Digital Fxs/Fxo Ground-Start Signaling; Table 6: Ground-Start Signaling At Fxo Interface For Call Initiated By The Fxo; Table 7: Ground-Start Signaling At Fxo Interface For Call Initiated By The Fxs - Juniper CTP2000 Series Hardware Documentation

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call, the CTP device sends A = B = 1 signaling bits. At the end of the call, the FXO device
goes back to on-hook, the CTP detects the on-hook, sends out A=0 and B=1 signaling
bits, and the circuit returns to the idle state.
For an analog CTP FXO interface, before the call starts the FXO is on-hook and no
ringing is generated by the attached FXS device. In this, the idle state, the CTP interface
generates B=1 and sends both A=0 and B=1. When the call comes in from the CO, the
CTP FXO interface detects ring voltage and starts toggling the B bit. When the other
end of the VCOMP bundle goes off-hook, it sends A=1 to the CTP. In response, the CTP
FXO interface closes the loop, going off-hook. With ringing stopped, the CTP interface
sends A = B = 1 during the call. At the end of the call the interface is again idle, and the
CTP interface sends A=0 and B=1 signaling bits.

Digital FXS/FXO Ground-Start Signaling

Unlike loop-start interfaces, in which the FXO and FXS each separately control a single
signaling bit, with ground-start signaling each side controls both the A and B bits. We
must consider each interface when the call is initiated by either interface. (See
Table 6 on page 23, Table 7 on page
all cases, when the FXO/FXS interface is idle, the FXO interface generates A = B = 1
signaling bits and the FXS A = 0, B = 1. When a call is in progress, either side initiates a
call termination if it receives these signaling bits. For example, an FXS goes on-hook if it
receives A=B=1 signaling bits.
Table 6: Ground-Start Signaling at FXO Interface for Call Initiated by the
FXO Interface
Idle (before call starts)
FXO grounds ring
FXS goes off-hook (grounds
tip)
FXO goes off-hook (closes
loop)
Duration of call
Table 7: Ground-Start Signaling at FXO Interface for Call Initiated by the
FXS Interface
Idle (before call starts)
FXS goes off-hook (grounds
tip and sends AC ring signal)
Chapter 2: CTP2000 Series Interface Modules
23, Table 8 on page 24, and
Signaling Bits to FXO
01 (A=0, B=1)
00
00
11
11
Signaling Bits to FXO
01 (A=0, B=1)
01
Table 9 on page 24). In
Signaling Bits from FXO
11 (A=1, B=1)
11
01
01
01
Signaling Bits from FXO
11 (A=1, B=1)
00/01 (B-bit toggles)
23