pill forward" system. It is called this because few backward acknowledgment signals are required. This is in contraposition to the R-2 system, where every transmitted digit must be acknowledged. CCITT NO. 5 SIGNALING CODE. Interregister signaling with the CCITT No. 5 code is very similar to the North American R-1 code. Variations with the R-1 code are shown in Table 7.3. The CCITT No. 5 line signaling code is also shown in Table 7.4. Table 7.2 North American R-1 Code Digit Frequency Pair (Hz) 700 + 900 700 + 1100 900 + 1100 700 + 1300 900 + 1300 1100 + 1300 700 + 1500 900 + 1500 1100 + 1500 10 (0) 1300 + 1500 Frequency Pair Explanation 1100 + 1700 Preparatory for digits 1500 + 1700 End-of-pulsing sequence 900 + 1700 1300 + 1100 Used with TSPS (traffic service position system) 700 + 1700 Coin collect 700 + 1100 Coin control Coin return 1100 + 1700 Coin control Ring-back 700 + 1700 Coin control Code 11 700 + 1700 Inward operator (CCITT No. 5) Code 12 900 + 1700 Delay operator 1100 + 1700 Terminal call 1300 + 1700 Transit call aPulsing of digits is at the rate of about seven digits per second with an interdigital period of 68 + 7 msec. For intercontinental dialing for CCITT No. 5 code compatibility, the R-1 rate is increased to 10 digits per second. The KP pulse duration is 100 msec. Source: Ref. 4. SIGNALING Table 7.3 CCITT No. 5 Code Showing Variations with the R-1 Code Signal Frequencies (Hz) Remarks 1100 + 1700 Terminal traffic 1300 + 1700 Transit traffic 700 + 900 700 + 1100 Same as Table 7.2 1500 + 1700 Code 11 700 + 1700 Code 11 operator Code 12 900 + 1700 Code 12 operator Lune signaling for CCITT No. 5 code is 2VF, with f1 2400 Hz and f2 2600 Hz. Line-signaling conditions are shown in Table 7.4. Source: Ref. 5. Table 7.4 CCITT No. 5 Line Signaling Code Recognition Time Signal Direction Frequency Sending Duration (msec) Seizing Continuous 40 + 10 Proceed to send Continuous 40 + 10 Busy flash Continuous 125 + 25 Acknowledgment Continuous 125 + 25 Answer Continuous 125 + 25 Acknowledgment Continuous 125 + 25 Cl