raffic. Terrestrial and systems. satellite voice systems support the SMS, which carries text The Ricochet system operates somewhat differently. messages up to about 150 characters over the cellular With low power transceivers mounted on light poles lo- network. Its successors, enhanced messaging service cated roughly every quarter mile, the system uses mostly (EMS) and multimedia messaging service (MMS), now unlicensed spectrum to offer 100-plus kbps Internet carry enhanced content such as pictures on some networks. access to subscribers. This proprietary system was de- ployed in 15 or 20 markets before being turned off for Private data financial reasons. At the time of this writing, it is back On an enterprise scale, VSAT and microwave technolo- online in two markets. gies exemplify high-performance data transfer. On a personal scale, technologies such as Bluetooth provide Cellular a medium for private data transfer over short ranges. The flagship wireless service is unquestionably cellular. Virtual private network (VPN) is an example of software From a few million North American users 10 years ago, that allows private communications across a shared in- the number of mobile subscribers exceeded the number frastructure (e.g., the Internet). of fixed telephone subscribers by the end of the 1990s as shown in Figure 1.0-2. Analog service offered by two Internet access carriers per market has given way to highperformance, Until recently, wireless Internet access involved feature-rich digital services (including SMS, caller ID, a 10 kbps cellular channel and a limited web view using three-way calling, etc.), often with four or more the text-based wireless application protocol (WAP) or carriers per market; additional base stations have similar mechanism. Today's digital cellular and wireless been installed to the point where dropped calls and LAN services now allow users full web access at rea- dead spots are exceptions to the rule of ubiquitous sonable speeds over a wide area. For higher rate fixed connectivity. acces