Codan Radio P25 Training Guide - Manual (Page 64)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 4: Anatomy of the Common Air Interface Page 56
LOGICAL LINK DATA UNIT 2
A diagram of Logical Link Data Unit 2 (LDU2) is given in Figure 4-9. LDU2 is the second half of a
superframe. LDU2 is composed of the FS (48 bits), NID (64 bits), nine voice code words, numbered
VC10 through VC18 (1296 bits), Encryption Sync Word (240 bits) and Low Speed Data (32 bits).
Twenty-Four Status Symbols are also interleaved throughout LDU1 yielding 1728 bits total. LDU2
takes 180 ms to transmit at 9.6 kbps (the standard bit rate of the P25 CAI).
Figure 4-9: Logical Link Data Unit 2
The Encryption Sync Word fi eld includes the Message Indicator (MI), Algorithm ID (ALGID) for the
encryption algorithm, and the Key ID (KID) for the encryption key. This information may be used to
support a multi-key encryption system, but is also used for single key and clear messages.
The Encryption Sync Word is constructed by serializing the information into 16 hex bits and then
encoding them with a (24,16,9) RS code to yield 24 hex bits. The 24 hex bits are then encoded with
a (10,6,3) shortened Hamming code to yield 240 bits total. The 240 bits of Encryption Sync (ES)
information is then inserted in between the voice code words (VC11 to VC17) in blocks of 40 bits (ES
1-4 is a block of 40 bits, etc.).
Low Speed Data is a serial stream of information. This information is provided for custom applications
that are not defi ned in the CAI. Low Speed Data is comprised of 32 bits of data, 16 bits of which are
inserted between VC8 and VC9 in LDU1 and 16 bits are inserted between VC17 and VC18 in LDU2.
Each group of 16 bits is encoded with a (16,8,5) shortened cyclic code, creating 32 bits total in each
LDU. Low Speed Data has a total capacity of 88.89 bps.
LDU2 with Embedded Encryption Sync (ES 96 bits + 144 bits parity = 240 bits)
1728 bits
1680 bits
Status Symbol 2 bits Each for Every 70 bits (24 X 2 = 48 bits)
VC10 VC11 VC12 VC13 VC14 VC15 VC16 LSD VC18
240 bits
FS 48 bits
NID 64 bits
VC10 ~ VC18 each 144 bits
(88 Digital Voice IMBE + 56 bits Parity) Low Speed
Data 32 bits
Each Block 70 bits
ES 1~4 ES 5~8 ES 9~12 ES13~16 ES 17~20
ES 21~24
Each 40 bits
16 bits Data
Shortened Cyclic Code (16,8,5)
Hamming Code (10,6,3)
240 bits
VC17
RS Code (24,16,9)
Message Indicator (MI)
Initialization vector for encryption algorithm
ALGID Key ID
72 bits 8 bits
144 bits
16 bits
Algorithm ID