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SYSTEMS AND SUBSYSTEMS CONTROL APPROACHES The airlink subsystem is defined as the span between the user host s connection point and the subscriber unit. It includes the service provider s message switch/area controller, base stations, and the radio modem. There are (at least!) four current philosophical control approaches: 1. Decentralized: Nearby units are able to communicate with each other with no intervening infrastructure. As the distance increases, disrupting the information flow, nearby nodes pick up the transmitting signal. They hand off the data from node to node until a base capable of connecting to the wireline network is found. The message is. | The Wireless Data Haadbaak Fourth Edition. James F. DeRose Copyright 1999 John Wiley Sons Inc. ISBNs 0-471-31651-2 Hardback 0-471-22458-8 Electronic 18 SYSTEMS AND SUBSYSTEMS CONTROL APPROACHES The airlink subsystem is defined as the span between the user host s connection point and the subscriber unit. It includes the service provider s message switch area controller base stations and the radio modem. There are at least four current philosophical control approaches 1. Decentralized . Nearby units are able to communicate with each other with no intervening infrastructure. As the distance increases disrupting the information flow nearby nodes pick up the transmitting signal. They hand off the data from node to node until a base capable of connecting to the wireline network is found. The message is then forwarded for action. An example of this approach is Metricom s Ricochet. 2. Partially Decentralized . The subscriber unit assisted by a high-function base station makes many channel assignment decisions. A base station knowing the location of the target subscriber unit can make the data transfer without going to higher levels. An example of this approach is the Ericsson system Mobitex used by BSWD. 3. Partially Centralized . High-function base stations are responsible for detailed control of the radio interface including channel allocation and access control. The subscriber units listen to commands broadcast by the base stations and follow those directions. A higher authority controls routing and relay functions for the base stations. The primary example of this approach is CDPD. 292 DECENTRALIZED METRICOM RICOCHET 293 4. Centralized High-level processors control all elements of channel assignment and message sequencing for a given geographical area which can range from citywide to multistate. Data transfers between subscriber units always rise to the highest level before descending again to the target. The primary example of this approach is the Motorola .