e constant expires. The call is then disconnected by both exchanges. Many different signaling systems are used for CAS and some of them include additional signals that are not present in Figure 2.10. Signals that carry signaling information indicated in Figure 2.10 depend on the signaling system in use and they may be, for example, as follows: Breaks of the loop between exchanges (loop/disconnect signaling); Tones with multiple frequencies, multifrequency code (MFC); Bit combinations of signaling channel of a PCM frame. CAS is still used in telephone networks, but it is gradually being replaced with a more efficient standardized method known as CCS. 2.6.2.2 CCS The modern interexchange signaling system is called CCS. It is based on the principles of computer communications in which data frames containing Introduction to Telecommunications Network Engineering information are exchanged between computers only when required. Signal- ing frames contain, for example, information about the connection to which the message belongs, the address of the destination exchange, dialed digits, and information about whether subscriber B has answered. In most cases only one data channel between two exchanges is required to serve all estab- lished calls. This is usually one 64-Kbps time slot of a primary 2- or 1.5-Mbps PCM frame, as explained in Chapter 4, and one channel is usually enough for all call-control communication between exchanges. A widely used international standard of CCS is called CCS7, also known as signaling system number 7 (SS7), CCITT#7, or ITU-T 7, and it is used in all modern telecommunications networks such as ISDN and GSM. Establishing a call requires the same signaling information as indicated in Figure 2.10, but in the case of CCS the signaling information is carried in data frames that are transferred between exchanges via a common data channel. In Figure 2.11 we see an example in which an ordinary fixed network subscriber, subscriber A, calls subscriber B when CCS is used between exchanges in the network.