The TIA-102 Documentation Suite covers 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)@ fixed 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 information necessary to enable an interoperable Common Air Interface is provided in this document or referenced in other documents as appropriate. An overview of the TIA-102 Documentation Suite is provided in [7]. This document assumes the reader has a good working knowledge of [7] which provides a general overview of LMR systems@ the Representative LMR Functional Network Model@ a Glossary@ and lists other applicable TIA-102 documents. 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 is 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 Common Air Interface defines a standardized interface at which communications between radios can take place. This interface 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 Um. The Common Air Interface uses a standard voice coder (see [3]) 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.