Codan Radio P25 Training Guide - Manual (Page 32)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 3: P25 Technical Information Page 24
P25 DIGITAL CODE DEFINITIONS
A P25 digital radio system uses many different codes, identifi cations, indicators and other digital
information in the Common Air Interface. Some of the codes are user accessible or programmable,
while others are meant for internal use inside of the CAI, or for specifi c applications.
Frame synchronization
A special sequence of 48 bits marking the location of the fi rst bit of the message provides frame
synchronization. Frame synchronization occurs at the beginning of every message (voice and data),
and is inserted every 180 ms throughout the voice message. This allows receivers to pick up voice
messages after the message has begun (late entry of receivers). Late entry can occur when a subscriber
unit selects a channel (or talk group) while there is already an active signal present. The subscriber unit
was not active when the transmission started, but is added when it detects the repeated frame sync
function. The frame synchronization is not accessible or programmable by the user.
(See Chapter 4; Figure 4-3)
Network ID (NID)
Every P25 data unit packet contains the 64 bit NID fi eld. The NID is composed of a 4 bit Data Unit ID
and a 12 bit NAC code. The NID is protected with a primitive BCH Code and a single parity bit is added
to fi ll out the NID code word to 64 bits.
(See Chapter 4; Figure 4-3)
Data Unit ID
The NID contains the 4 bit Data Unit ID fi eld. The Data Unit ID is used to determine the “type” of packet
information (eg. Header Data Unit, Logical Link Data Unit 1, etc.). The Data Unit ID is not accessible
or programmable by the user.
(See Chapter 4; Figure 4-3)