er signaling. 7.2.3 Call Progress: Audible-Visual This type of signaling we categorize in the forward direction and in the backward direc- tion. In the forward direction there is alerting. This provides some sort of audible-visual means of informing the called subscriber that there is a telephone call waiting. This is often done by ringing a telephone's bell. A buzzer, chime, or light may also be used for alerting. The remainder of the techniques we will discuss are used in the backward direc- tion. Among these are audible tones or voice announcements that will inform the calling subscriber the following: 1. Ringback. This tells the calling subscriber that the distant telephone is ringing. 2. Busyback. This tells the calling subscriber that the called line is busy. 3. ATB-All Trunks Busy. There is congestion on the routing. Sometimes a recorded voice announcement is used here. 4. Loud Warble on Telephone Instrument-Timeout. This occurs when a telephone instrument has been left off-hook unintentionally. 7.3 SIGNALING TECHNIQUES 7.3.1 Conveying Signaling Information Signaling information can be conveyed by a number of means from a subscriber to the serving switch and between (among) switches. Signaling information can be transmitted by means such as Duration of pulses (pulse duration bears a specific meaning) Combination of pulses Frequency of signal Combination of frequencies Presence or absence of a signal Binary code For dc systems, the direction and/or level of transmitted current 7.3 SIGNALING TECHNIQUES 7.3.2 Evolution of Signaling Signaling and switching are inextricably tied together. Switching automated the network. But without signaling, switching systems could not function. Thus it would be better said that switching with signaling automated the network. Conventional subscriber line signaling has not changed much over the years, with the exception of the push-button tones that replaced the dial for address signaling. ISDN, being a full digital service to the subscriber, uses a unique digital signaling sy