Customs Over The Horizon Enforcement Network
From The RadioReference Wiki
COTHEN Network Frequencies
Frequency (Khz) Mode --------------------------- 5732.0 USB 7527.0 USB 8912.0 USB 10242.0 USB 11494.0 USB 13907.0 USB 15867.0 USB 18594.0 USB 20890.0 USB 23214.0 USB 25350.0 USB
The US Customs COTHEN Network uses a Rockwell/Collins ALE (Automatic Link Establishment) protocol to connect users to each other over the HF spectrum. If you listen to frequencies in use on the COTHEN network, you will hear the ALE sounding calls which units periodically and automatically initiate to determine the best frequency to use for calls to other units.
The COTHEN network now uses the MIL-STD 188-141A protocol. It formally used the FED-STD-1045A ALE protocol, developed by Rockwell Collins. The FED-STD protocol is Rockwell Collins proprietary and goes by the tradename Selscan. It is no longer actively used by the COTHEN network.