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4.4 MFC-R2 SIGNALING 95Blocking. An exchange can block an idle trunk by changing its status bits froma,b ¼ 1,0 to a,b ¼ 1,1. Exchanges do not seize trunks that are in this state at thedistant end. To end blocking, the exchange returns the bits to a,b ¼ 1,0 (idle).Double Seizure. After sending a seizure signal, the outgoing exchangeexpects to receive a change to a b ,b b ¼ 1,1 from the incoming exchange. Theresponse a b ,b b ¼ 0,0 indicates a double seizure. Both exchanges then abort theircall setups and—depending on the procedures of the telecom—either make asecond attempt or abort the call setups, sending congestion indications to thecalling subscribers.4.4.3 Interregister SignalingIn R2 signaling, the equipment units at the exchanges that send and receive digits,and the signaling between these units, are usually referred to as registers, and interregistersignaling.R2 uses forward and backward in-band MF (multifrequency) signals. On digitaltrunks, these signals are PCM-coded. Signaling is compelled: a forward MF signal,sent by an outgoing register (OR) at outgoing exchange A, is held “on” until thereceipt of a backward MF acknowledgment from incoming register (IR) at incomingexchange B—see Fig. 4.4-3. R2 registers are transceivers: they both send andreceive register signals.Figure 4.4-3. Compelled interregister signal and acknowledgment.