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New Member

SIP to Q931 Gateway - Delayed response to 486 User Busy


I have 28xx and 5350 routers with E1 cards, acting as Voice Gateways, converting from SIP to Q931.

(124-16b IOS).

I have the following problem:

1) Inbound call on the Q931 interface (SETUP)

2) SIP INVITE sent by VGW to customer SIP Endpoint

3) Endpoint replies with 100 Trying

4) VGW sends Call Proceeding and Progress via Q931

5) Endpoint sends 486 User Busy

6) VGW ACKs the 486

7) VGW sends DISCONNECT via Q931

The problem I am having is that between steps 6 & 7 there is a 10 second delay. There is no delay on the SIP leg of the call (from INVITE to ACK takes 0.17 seconds) so it seems that the VGW is not acting on the 486 for 10 seconds.

Has anyone got any suggestions regarding this problem?

Any help would be much appreciated




Q931 trace

15:47:45.833 BST: ISDN Se0/1/0:15 Q931: RX <- SETUP pd = 8 callref = 0x001A

Sending Complete

Bearer Capability i = 0x9090A3

Standard = CCITT

Transfer Capability = 3.1kHz Audio

Transfer Mode = Circuit

Transfer Rate = 64 kbit/s

Channel ID i = 0xA9839B

Progress Ind i = 0x8581 - Call not end-to-end ISDN, may have in-band info

Calling Party Number i = 0x21A3, '7825123456'

Called Party Number i = 0x81, '0207123456'

15:47:45.861 BST: ISDN Se0/1/0:15 Q931: TX -> CALL_PROC pd = 8 callref = 0x801A

Channel ID i = 0xA9839B

15:47:45.861 BST: ISDN Se0/1/0:15 Q931: TX -> PROGRESS pd = 8 callref = 0x801A

Progress Ind i = 0x8188 - In-band info or appropriate now available

15:47:56.150 BST: ISDN Se0/1/0:15 Q931: TX -> DISCONNECT pd = 8 callref = 0x801A

Cause i = 0x8091 - User busy


Re: SIP to Q931 Gateway - Delayed response to 486 User Busy

Cause i = 0x8091 - User busy.

This is because the called system acknowledges the connection request. However, the system cannot accept the call because all B-channels are in use. The user equipment is compatible with the call in this situation.

If you have multiple ISDN circuits, the telco can configure them in a "hunt-group", in which calls switch to the next available circuit.

New Member

Re: SIP to Q931 Gateway - Delayed response to 486 User Busy


Thanks for your comments on this issue, however this does not answer the question.

The Q931 "user busy" is being generated due to the SIP "486 User Busy", rather than a limitation on B-channels.

My question is; why does it take 10 seconds from the VGW receiving the 486 until it sends the Q931 user busy?



New Member

Re: SIP to Q931 Gateway - Delayed response to 486 User Busy

Re: SIP to Q931 Gateway - Delayed response to 486 User Busy

Hi Richard,

I would do this:

debug voip ccapi inout

You'll see 2 large 'gaps' in the output with things like:







(info info info incoming dial-peer=x ..........)

Those are the call legs being established. You will see the incoming dial peer directly below that for the pots dial peer and the outgoing dial peer for the SIP leg for the second.

My guess is that you'll see some more with the same ani/dest because your IOS may be hunting for other solutions.

Because otherwise, I don't know of a good reason for this delay to be there. It may be trying to send it out another dial peer that it just can't contact and it's waiting for the TCP SYN timer to pop.


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