The demand for wireless data services appears to be inescapable and relentlessly increasing. This demand is driven by both a growing reliance on data communications, especially in the form of Email, and an ever-increasing desire for mobility. Recent technical developments are now allowing this demand to be met at a commercially reasonable cost.
In earlier years, wireless data communications were expensive and justifiable only by those willing to pay the price, such as the military, public safety or other governmental authorities. Other early adopters of wireless data technology had very specific needs, typified by services such as package delivery or taxi dispatch, or applications such as remote telemetry. These early wireless data solutions were typically mission-critical to their customers. They were also typically private, closed systems, which were dedicated to a single purpose rather than serving multiple user constituencies.
Early wireless data communications systems were physical layer-centric by design. Radio-based systems were developed from an RF (radio frequency) technology perspective, with customizations enhancing the efficiency of the scarce resource-the RF channel. These customizations often included shared RF channels, canned messages and proprietary protocols at all levels. Reminiscent of the early days of computing, applications were developed with full consideration of physical resource limitations; the RF protocols in use were clearly visible to applications, countering today's prevailing design wisdom of layered protocol architectures.
RF bandwidth has always been a key constraint in wireless data systems. Efficient use of limited bandwidth has typically involved the reuse of scarce radio frequencies with geographic separation of simultaneously-transmitting-on-the-same-frequency units to prevent interference. This is the architectural basis of current cellular systems, as we discussed in Chapter 2. Increasing system bandwidth is most easily accomplished by subdividing coverage areas called cells into smaller cells or microcells, with corresponding decreased transmission power levels.
From the earliest days of commercial wireless data services, expectations have been high for growth of customer bases and revenues. While these high expectations remain largely unsatisfied, there is little doubt as to their eventual fruition: the demand for mobile data communications appears insatiable. As we discussed in Chapter 1, mobility is enhanced by wirelessness. Continuing improvements in technologies as disparate as batteries and digital signal processors are increasing both the effective duty cycle and reliability of wireless data transmission.
The following sections will describe non-cellular approaches to mobile data networking. Non-cellular in this context refers to services and technology provided by organizations other than cellular carriers, which we discussed in Chapter 2.