Codan Radio P25 Training Guide - Manual (Page 25)

P25 RADIO SYSTEMS | TRAINING GUIDE
Chapter 2: P25 Interface Standards Page 17
The P25 Interface Standards as shown on the General System Model are as follows:
RF Sub-System (RFSS) Core Infrastructure
Common Air Interface (Um) Radio to radio protocol
Inter Sub-System Interface (ISSIg) RFSS to all other system interconnections (In progress)
Telephone Interconnect Interface (Et) PSTN to RFSS defi nition
Network Management Interface (En) Network to RFSS defi nition (In progress)
Data Host or Network Interface (Ed) Computer aided dispatch to RFSS defi nition
Data Peripheral Interface (A) Radio to Data Peripheral defi nition
Fixed Station Interface (Ef) Base station to RFSS / Console Sub-System defi nition (in progress)
Console Sub-System Interface (Ec) Console to RFSS defi nition (In progress)
RF SUB-SYSTEM
The P25 interfaces bound the RF Sub-System (RFSS) infrastructure. The RF Sub-System can be
made from any collection of site equipment (single station/site or multiple station/site), whose only
requirement is that the equipment supports the Common Air Interface, and contains all necessary
control logic to support the open intersystem interfaces and call processing. The RF Sub-Systems are
the building blocks for wide-area system construction and will connect with any other confi guration of
equipment or RF Sub-Systems.
COMMON AIR INTERFACE
The Common Air Interface (Um) or CAI defi nes a standard (or reference point) at which communications
between P25 radios can take place. The CAI is the core element of the P25 standard that assures
the ability of one company’s P25 digital radio to communicate with another company’s P25 digital
radio. Communications between P25 radios are done at a gross bit rate of 9.6 kbps and with FDMA
channel access. Several processes take place to convert information for transmission. The Common
Air Interface uses an IMBE™ voice coder (vocoder) to convert (compress) speech to a digital format
for communication. This voice information is then protected with error correction coding to provide
protection over the channel. The voice information and error correction is then transmitted with
additional encryption information, unit identifi cation, and low speed data to fully utilize the 9.6 kbps of
channel capacity in the Common Air Interface.
A breakdown of the information contained in the Common Air Interface can be found in Chapter 4:
Anatomy of the Common Air Interface. Chapter 8 contains some detailed information on the operation
and theory of the IMBE™ Vocoder.