ear back Continuous 125 + 25 Acknowledgment Continuous 125 + 25 Forward transfer 850 + 200 msec 125 + 25 Clear forward Continuous 125 + 25 Release guard f1+f2 Continuous 125 + 25 Source: Ref. 4. R-2 CODE. The R-2 code has been denominated by CCITT (CCITT Rec. Q.361) as a European regional signaling code. Taking full advantage of combinations of two-out-of- six tone frequencies, 15 frequency pair possibilities are available. This number is doubled in each direction by having meaning in groups I and II in the forward direction (i.e., toward the called subscriber) and groups A and B in the backward direction, as shown in Table 7.5. Groups I and A are said to be of primary meaning, and groups II and B are said to be of secondary meaning. The change from primary to secondary meaning is commanded by the backward signal A-3 or A-5. Secondary meanings can be changed back to primary meanings only when the original change from primary to secondary was made by the use of the A-5 signal. Turning to Table 7.5, the 10 digits to be sent in the forward direction in the R-2 system are in group I and are index numbers 1 through 10 in the table. The index 15 signal (group A) indicates "congestion in an international exchange or at its output." This is a typical backward information signal giving circuit status information. Group B consists of nearly all "backward information" and, in particular, deals with subscriber status. The R-2 line-signaling system has two versions: the one used on analog networks is discussed here; the other, used on E-1 PCM networks, was briefly covered in Chapter 6. The analog version is an out-of-band tone-on-when-idle system. Table 7.6 shows the line conditions in each direction, forward and backward. Note that the code takes advantage of 7.3 SIGNALING TECHNIQUES Table 7.5 European R-2 Signaling System Frequencies (Hz) Forward Direction I/II Index No. for Backward Groups I/II and A/B Direction A/B Source: Ref. 6. Table 7.6 Line Conditions for the R-2 Code Signaling Conditions Operating Condition of the C