connecting sound (Fax) (CNG)

Connecting Sound(CNG) is a detection tone with a frequency of 1,100 Hz, a duration of 0.5 s, followed by a pause of 3s, which is used in fax and transmitted at the beginning of a fax transmission.

The 1,100 Hz signal tone is used to establish a connection with the receiving fax machine. The whistle tone is followed by a pause of three seconds duration. By means of this signal, a fax machine or a fax switch recognizes that it is a fax and that it must go on reception.

If the subscriber does not have a fax machine, an error message is sent to the sender in the case of ISDN, indicating that the receiving fax cannot be reached.