CHAPTER 6. Digital Voice Port
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The biggest disadvantage of CAS signaling is that the network uses bits from
information IP packets, such as voice packets, to perform signaling functions.
CAS signaling is often referred to as robbed-bit signaling.
Digital T1 trunk to be used in connection of most switch-to-switch is E & M
signaling type, but there is the case partially forming connection with FXS and FXO.
Figure 6.8 T1 CAS signaling network
1.
Loopstart signaling
Is the simplest type of CAS signaling, which change the operation type of
analog FXS into digital signaling type as it is. Thus, there are more defects
than other CAS signaling.
One of the defects is that there is no way to notify users of far-end
disconnect or answer, making users impossible to know about the status.
In the case of loop-start signaling, FXS only uses the A-bit and the FXO
side only uses the B-bit to communicate call information..
2.
Groundstart signaling
Is very similar with loop-start signaling in many ways.
Its merits compared with loop-start signaling are its supporting far-end
disconnect supervision loop-start does not provide. This is done by using the
A- and B- bit on the network side instead of just the B-bit. The A-bit is also
used on the CPE side. However, the B-bit can also be involved, based on the
switch’s implementation.
PSTN
PBX, K/P
T1-R1, T1-FXS
T1-FXO, T1-E & M
T1-R1, T1-FXS
T1-FXO, T1-E & M
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