Overview

The Specialized Mobile Radio (SMR) service was first established by the Wireless Radio Commission (part of the Federal Communications Commission [FCC]) in 1979 to provide land/mobile communications on a commercial basis. An SMR system consists of the following:

• One or more base station transmitters

• One or more antennas

• End user radio equipment that usually consists of a mobile radio unit, either provided by the end user or obtained from the SMR operator for a fee

SMR users operate in either an interconnected mode or a dispatch mode:

• Interconnected mode Links the mobile radio with the Public Switched Telephone Network (PSTN). An end user transmits a message via the mobile radio unit to the SMR base station. The call is then routed to the local dial-up telephone network, which enables the mobile radio unit to function as a mobile telephone.

• Dispatch mode Allows two-way, over-the-air, voice communications between two or more mobile units or mobile units and fixed locations.

SMR customers using dispatch communications include construction companies with several trucks at different jobs or an on-the-road trucking company with a dispatch operation in a central office.

SMR systems consist of two distinct types: conventional and trunked systems. A conventional system allows the use of only one channel. If someone else is already using that end user's assigned channel, the end user must wait until the channel is available, as shown in Figure 19-1. In contrast, a trunked system combines channels and contains microprocessor capabilities...