Codan Radio P25 Training Guide - Manual (Page 62)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 4: Anatomy of the Common Air Interface Page 54
LINK CONTROL WORD
The Link Control Word is a portion of the voice message that resides in the Logical Link Data Unit 1
(LDU1) and the expanded Terminator Data Unit. There is a basic Link Control format that implicitly
assumes the Standard Manufacturers ID is used, and one that is used when the Manufacturers ID is
specifi ed explicitly.
The Link Control Word fi eld may include a Talk-group ID (TGID or Group Address), a Source ID
(Source Address), a Destination ID (Target Address), Service Options, including an Emergency
indicator, a Manufacturer’s ID (MFID) and any other necessary call ID information. The Link Control
Word uses a variable format (both implicitly and explicitly) since there is too much information for a fi xed
fi eld format. The type of format is identifi ed by the Link Control Format (LCF). The LCF specifi es
the the content of the Link Control Word’s information and contains the P, SF and LCO sub-fi elds. Two
format examples are diagrammed in Figure 4-7. All of the information fi elds (including the LCF) total
72 bits.
Figure 4-7: Link Control Words
P - Protected fl ag
%0 Link Control is not encrypted
%1 Octets 2 through 8 are encrypted
SF - Implicit / Explicit MFID Format
%0 Explicit MFID Format.
%1 Implicit MFID Formats, the standard Manufacturers ID ($00) is implied.
LCO - Link Control Opcode
This indicates the Link Control Format opcode.
The Link Control Opcode has values of $00 through $3F and is set appropriately for the different
commands or information determined by the manufacturer since this is a non-standard format.
P SF
Bit 7 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 0 Bit 1
Octet 0
Octet 6
Octet 5
Octet 4
Octet 3
Octet 2
Octet 1
Octet 8
Octet 7
Service Options
LCO
Group Voice Channel User
MFID
Reserved S
Group
Address
Source
Address
P SF
Bit 7 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 0 Bit 1
Octet 0
Octet 6
Octet 5
Octet 4
Octet 3
Octet 2
Octet 1
Octet 8
Octet 7
Service Options
LCO
Unit to Unit Voice Channel User
MFID
Target
Address
Source
Address