Digital Signaling; Digital Fxs/Fxo Loop-Start Signaling - Juniper CTP2000 Series Hardware Documentation

CTP2000 Hardware Documentation

Digital Signaling

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This time constraint is used to minimize glare. Once the FXO has closed the loop, the call
proceeds as in the loop-start case.
When a call is initiated by the customer (FXO) side, the FXO starts by grounding the R
lead. The FXS side responds by grounding its T lead. After the FXO has detected the
grounded T lead, it closes the loop by removing the R lead from ground and closing the
loop. With ground-start circuits, a far-end disconnect (FXS side) is indicated by the FXS
disconnecting the tip from ground. The FXO senses the tip disconnect and goes on-hook
by opening the loop.
Channel banks are often used to multiplex and demultiplex FXS or FXO interfaces onto
T1 or E1 digital circuits. In the process, the analog signal is converted into pulse code
modulation (PCM) and carried by one of the channels in the time-division multiplexing
(TDM) circuit. For the interface to function properly, it must be able to signal the remote
end of the T1/E1 connection as well as respond to signals from the remote end. Signaling
is carried over the TDM circuit using either channel-associated signaling (CAS) or
common-channel signaling (CCS). Generally, four signaling bits (A, B, C, and D) may be
used; however, two signaling bits are most common (A and B).
For CTP analog voice products to work with digital devices, A and B bits are generated
and transported across the network.

Digital FXS/FXO Loop-Start Signaling

For loop-start signaling of FXS and FXO interfaces, the A bit is used to indicate the state
of the current loop, whereas the B bit is used for ringing. In the idle state (no ringing, FXO
on-hook), A=0 and B=1. A=1 when the FXO is off-hook. Ringing is signaled by the B bit
toggling between 0 and 1. Typically the toggling is 2 seconds off and 4 seconds on. For
digital loop-state, the signaling is bidirectional.
Because hook indication is detected by the analog FXS interface, this device is responsible
for generating the A bit. Likewise, because the analog FXS interface generates the ring
voltage, this device must respond to the B bit. Because the signaling is bidirectional, the
FXS side must echo the B bit when sending out the A bit.
The same logic applies to the FXO interface. The FXO device goes on/off-hook. It must
respond to the A bit, going off-hook when A transitions from 0 to 1, and going on-hook
when A goes from 1 to 0. The analog FXO interface detects ringing; therefore, it is
responsible for generating the B bit value. The FXO device must echo the A value when
sending out the digital signaling over the network.
Two situations for this call sequence must be considered: when the CTP is the FXO device
and when the CTP platform is the FXS device. In both cases, before the call starts, the
FXO is on-hook (A=1) and there is no ringing (B=1).
For an analog CTP FXS interface, before the call starts, the CTP interface must generate
A=0 and send both A=0 and B=1. When the call initiates from the CO, the B bit is
toggled. In response to this toggling, the CTP device generates a ring voltage on the
analog FXS interface. When the attached FXO device goes off-hook, the CTP FXS
interface detects the off-hook, sets the A bit to 1, and stops the ring voltage. During the
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