Note 1 to entry: If the telecommunication is by nature unidirectional, e.g. long-distance television transmission, the term “circuit” is sometimes used to designate the single transmission channel providing the facility, but this usage is deprecated.

Note 2 to entry: In some countries the term “communication channel” or its abbreviation “channel” is used to mean “telecommunication circuit”, i.e. to encompass the two directions of transmission. This usage is deprecated.

Note 3 to entry: The adjective "telecommunication" can be replaced by an adjective indicating the nature or characteristics of the transmitted signals; for example, telephone circuit, telegraph circuit, data circuit, digital circuit.

Note 4 to entry: Characteristics of the transmission channels such as frequency bandwidth, or digit rate, can be different in the two directions of transmission.

Note 5 to entry: In telephony, usage of the term “telephone circuit” is generally limited to a telecommunication circuit directly connecting two switching centres.

Note 6 to entry: In switching, the two transmission channels associated in a telecommunication circuit can be: a) permanently associated together; b) selected from separate sets for association together throughout a call; c) selected from time to time during a call and then released, the selection and release processes being determined, separately in the two directions, by the varying characteristics of the information being transmitted over the telecommunication circuit.