Codan Radio P25 Training Guide - Manual (Page 38)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 3: P25 Technical Information Page 30
Message Indicator (MI)
The Header Code Word and Encryption Synchronization (LDU2) contain the 72 bit MI fi eld. The MI is
the initialization vector (synchronization for key stream generator) for a Type 1, Type 2, Type 3 or Type
4 encryption algorithm. Clear messages are denoted $000000000 while secure (encrypted) messages
are variable. The Message Indicator is not accessible or programmable by the user.
(See Chapter 4; Figure 4-4 and Figure 4-9)
Talk-group ID (TGID)
The Header Code Word and Link Control Word (LDU1) contain the 16 bit TGID fi eld. The TGID identifi es
the talk-group for the message. The purpose of a talk group is to allow logical groupings of radio users
into distinct organizations. The TGID could also be used to minimize co-channel interference and allow
subscriber addressing.
TheTGID ranges from hexadecimal $0000 to $FFFF and contains 65,536 addresses.
The following TGID’s have specifi c functions:
$0001 specifi ed as the default TGID value and should be used in systems where no other talk
groups are defi ned.
$0000 no-one or a talk group with no users. Used when implementing an individual call.
$FFFF reserved as a talk group which includes everyone.
(See Chapter 4; Figure 4-4 and Figure 4-6)
Low Speed Data
Each Logical Link Data Unit in a voice message contains the 16 bit Low Speed Data fi eld. The Low
Speed Data is intended for custom user applications not defi ned by the CAI (possibly GPS location
data, infrastructure status information, etc.) and has a total capacity of 88.89 bps. The Low Speed Data
is encoded with a shortened cyclic code to create 64 bits per superframe.
(See Chapter 4; Figure 4-6 and Figure 4-9)