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4.4 MFC-R2 SIGNALING 93Figure 4.4-2. Release-guard signals: (a) clear-forward, (b) release-guard signal when clear-backsignal has been sent, and (c) release-guard signal when clear-back signal has not been sent.As shown in Fig. 4.4-2, if the incoming exchange receives a clear-forward after ithas sent a clear-back, the release-guard signal is a 450-ms tone-off pulse (b). Ifthe incoming exchange has not sent a clear-back when it receives the clear-forward,the release-guard signal is a change to on-hook, 450 ms after the receipt of the clearforward(c).National R2 Supervision Signals. Most national R2 systems use pulsedout-of-band supervision signals. There are several country-specific pulsed signalingsystems, for example:Forward SignalsPulse Duration (ms)Seizure 150Clear-forward 600Backward SignalsPulse Duration (ms)Answer 150Clear-back 600Release-guard 600BlockingContinuous

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