The Project 25 standard will cover all of the parts of a system for public safety land mobile radio communications. These systems have subscriber units (which include portable radios for hand held operation and mobile radios for vehicular operation), base stations (for fixed installations), and other fixed equipment (for wide-area operation and console operator positions), as well as computer equipment (for data communications). There are interfaces between each of these equipment items. The Common Air Interface allows these radios to send and receive digital information over a radio channel.

The Common Air Interface is part of an intended suite of standards necessary for interoperability. A general overview of the Project 25 standard is in the Project 25 System and Standard Definition given as reference [1]. The reader should have familiarity with this document. Included in this reference are the Project 25 Overview, General System Model, Standards Organization, Glossary, and the Statement of Requirements. The Standards Organization is a list of other applicable Project 25 standards. Sections of reference [1] are also listed as references. They are reference [1.1], STATEMENT OF REQUIREMENTS, and reference [1.2], General System Model.

This document directly addresses many of the requirements in the STATEMENT OF REQUIREMENTS and it uses the elements of the model given in the General System Model document. Specifically, this document addresses the Physical Layer and Data Link Layer of the Common Air Interface.

The objective of the Common Air Interface is to ensure that subscriber unit equipment that conforms to this document will be interoperable at the Physical  Layer and Data Link Layer with subscriber unit equipment from different manufacturers, and compatible with radio systems for different agencies. The full capability of Project 25 will be achieved when these subsequent specifications and Network Layer definitions are developed. This will allow effective and reliable intra-agency and inter-agency communications in an all-digital mode for voice and data. This is in conformance to the STATEMENT OF REQUIREMENTS.

This document will not provide the specification or operational detail for system implementations which include but are not limited to trunking, roaming, network management, vocoder, security, data, subsystems interfaces and data between private and public switched telephone networks. It will provide only the appropriate access requirements to ensure common air interface compatibility. This document will be updated as necessary to ensure Physical and Data Link Layer compatibility for the purpose of providing a common air interface as additional system implementation requirements are available.

The Common Air Interface defines a reference point at which communications between radios can take place. This reference point is designated Um. Communications through Um are done at a gross bit rate of 9.6 kbps. Several processes take place to convert information for transmission through the Um reference point. The Common Air Interface uses a standard voice coder (reference [2]) to convert speech to a digital format for communication through Um. This voice information is then protected with additional error correction coding to provide protection over the channel.