or planned. And to make it quite clear that work in this field is far from complete, we note that 3.5G and 4G standards are also in the works.

Finally , it should be noted that there is a class of mobile phones that are not cellular phones but satellite phones, or satphones . Satphones use communication satellites as base stations, communicating on frequency bands that have been reserved internationally for satellite use. Consequently, service is available even where there are no cellular base stations. Satphones are rarely used where cellular is available, since service is typically much more expensive. Satphones are also larger and heavier than modern cell phones because of the need to transmit and receive over much longer distances, to reach satellites rather than cellphone towers. Satellite communication is more extensively used in television and radio broadcasting, taking advantage of the fact that the signal is broadcast, not point-to-point. High-bandwidth data communication via satellite are commercially available, but its relatively high price (for both equipment and service) limits its use to regions where no alternative is available.

Source of Information : Elsevier Wireless Networking Complete 2010