Comrex ACCESS MultiRack Product Manual page 61

Finally, CrossLock can be configured to use one network only when the primary network fails. This is called
"HotSwap" mode and is described in a later section.
As long as an incoming CrossLock call is possible on the primary network attached to a codec, the call may be
initiated from the remote end of the connection. As an example, at a transmitter site the primary network is a DSL
line, and port UDP 9001 (the default port for CrossLock connections) is open from the public Internet. The backup
network is a 4G modem, which on its own would not accept an incoming connect request. Even without using
Switchboard, the connection can be initiated from the studio side, and the 4G modem will be automatically added
to the CrossLock channel.
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