This document specifies the procedures for conventional LMR systems that permit the conveyance of digital voice, data packets, and supplementary data messages over the common air interface, and permit interconnect calls to a PSTN.

This document does not explicitly address any procedures required in support of any non-conventional (e.g., trunking) modes of operation.

A conventional LMR system is one that does not assign radio channels to SUs through the use of a control channel and over-the-air signaling. It is different from a trunking LMR system which includes some means of assigning radios to channels.

A digital conventional LMR system is one which meets the minimum requirements specified in this document. Additional functions or features can be added as required. The operation of these additional functions is not discussed in this document, but it is expected that these are compatible with the operation as described here. Procedures covered in this standard are further restricted to the lower 2 OSI layers of the digital LMR system. Operation of the encryption function, vocoder function, or user data applications is not explicitly defined here. Operation of trunking control is also not included.

There are several different types of conventional LMR systems, depending on whether or not a repeater is used. These are diagrammed in Figure 1 below.

For the purposes of this document, conventional LMR systems can be classified as either repeater LMR systems or direct LMR systems. Repeater LMR systems make use of a full duplex fixed station that is configured so that all of the signals that are received are re-transmitted. Direct LMR systems transmit directly from one unit to another without the assistance of any intervening repeater. Radio units in an LMR system consist of mobile or portable units, and fixed stations. The mobile or portable units are represented as SU elements and the fixed stations are represented as FS elements in Figure 1 below. The communication paths are represented as arrows. They are intended to show a simplex (i.e. one direction only) path. Generally, the SU is capable of operating half duplex, which is to say that it can transmit or receive, but not both at the same time. The FS is always full duplex. Full duplex operation for portables or mobiles is not discussed for a conventional LMR system.