Common Associated Signaling (CAS) is a type of in-band signaling used in telecommunications to transmit call setup and control information along with the voice or data signal. CAS signaling can be used in several ways depending on the specific application, but one common example is the use of Multi-Frequency (MF) signaling in telephone systems.
In this example, when a person dials a phone number on their phone, the phone generates a series of multi-frequency tones (MF tones) that encode the dialed digits. The tones are then transmitted along with the voice signal to the telephone exchange using CAS signaling.
At the exchange, the signal is routed to the appropriate destination based on the dialed digits. If the call is being transferred to another telephone line, the exchange generates a ringing signal by superimposing a specific frequency tone onto the voice signal. The ringing tone is then transmitted to the called party along with the voice signal, and the called party’s telephone rings.
When the call is answered, the exchange generates a tone to signal that the call has been connected. During the call, other signaling information can be sent in-band to control various features such as call hold, call transfer, and conference calling.
For example, if a user wants to put a call on hold, they can press the “Flash” button on their phone. The phone generates a specific MF tone sequence that indicates to the exchange to put the call on hold. The exchange responds by generating another tone to indicate that the call is on hold, and the user can then make another call or perform other actions.
When the user wants to resume the call, they can press the “Flash” button again, and the phone generates another MF tone sequence that indicates to the exchange to resume the call. The exchange responds by generating another tone to indicate that the call is no longer on hold, and the user can continue the call.
Difference in Signaling after that
CAS was one of the earliest methods of in-band signaling used in analog telecommunications systems. It was used in the first generation of telephone networks and was widely adopted in the mid-20th century.
CCS1, on the other hand, was developed in the 1970s as an out-of-band signaling system for digital telecommunications networks. It was designed to be more efficient and flexible than in-band signaling systems like CAS, and to support new features like call forwarding and caller ID.
In summary, CAS was developed first and was used in early analog telecommunications systems, while CCS1 was developed later as an out-of-band signaling system for digital telecommunications networks.
