Information Technology Reference
In-Depth Information
Q.931 messages used to establish a call include SETUP, CALL PROCEEDING, ALERTING, CONNECT, and CONNECT ACK.
The SETUP message includes information such as bearer capability, transfer rate, calling party number, and called party number.
Issues that occur with ISDN are often caused by misconfiguration. But, it is worth briefly looking at one or two issues that are not
caused by misconfiguration.
One issue involves a user of an IP phone not hearing a busy signal when calling a busy destination on the PSTN. This occurs because
the PSTN has disconnected the call with a normal call-clearing cause code instead of using the (appropriate) user busy cause code.
This issue can be remedied by configuring the voice call convert-discpi-to-prog command on the gateway. This command
configures the gateway to change a Disconnect message with a progress indicator to a progress message with a progress indicator.
(Audio cut-through will result, and the busy tone will be heard from the PSTN.)
Another issue can occur when using MGCP to backhaul ISDN Q.931 signaling to CUCM. In this case, there are certain cases in
which ISDN numbering type and plan mismatches can occur. Usually, ISDN switches ignore numbering plan and numbering type
fields in ISDN messages, but if they are configured to route calls partially on the basis of these fields, for example, problems can
occur.
If CUCM is sending the proper digits, but calls are not being correctly routed, it might be a good idea to change the calling and called
party numbering plan and numbering type to other values. If route patterns contain the @ wildcard, the numbering plan is national
(or international if 011 is dialed) and the numbering type is ISDN. If route patterns do not contain the @ wildcard, the called party
numbering plan and numbering type are both unknown.
T1/E1 CAS
In CAS, signaling operates by “robbing” the least significant bit of information from voice channels and using this to send clocking
and framing information. CAS is also known as robbed-bit signaling. So, unlike ISDN, T1 CAS uses in-band signaling.
There are several types of CAS signaling, including the following:
Loop-start
Ground-start
 
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