Codan Radio P25 Training Guide - Manual (Page 58)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 4: Anatomy of the Common Air Interface Page 50
DATA
Data messages are transmitted over the P25 CAI using a packet technique. The data information is
broken into fragments, packets and blocks are then error coded and sent as a single packet called a
Packet Data Unit. The Packet Data Unit can be of varying lengths and includes a header block that
contains the length of the data message.
Figure 4-2: P25 Data Message Structure
FRAME SYNCHRONIZATION AND NETWORK IDENTIFIER
Each data unit (Header Data Unit, Logical Link Data Unit 1, Logical Link Data Unit 2, Packet Data Unit
and Terminator Data Unit) begins with a Frame Synchronization (FS) and Network Identifi er (NID).
Figure 4-3: Frame Synchronization and Network Identifi er
Packet length is variable
PDU
Packet Data Unit
Remainder of Header Data Unit, Logical Data Unit 1 or 2, Packet Data Unit or Terminator Data Unit
NID 63 bits+1Parity bit FS 48 bits
(63,16,23) primitive BCH Code Plus one Parity bit
A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1 A0 S3 S2 S1 S0
12 bits 4 bits
Data Unit ID
$5575F5FF77FF
NAC
Data Unit ID P Data Unit Usage
%0000 0 Header Data Unit
%0011 0 Terminator without subsequent Link Control
%0101 1 Logical Link Data Unit 1
%0111 0 Trunking Signaling Block
%1010 1 Logical Link Data Unit 2
%1100 0 Packet Data Unit
%1111 0 Terminator with subsequent Link Control
The P bit is the last (64-th) parity bit in the code word.
Total Number of Bits Minus Status Symbol Bits
Status Symbol Each 2 bits for Every 70 Bits
Header Data Unit, Logical Link Data Unit 1 or 2, Packet Data Unit or Terminator Data Unit
Total Number of Bits