Codan Radio MT-4E Analog and P25 Digital - User Guide - Manual (Page 19)

MT-4E ANALOG & P25 DIGITAL RADIO SYSTEMS | USER GUIDE
Chapter 2: Technical Information Page 11
REPEATING DIGITAL SIGNALS
A P25 digital signal is received and retransmitted by the repeater completely intact. The digital codes
such as TGID, Source ID, Destination ID, Algorithm ID, Key ID, etc. all pass transparently through the
repeater system.
The TGID and Unit ID programmed into the transmitter are normally overwritten with the incoming TGID
and Source ID when the transmitter is used in a repeater. The TGID and Unit ID are only transmitted
when the transmitter is keyed in a non-repeater mode (no input from the receiver) or in a repeating
mode using the analog audio repeat path (where all digital information is stripped off from the receiver).
A non-repeater mode would include keying by the front panel microphone or base station keying (eg.
from a tone remote).
The Network Access Code (NAC) does not normally pass transparently through the repeater. NACs
are similar to analog CTCSS tones. The NAC is typically programmed into the receiver and transmitter
independently, allowing the user to program different receive and transmit NACs. To pass the NAC
through the repeater transparently, program the receiver with the special NAC $F7F. This will allow any
incoming NAC to unsquelch the receiver and will overwrite the NAC programmed into the transmitter
with the incoming NAC, allowing transparent operation.
Status Symbols are not passed through the repeater transparently. Status Symbols are changed by the
repeater to indicate inbound channel status (busy). In Base Station mode the Status Symbols are set
to show unknown status of the inbound channel. Status Symbols are not programmable.
Talk Groups are typically used in subscriber units, but a repeater could also be programmed as part of
a Talk Group. The receiver can be programmed to unsquelch on a specifi c NAC and TGID.
NOTE: If the receiver is programmed for use in a Talk Group, the NAC of that receiver must not be
set to $F7E or $F7F, as the NAC unsquelch will take precedence over the TGID and the
receiver will unsquelch on any incoming NAC (ignoring the TGID programmed setting).
Encrypted (AES or DES-OFB) voice signals will pass transparently through the repeater.
Packet Data Units (confi rmed or unconfi rmed data messages) such as Over The Air Rekeying (for
encrypted systems) and GPS position information will pass transparently through the repeater.
For a more detailed explanation of the P25 protocol, signaling and terminology, please see the Codan
P25 Training Guide.