Network Access Code (NAC)
"...can someone explain what the NAC means?
In its simplest form, the NETWORK ACCESS CODE (NAC) is the digital equivalent of a subaudible tone (CTCSS, Quiet Channel, Channel Guard, Private Line, etc.).
In Analog mode, a CTCSS tone or Digital Squelch codeword (DCS, Digital Channel Guard, Digital Private Line, etc.) is transmitted with the audio to open receivers on another radio, or a repeater's input.
In non-P25 Digital mode, due to their not being any Analog component to convey the subaudible component (analog tone or digital codeword), a digital address code is transmitted once a second in the data stream to accomplish the same task. In GE radios (now owned by Harris), this was referred to as the "TX and RX Address". Transmit and Receive codes can be different!
An example would be a fire department who had two repeater sites, one east and one west, on the same input and output RF frequencies. Both repeaters therefore could have the same output RF frequency and digital address (or CTCSS/DCS if analog), and could have the exact same input RF frequency, but have a different digital address (or CTCSS/DCS if analog) so the users could selectively access either repeater site. This is very common in state forestry systems.
In P-25 Conventional digital systems, the NAC code is transmitted continuously as part of the preamble in the message data superframe. NAC code function for conventional P-25 and trunked P-25 systems are quite different. The DEFAULT NAC code for conventional P-25 is $293 (hex 293), and is often used for Amateur Radio repeaters. Using the default for commercial or public safety systems is not a wise decision, as it is the digital equivalent of Carrier Squelch.