Nortel Circuit Card Installation Manual page 370

Page 370 of 906
Signaling levels
Forward and backward signals
553-3001-211 Standard 3.00
NT5K21 XMFC/MFE card
MFC signaling uses pairs of frequencies to represent digits, and is divided
into two levels:
• Level 1: used when a call is first established and may be used to send the
dialed digits.
• Level 2: used after Level 1 signaling is completed and may contain such
information as the status, capabilities, or classifications of both calling
parties.
When one NT5K21 XMFC/MFE card sends a pair of frequencies to a
receiving XMFC/MFE card (forward signaling), the receiving XMFC/MFE
card must respond by sending a different set of frequencies back to the
originating XMFC/MFE card (backward signaling). In other words, the
receiving card is always "compelled" to respond to the originating card.
In summary, the signaling works as follows:
• The first XMFC/MFE card sends a forward signal to the second card.
• The second card hears the forward signal and replies with a backward
signal.
• The first card hears the backward signal and "turns off" its forward
signal.
• The second card hears the forward signal being removed and removes its
backward signal.
• The first XMFC/MFE can either send a second signal or drop out of the
call.
August 2005