Codan Radio P25 Training Guide - Manual (Page 66)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 4: Anatomy of the Common Air Interface Page 58
PACKET DATA UNIT
A diagram of the Packet Data Unit is given in Figure 4-12. There are two different types of delivery
for data packets. Confi rmed delivery is used when the recipient of the packet is required to send an
acknowledgment of receipt. Unconfi rmed delivery does not require an acknowledgment of receipt.
Confi rmed or unconfi rmed delivery is defi ned in the header block.
Figure 4-12: Data Packet Unit
Data is sent in variable length packets and the length of the data packet is defi ned in the header block.
When a data packet ends, nulls are added until the next status symbol.
The data message is split into fragments, and then formed into packets, and the packets are then split
into a sequence of information blocks that are error protected by a Trellis code. These blocks are then
transmitted as a single data packet.
The data packets include a CRC in both the Header block and the last Data Block. The CRC verifi es
the accuracy of the error correction in the receiver. If the packet is corrupted in confi rmed data mode
during reception, then an automatic retransmission request is generated to repeat parts of the packet.
The receiver then reassembles the packets into a continuous message.
A data packet may have any number of fragments. The fragments must not be longer than the storage
capability of the subscriber radio. The minimum storage capacity for a fragment in the subscriber is
512 octets.
PDU (arbitrary length)
Data Message, arbitrary length
Fragment Fragment Fragment
Break into Fragments
Header Block
12 Octet
Block 1
M Octets
Block 2
M Octets
Block n
M Octets
FS 48 bits NID 64 bits
Trellis coded Header Block
196 bits
Trellis coded Optional Block
196 bits
Trellis coded Optional Block
196 bits
70 bits 70 bits 70 bits 70 bits 70 bits 70 bits 70 bits 70 bits
expand with status symbols
Status Symbols as required
2 bits after every 70 bits
Rate 1/2 Trellis
Encoder (196,96)
Rate 1/2 Trellis Encoder
if Unconfirmed
Rate 3/4 Trellis Encoder
if Confirmed
M = 12 octets for unconfirmed
M = 16 octets for confirmed