Codan Radio P25 Training Guide - Manual (Page 57)

P25 RADIO SYSTEMS | TRAINING GUIDE
Chapter 4: Anatomy of the Common Air Interface Page 49
CHAPTER 4: ANATOMY OF THE COMMON AIR INTERFACE
VOICE
The P25 standard requires the use of the IMBE™ Vocoder to encode speech (tone and audio level) into
a digital bit stream. The IMBE™ digital bit stream is broken into voice frames where each voice frame
is 88 bits in length (representing 20 ms of speech). The voice frames are protected with error correction
codes which add 56 parity check bits resulting in an overall voice frame size of 144 bits. The voice
frames are grouped into Logical Link Data Units (LDU1 and LDU2) that contain 9 voice frames each.
Each Logical Link Data Unit is 180 ms in length and can be consecutively grouped into Superframes
of 360 ms. The superframes are repeated continuously throughout the voice message after a Header
Data Unit has been sent. Additional information (encryption, Link Control information and Low Speed
Data) is interleaved throughout the voice message.
The voice message structure for a P25 CAI voice transmission is shown in Figure 4-1. The voice
message begins with a Header Data Unit (to properly initialize any encryption and link control functions
for the message), and then continues with Logical Link Data Units or LDUs. The LDUs alternate until
the end of the voice message. The end of the message is marked with a Terminator Data Unit. The
Terminator Data Unit can follow any of the other voice data units.
Figure 4-1: P25 Voice Message Structure
Terminator Data Unit
180 ms 1728 bits 180 ms 1728 bits 82.5 ms 792 bits
Superframe 360 ms 3456 bits
HDU LDU1 LDU2 TDU
15 or 45 ms
Header Data Unit Logical Link Data Unit 1 Logical Link Data Unit 2