Codan Radio MT-4R - Manual (Page 19)

UG-001 Daniels MT-4R and MT-4D Radio Systems
www.danelec.com
Chapter 2: Technical Information 11
User
Guide
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 (from a tone remote, etc.).
The Network Access Code (NAC) does not normally pass transparently
through the repeater. NAC codes 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 NAC codes. To pass the NAC code through the repeater
transparently, program the receiver with the special NAC code $F7F.
This will allow any incoming NAC code to unsquelch the receiver and
will overwrite the NAC code programmed into the transmitter with the
incoming NAC code, 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.
If the receiver is programmed for use in a Talk Group, the NAC code
of that receiver must not be set to $F7E or $F7F, as the NAC code
unsquelch will take precedence over the TGID and the receiver will
unsquelch on any incoming NAC code (ignoring the TGID programmed
setting).
Encrypted (AES or DES-OFB) voice signals will pass transparently
through the repeater. Since the signal is encrypted, the analog audio
output from the receiver will be “garbled”. The System Monitor speaker
can be used to monitor the audio output; but the encrypted signal will
be unintelligible.
For a more detailed explanation of the P25 protocol, signaling and
terminology, please see Daniels P25 Training Guide.