Codan Radio P25 Training Guide - Manual (Page 84)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 6: P25 Trunking Page 76
CONTROL CHANNEL MESSAGES
The control channel packet structure is based on the data packet structure of the Common Air Interface.
The trunking control channel consists of both an inbound and an outbound path. A control channel
packet sent from the subscriber unit to the trunking controller is called an Inbound Signaling Packet
(ISP) and a control channel packet sent from the trunking controller to the subscriber unit is called
an Outbound Signaling Packet (OSP). These messages are typically formatted as a single block
message called a Trunking Signaling Block (TSBK). The TSBK uses the same Trellis coding as an
unconfi rmed data packet. A Multiple Block Trunking (MBT) packet structure is only used when there is
more information than normally sent by TSBK and uses the unconfi rmed data packet structure.
INBOUND SIGNALING PACKET ACCESS VIA SLOTTED
ALOHA
Subscribers access the inbound control channels to send ISP’s using a technique called Slotted ALOHA.
Slot boundaries are set by the status symbols in the outbound control channels OSP’s. The fi rst status
symbol of every OSP is 11 allowing the subscriber to synchronize ISP transmissions accordingly. The
status symbols occur every 7.5 ms, which is called a microslot. An ISP lasts for 32.92 ms, so the
slot needs to exceed this in order to allow for the ISP. The slot boundaries need to be a multiple of a
microslot making the minimum required slot 37.5 ms (5 microslots). The trunking system may utilize slot
times greater than the minimum to accommodate slower subscribers. OSP and ISP Status Symbols
are shown in Table 6-1.
OSP Status Symbols
Symbol Status of Inbound Channel Description
%00 Unknown Not Used
%01 Busy Inbound control channel is not available
%10 Unknown Used between slot boundaries
%11 Idle Used to indicate the start of Inbound slot
ISP Status Symbols
Symbol Status of Inbound Channel Description
%00 Unknown Not Used
%01 Busy Not Used
%10 Unknown Used for all ISPs
%11 Idle Not Used
Table 6-1: OSP and ISP Status Symbols