Codan Radio P25 Training Guide - Manual (Page 87)

P25 RADIO SYSTEMS | TRAINING GUIDE
Chapter 6: P25 Trunking Page 79
ENCRYPTION
The traffi c channels of a trunked system can be encrypted in the same manner as a conventional system
encrypted system, using the Algorithm ID, Key ID and Message Indicator for encryption variables.
The control channel of a trunked system may also be optionally encrypted. In a TSBK message, the
Opcode and Arguments may be encrypted and the Protected trunking block fl ag (P) is set to indicate
encryption. In an MBT message, the Logical Link ID, (and in the alternate header format, the Opcode
and Octets 8 and 9) as well as all Data (except the CRC) may be encrypted, and the SAP Identifi er is
set to indicate encryption.
The Algorithm ID, Key ID and initial Message Indicator (which is incremented each microslot) may be
set by sending control channel information in the form of a Protection Parameter Update using standard
trunking control channel formatted information.
DATA PACKET STRUCTURES FOR SINGLE AND MULTI
BLOCK MESSAGES
The control channel message is composed of one or more information blocks protected by a rate
1/2 trellis code, and the sequence of blocks is transferred over the common air interface as a single
data packet. The last part of the data packet is a CRC to be used to verify the accuracy of the error
correction at the received end
The structure for conventional data packets is the same structure used for ISP and OSP control channel
packets. The Frame Synchronization (FS) and Network Identifi er (NID) are sent before the information
block(s) for the packet. There is one Status Symbol (SS) consisting of two(2) bits inserted after every
70 bits in a packet.
The 4-bit Data Unit ID portion of the 16-bit Network Identifi er(NID) indicates the format of the control
channel packet as either:
$7 Indicating the single block format (TSBK)
$C indicating the multiple block format (PDU)
A special abbreviated data packet, the Trunking Signaling Block (TSBK) is used for control channel
messages that are time sensitive (requests and grants for traffi c channels) and is the typical data
packet used. The ISP is limited to one TSBK where the number of TSBKs in the OSP is variable
(single, double or triple block) and determined by the trunking controller to maximize the control channel
resource. Figure 6-4 shows the Trunking Signaling Block structure and Figure 6-5 shows the contents
of the TSBK.
Control channel messages that are not time sensitive, and that contain more information than a TSBK
can handle, such as registration, can be sent as a Multiple Block Trunking (MBT) message. MBT uses
the unconfi rmed data packet format with the packet split into blocks of 12 octets. The fi rst block is a
header block, with up to 3 data blocks following. Figure 6-6 shows the Multiple Block Trunking structure
(unconfi rmed data packet) and Figure 6-7 shows the contents of the two MBT formats.