because of the minimum time required for recognition of the incoming signal. After the initial forward signal, further forward signals are delayed for a short recognition time (see Figure 7.4). Recognition time is normally less than 80 msec. Fully compelled signaling is advantageous in that signaling receivers do not have to measure duration of each signal, thus making signaling equipment simpler and more eco- nomical. Fully compelled signaling adapts automatically to the velocity of propagation, to long circuits, to short circuits, to metallic pairs, or to carrier and is designed to with- stand short interruptions in the transmission path. The principal drawback of compelled signaling is its inherent lower speed, thus requiring more time for setup. Setup time over Outgoing register Incoming register Forward signal Acknowledging backward signal Next forward signal Acknowledging backward signal Sending Receiving Figure 7.4 Fully compelled signaling procedure. SOCOTEL is a European multifrequency signaling system used principally in France and Spain. SIGNALING space-satellite circuits with compelled signaling is appreciable and may force the system engineer to seek a compromise signaling system. There is also a partially compelled type of signaling, where signal duration is fixed in both forward and backward directions according to system specifications; or the forward signal is of indefinite duration and the backward signal is of fixed duration. The forward signal ceases once the backward signal has been received correctly. CCITT Signaling System No. 4 (not discussed in this text; see CCITT Recs. Q.120 to 130) is an example of a partially compelled signaling system. 7.5 CONCEPTS OF LINK-BY-LINK VERSUS END-TO-END SIGNALING An important factor to be considered in switching system design that directly affects both signaling and customer satisfaction is postdialing delay. This is the amount of time it takes after the calling subscriber completes dialing until ring-back is received. Ring-back is a backward signal to th