The Ubiquity of Cellular

Rapid and accelerating growth of the cellular telephone industry over its first 12 years has resulted in extensive coverage of populated areas by cellular services. Cellular is now the dominant two-way mobile communications technology, with more than eighteen thousand cell sites covering over 95% of the U.S. population and serving over 33 million subscribers -a 13% market penetration-at year-end 1995, as depicted in Table 2.1.^2.1

  

Table 2.1: Growth of AMPS Subscriber Base

2#2

This extensive coverage provided by cellular voice services would seem to make cellular the ideal medium for providing ubiquitous wireless data services. Unfortunately, the cellular industry's voice heritage impacts its ability to support data applications. Cellular's circuit-switched orientation and radio channel characteristics conflict somewhat with the needs of data applications.

Cellular systems have followed the traditional circuit-switched channel model of telephony. In this model, the end-to-end circuit, including the cellular channel, is dedicated to a single user or application before they can transmit on the channel. The channel remains dedicated to the user or application for the duration of the transmission, until it is explicitly released.

A single dedicated channel per user may be suitable for voice applications albeit somewhat inefficient from a channel perspective. However, a dedicated channel per data user is extremely inefficient and thus prohibitively expensive, unless the data application involves bulk transport of large quantities of data^2.2 or the application is of a high-performance mission-critical nature^2.3 . In support of the circuit-switched nature of cellular, billing systems have been oriented toward billable units of time on the order of minutes of airtime, rather than quantities which better match the activities of data users.^2.4

The characteristics of the radio channel used by cellular systems have also challenged data applications attempting to use the cellular channels. As we shall see, these radio characteristics are further exacerbated by call control messages which are transmitted in-band while the call (data transmission) is in progress.