The authentication service described in this document is applicable to FDMA and TDMA trunking systems using an FDMA trunking control channel. Authentication is a supplementary service for trunked radio systems. This document describes two forms of authentication: unit authentication and mutual authentication. If unit authentication is implemented it shall be implemented as specified herein. If mutual authentication is implemented @ it shall be implemented as specified herein. This document defines the full standard across the Um interface.