This document defines the control messages of trunking for APCO Project 25 that may be applied to conventional systems. These control messages are extensions to the basic Common Air Interface. Conventional systems for APCO Project 25 are defined to be any non-trunked system which conforms to the standards and specifications of reference [1] or its subordinate documents, where a trunked system is defined to include a centralized controller that assigns requests for service by subscribers to specific radio channels. Conventional systems therefore do not have a controller which assigns radio channels in the same sense as defined in reference [2].