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Busy tone delayed on voice gateway

Hi there, I've a little issue in my production environment, maybe someone can help me.

Here's the scenario :

The 3845 VG is backhauled by the CUCM 6.1 by means of MGCP.

The VG sees the PSTN by means of a PRI vs. a Siemens HiPath PBX (QSIG trunk).

Users observe that sometimes after calling busy PSTN nunmbers they have to wait more than 10 seconds before hearing the busy tone.

My hypothesis is that in moments of traffic congestion on the PBX, the VGW receiva a "wait until there's a free channel" isdn signal from the PBX (0x8091 - User busy). Is this correct/possible?

I've captured the VG's debug isdn q931 output in both the normal case and the "slow" case (the facility messages are cropped out for space reasons):

Normal behavior (busy tone sent immediately to user) :

*Oct 6 15:22:53.918: ISDN Se0/0/0:15 Q931: TX -> SETUP pd = 8 callref = 0x10DD

Sending Complete

Bearer Capability i = 0x8090A3

Standard = CCITT

Transfer Capability = Speech

Transfer Mode = Circuit

Transfer Rate = 64 kbit/s

Channel ID i = 0xA9839E

Exclusive, Channel 30

Calling Party Number i = 0x0081, '4XXXX'

Plan:Unknown, Type:Unknown

Called Party Number i = 0x80, '0XXXXXXXX'

Plan:Unknown, Type:Unknown

*Oct 6 15:22:53.962: ISDN Se0/0/0:15 Q931: RX <- CALL_PROC pd = 8 callref = 0x90DD

Channel ID i = 0xA9839E

Exclusive, Channel 30

*Oct 6 15:22:53.978: ISDN Se0/0/0:15 Q931: RX <- FACILITY pd = 8 callref = 0x90DD

*Oct 6 15:22:55.258: ISDN Se0/0/1:15 Q931: RX <- FACILITY pd = 8 callref = 0x90D3

*Oct 6 15:22:58.838: ISDN Se0/0/0:15 Q931: RX <- PROGRESS pd = 8 callref = 0x90DD

Cause i = 0x8091 - User busy

Progress Ind i = 0x8088 - In-band info or appropriate now available

Progress Ind i = 0xA090 - Reserved value

---------------------------------------------------------

Slow behavior (busy tone sent to the user after couple of secs) :

*Oct 7 12:21:42.097: ISDN Se0/0/0:15 Q931: TX -> SETUP pd = 8 callref = 0x11C8

Sending Complete

Bearer Capability i = 0x8090A3

Standard = CCITT

Transfer Capability = Speech

Transfer Mode = Circuit

Transfer Rate = 64 kbit/s

Channel ID i = 0xA9839E

Exclusive, Channel 30

Calling Party Number i = 0x0081, '4XXXX'

Plan:Unknown, Type:Unknown

Called Party Number i = 0x80, '0XXXXXXXX'

Plan:Unknown, Type:Unknown

*Oct 7 12:21:42.141: ISDN Se0/0/0:15 Q931: RX <- CALL_PROC pd = 8 callref = 0x91C8

Channel ID i = 0xA9839E

Exclusive, Channel 30

*Oct 7 12:21:42.153: ISDN Se0/0/0:15 Q931: RX <- FACILITY pd = 8 callref = 0x91C8

*Oct 7 12:21:46.437: ISDN Se0/0/0:15 Q931: RX <- PROGRESS pd = 8 callref = 0x91C8

Cause i = 0x8091 - User busy

Progress Ind i = 0x8088 - In-band info or appropriate now available

Progress Ind i = 0xA090 - Reserved value

*Oct 7 12:22:09.969: ISDN Se0/0/1:15 Q931: RX <- DISCONNECT pd = 8 callref = 0x003E

Cause i = 0x8090 - Normal call clearing

*Oct 7 12:22:09.981: ISDN Se0/0/1:15 Q931: TX -> RELEASE pd = 8 callref = 0x803E

*Oct 7 12:22:10.009: ISDN Se0/0/1:15 Q931: RX <- RELEASE_COMP pd = 8 callref = 0x003E

Cause i = 0x8090 - Normal call clearing

*Oct 7 12:22:11.637: ISDN Se0/0/1:15 Q931: RX <- FACILITY pd = 8 callref = 0x91C5

*Oct 7 12:22:16.397: ISDN Se0/0/0:15 Q931: RX <- DISCONNECT pd = 8 callref = 0x91C8

Cause i = 0x8091 - User busy

*Oct 7 12:22:16.413: ISDN Se0/0/0:15 Q931: TX -> RELEASE pd = 8 callref = 0x11C8

*Oct 7 12:22:16.429: ISDN Se0/0/0:15 Q931: RX <- RELEASE_COMP pd = 8 callref = 0x91C8

Cause i = 0x8090 - Normal call clearing

-----------------------------------------------------------

Thanks in advance,

Alberto

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