The communication (back channel) from the transponder to the decoder operates on 2.4 GHz which is a worldwide public band also used for many purposes, such as Wi-Fi.

Within this band, the active system can use 8 different frequency channels. When the loop activates the transponder, it also encodes the channel ID on the loop signal so that the transponder knows on which channel the decoder is listening for the reply from the transponder.

It is adviseable to always use the channel with the least noise / interference, for this you can run a channel scan on whichever system you are using. Additionally, when using multiple systems you should try to keep these on different channels, particularly when in close proximity. 

Backup Channels

Active Transponders from V2 and above (except ActiveBasic before 2023) use a primary and backup channel for sending passing data to the Active Extension, this can help in areas where there is high noise on the 2.4 Ghz frequency.

Additionally ActivePro V3 Performance transponders support Quad Channel Backup.