Circuit Signaling

All voice circuits use signaling methods to communicate. These signaling methods vary based on the type of circuit, but all circuits must communicate the same types of information. This information is grouped by the type of information and signaling method:

• Supervisory signaling Supervisory signaling is used to indicate the state or current status of a circuit. A circuit, or circuit channel, can be either on-hook, which indicates that the circuit is available, or off-hook, which indicates that the circuit is in use.
• Address signaling Address information consists of the digits that are assigned to an end station. Address signaling methods are required to transmit this information from the call originator.

  Multifrequency (MF) tones are commonly used for address signaling. MF signaling uses numerous reference frequencies sent two at a time to represent each digit. Each protocol defines the reference frequencies and how each pair of frequencies, or tone-pair, maps to a digit.

• Informational signaling Informational signaling is used to provide feedback to the calling or called party. Common informational signaling includes dial tone, ring indication, and busy signal.

Various terms describe the address information. People often use these terms interchangeably, although they actually refer to specific standards or signaling methods.

• Dialed number identification service (DNIS) The telephone number of the called party. Specifically, DNIS refers to the number that the service provider sends to the destination. DNIS might or might not be the digits that the calling party dials to reach the destination.
• Automatic number identification (ANI) The telephone number of the calling party.
• Calling number identification or calling name identification (CNID) CNID is used as a synonym to ANI.
• Calling line identification (CLID) Similar to ANI. Refers to the number of the calling party.
• Caller ID Generic term that describes the delivery of calling number or name. Also used as a marketing term by many service providers to refer to a chargeable feature offered on analog phone service.