NXDN Simplified Overview Theory

Local Repeater Operation


No attempt has been made to go into detail. This is just a very basic overview.


An IDAS/NXDN signal is processed like a letter being placed in multiple envelopes. Understand the theory will help you with your radio programing.


  >When you talk, your message is digitized and placed in an envelope. We will call this envelope1. On the front of envelope1 is placed the Talkgroup number that you are using and your (originator) user/radio ID (like the return address).

l     >Envelope1 is placed in another envelope (envelope2). On the outside of envelope2 is the RAN you are using.

l    >Envelope2 is now sent to the repeater.

l    >The repeater looks at the incoming envelope2. If the RAN is open on the repeater it will process the message. (This is just like CTCSS). If the RAN is closed on the repeater, it will ignore the message. (In the ICOM repeater, this information is stored in the Multi-table).

l    >The repeater now rebroadcasts envelope2 over the air and over the network. The repeater does not open envelope2 and look at the inner envelope1 (which contains the Talkgroup, User/Radio ID, or the message). The repeater is transparent to the Talkgroup and Radio/User ID.

l    >The receiving radio hears the incoming signal and looks at envelope2.

l    >The receiving radio first looks at the RAN and sees if it should decode this signal.

l    >If the RAN is good, the radio now looks at envelpe1, containing the incoming Talkgroup number to see if it should decode this signal.

l    >The radio now looks at the (originator) User/Radio ID that was placed on envelope1. If the (originator) User/Radio ID is the same ID as the receiving radio, the radio assumes that this is an echo of it's own transmission and ignores the incoming signal.

l    >If the channel you are on is programed to to receive both the correct RAN, Talkgroup number, and you do not have the same Radio/User ID of the sender, you will hear the conversation. Otherwise, you will just see a frequency busy indication on your radio.