Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 4 Troubleshooting with Call Flows
Call Flow Analysis
Step 2 Check to see whether the Call Agent can reach the terminating TGW—that is, can you see repeated
attempts to send CRCX messages to the trunk gateway? This may indicate a problem communicating
with the terminating TGW.
Note This failure should normally cause the Call Agent to route the originator to an announcement.
Verify L3 connectivity between the Call Agent and the terminating TGW.
Step 3 If the MGCP messaging looks correct, check to see whether the Call Agent sent an IAM to the PSTN
switch. The PSTN switch should have replied with an ACM. If there are no SS7 messages, check L3
connectivity between the connection manger and SS7 gateway. If there is L3 connectivity, contact Call
Agent technical support. The connection manager is not communicating with the SS7 gateway.
Step 4 If the CRCX messages are not seen by the originating MTA and trunk gateway, the Call Agent may be
having a problem. Contact Cisco technical support.
No Post-Dial Cut-Through (No Ringback)—Off-Net to On-Net
In this scenario, a caller from the PSTN calls a subscriber with an MTA. The PSTN caller reports hearing
“dead air.” There is no ringback or network announcement.
All incoming calls from the PSTN start with an SS7 IAM message. The receipt of an IAM causes the
Call Agent to send a CRCX message to both the trunking gateway, where the call enters and the
terminating MTA, as is shown in the call flow below. When the Call Agent sends the CRCX to the
terminating MTA requesting power ring and ringback tone, it also sends an SS7 ACM message back to
the PSTN switch. The PSTN caller should hear ringback tone provided by the terminating MTA.
If the Call Agent detects an error in processing the call (for example, the subscriber number is
unallocated), it sends an SS7 REL message with the appropriate cause code. This causes the PSTN
switch to play a local announcement. The caller’s receiving dead air most likely indicates that the Call
Agent proceeded with the call and sent an ACM within the acceptable GR317 time limits, or else the
PSTN switch would have routed the caller to reorder tone. When the PSTN switch receives the ACM, it
cuts through the backward direction of the call and expects ringback from the terminating MTA. If the
caller does not hear ringback tone, there is a problem in making the VoIP connections, or the terminating
MTA is having a problem playing ringback tone.