Conventional Implementation
P25 systems do not support Continuous Tone-Coded Squelch System (CTCSS) tone or Digital-Coded Squelch (DCS) codes for access control. Instead they use what is called a Network Access Code (NAC). This is a 12 bit code that prefixes every packet of data sent (including voice packets).
The NAC is a feature similar to CTCSS or DCS for analog radios. That is, radios can be programmed to only pass audio when receiving the correct NAC. NACs are programmed as a three-hexadecimal-digit code that is transmitted along with the digital signal being transmitted.
Since the NAC is a three-hexadecimal-digit number (12 bits), there are 4096 possible NACs for programming, far more than all analog methods combined.
Three of the possible NACs have special functions:
- 0x293 ($293) – the default NAC
- 0xf7e ($F7E) – a receiver set for this NAC will pass audio on any decoded signal received
- 0xf7f ($F7F) – a repeater receiver set for this NAC will allow all incoming decoded signals and the repeater transmitter will retransmit the received NAC.
Read more about this topic: Project 25
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