Monitoring the Signaling for Digital CAS Protocols
- Nathaniel Halbrooks
Overview
PSTN digital trunks use signaling bits associated with voice channels to carry signaling information. The digital Channel Associated Signaling (CAS) bits are also known as ABCD bits. The meaning of these signaling bits will depend on the digital CAS protocol used on the E1/T1 trunk.Â
showsg95  is an utility included with Dialogic® NaturalAccess™ that can be used to monitor the signaling bits from a Digital CAS protocol in a network interface of a Dialogic CG Series Media Board
The command line options are as follows:
-b{board}
Board number [default 0]
-d{AGSW|WTI8|QXSW}
Device type [default AGSW]
-k{t}
Quad-T1/E1 Trunk Number [default 0]
-l{m..n}
Local timeslot range to display
-o
one sample only
-r
show Received signaling only
-s
show Sent signaling only
-t{time}
Sampling interval, in seconds if < 10, else msec [default 1 sec]
-T
Time stamp each interval
-4
Display ABCD signaling [default for E1 boards; all others display AB signaling only]
-2
Display only AB signaling on E1 boards |
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The following is an example of the AB signaling digits captured on board 0, trunk 0, channel 4. The value 14:58:59 represents a time stamp for this message. In the output text, snd shows the bits sent and rcv the bits received.
This trace was captured using the Off-Premises Station (OPS) CAS protocol. The line state changes can be mapped to call status changes according to the protocol specifications, briefly summarized in the "CAS for Natural Call Control Developer's Manual".
E:\>showsg95 -b0 -l3 -k0 -2 -T -t10
SHOWSG95 Version 1.0 Jun 23 2003
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Monitoring Trunk 0 slot 3 of agsw board 0, sending and receiving
Sampling interval 10 milliseconds |
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In the trace, the symbol "." indicates that the corresponding bit is reset, the symbols "A" and "B" indicate that the corresponding bits are set.
3 14:58:59 snd: .B rcv: .B
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The combination 01/01 indicates that the line is idle.
14:59:04 snd: .B rcv: ..
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The combination 01/00 indicates that a "ring-on" signal has been received from the Switch. Incoming call.
14:59:06
snd: .B
rcv: .B
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The combination 01/01 indicates that a "ring-off" signal has been received from the Switch.
14:59:09 snd: .B rcv: ..
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The combination 01/00 indicates that a "ring-on" signal has been received from the Switch.
14:59:10
snd: .B
rcv: .B
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The combination 01/01 indicates that a "ring-off" signal has been received from the Switch.
14:59:12 snd: AB rcv: .B
The combination 11/01 indicates that the application answered the call and the line is connected.
The call then continues from here.
Product List
Dialogic CG Series Media Boards
Glossary of Acronyms / Terms
CAS -- Channel associated signaling; a signaling method in which signaling information is provided on a line-by-line or channel-by-channel basis. Signaling information is sent for each channel at regular intervals, regardless of state changes. Each transmission channel is associated with one or more transmission paths. In-band signaling is a type of channel associated signaling.Â
OPS protocol -- A protocol for controlling channel associated signaling for T1 or E1 digital trunks, of signaling types FX (foreign exchange) or SA (special access).Â