Codan Radio P25 Training Guide - Manual (Page 35)

P25 RADIO SYSTEMS | TRAINING GUIDE
Chapter 3: P25 Technical Information Page 27
Status Symbols
Throughout the Data Units, 2 bit status symbols are interleaved so that there is one status symbol for
every 70 bits of information. The status symbols allow repeaters to indicate the status of the inbound
channels to subscribers. The repeaters assert the status symbols on both voice and data messages,
indicating inbound activity for both voice and data calls.
The subscribers set the value of the status symbol to signify an Unknown status in their messages
since they are unable to indicate the status of any inbound channel.
There are 4 possible values for the status symbol; 01 (for busy), 11 (for idle), 00 (unknown, used by
talk-around) and 10 (unknown, used for inbound or outbound). Repeaters use status symbols 01 and
11, and subscribers use status symbols 00 and 10.
There is one value for Busy (01), one for Idle (11), and two values to indicate Unknown status. When
the subscriber sends a message on a direct channel, it will use the Unknown value for direct mode
operation (00). When the subscriber sends a message inbound to a repeater, it will use the Unknown
value for repeater operation (10).
Status Symbols are used on a P25 trunking system for subscriber access to the inbound control channel
using the Slotted ALOHA technique. The Status Symbols are transmitted on the outbound control
channel, and the subscriber uses them to identify the slot boundaries for the inbound control channel.
The reference oscillator stability for repeaters and base stations is often better than for subscriber
radios. Subscribers may compare the frequency of their local reference oscillator with the carrier
frequency from a repeater or base station transmitter, in order to adjust and improve their local reference
oscillator. This adjustment is called Automatic Frequency Control (AFC). AFC operation is anticipated
by the FCC regulations for the 746-806 MHz band. Subscribers may detect a repeater or base station
transmission by checking the values of the status symbols on slot boundaries. A repeater or base station
will transmit Busy or Idle indications on slot boundaries. When a subscriber detects these values, it can
average enough data symbols from a transmission to obtain an estimate of the carrier frequency used
by the repeater or base station. It can then compare this to the receiver local oscillator to determine
any frequency corrections to improve local reference stability. After the repeater or base station stops
transmitting, the subscriber units will be in an unlock state. AFC locking resumes when a repeater or
base station restarts its transmissions.
Status Symbols are not widely used at this time, however there are many possible uses for them in the
future (such as data / voice priority).
(See Chapter 4; Figure 4-3)