Collaboration

Conferencing – Troubleshooting failed conference

Simon Birtles

BR1 Conference Host (CUCM)
HQ Participant (CUCM)
BR3 Participant (CME)

BR1 used local 2811 Conference Bridge as in MRGL in CUCM.

BR3 calls international +34966798100 to BR1 phone; so via BR3-GW to PSTN to BR1-GW to CUCM to Phone. Connects OK.

BR1 presses conference button and dials internal 2001000 to HQ phone over WAN. Connects OK.

BR1 presses conference soft key, ‘Unable to conference‘ briefly appears and drops BR3, call continues between BR1 and HQ. Failed.

Troubleshoot

BR1 phone has DP (DP_COLL) and <none> for MRGL; checking DP settings:
  • MRGL is MRGL_BR1
  • Location is LOC_COLL
  • Regions is REG_COLL

MRGL_BR1 >> MRG_BR1 >> CFB_BR1_GW:

  • Device Pool is DP_COLL
  • Location is LOC_COLL
  • Is registered with CUCM.
Checking BR1 router:

 

BR1#sh voice dsp capabilities slot 0 dsp 1
DSP Type: C5510 -16
Card 0 DSP id 1 Capabilities:
Credits   240 ,  G711Credits 15,  HC Credits  40,  MC Credits  30,
FC Channel  16,  HC Channel  6 ,  MC Channel  8 ,
Conference 8-party credits:
G711  30 ,  G729  120,  G722  120,  ILBC  240
Secure Credits:
Sec LC Xcode  30,     Sec HC Xcode  80,
Sec MC Xcode  60,     Sec LC UNIV Xcode  30,
Sec HC UNIV Xcode  80,Sec MC UNIV Xcode  60,
Sec G729 conf 120,    Sec G722 conf 240,     Sec ILBC conf 240,
Sec G711 conf 60 ,
Max Conference Parties per DSP:
G711 64,  G729 16,  G722 16,  ILBC 8 ,
Sec G711 32,  Sec G729 16,
Sec G722 8   Sec ILBC 8 ,
Voice Channels:
g711perdsp = 16,  g726perdsp = 8 , g729perdsp = 6 , g729aperdsp = 8 ,
g723perdsp = 6 ,  g728perdsp = 6 , g711_5msperdsp = 14, gsmamrnbperdsp = 6 ,
ilbcperdsp = 6 ,  isacperdsp = 3 , modemrelayperdsp = 6 ,
g72264Perdsp = 8 , h324perdsp = 5 ,
m_f_thruperdsp = 16, faxrelayperdsp = 8 ,
maxchperdsp = 16, minchperdsp = 3 ,
srtp_maxchperdsp = 10, srtp_minchperdsp = 6 , faxrelay_srtp_perdsp = 8 ,
g711_srtp_perdsp = 10, g729_srtp_perdsp = 6 , g729a_srtp_perdsp = 8
gnx64_srtp_perdsp = 10

so, 240 credits / 30 credits per G711 participant = 8 participants per conference call. 

BR1#sh voice dsp group all          
DSP groups on slot 0:
dsp 1:
State: UP, firmware: 28.3.14
Max signal/voice channel: 16/16
Max credits: 240
num_of_sig_chnls_allocated: 0
Transcoding channels allocated: 0
  Group: FLEX_GROUP_CONF, complexity: CONFERENCE
Shared credits: 0, reserved credits: 240
Codec: CONF_G711, maximum participants: 8
Sessions per dsp: 8
dsp 2:
State: UP, firmware: 28.3.14
Max signal/voice channel: 16/16
Max credits: 240
num_of_sig_chnls_allocated: 7
Transcoding channels allocated: 0
Group: FLEX_GROUP_VOICE, complexity: FLEX
Shared credits: 240, reserved credits: 0
Signaling channels allocated: 7
Voice channels allocated: 0
Credits used (rounded-up): 0

0 DSP resource allocation failure

So conference bridge supports only G.711 and supports up to 8 participants.
Check each phone codec, assigned region and location (CAC).
  • BR1 phone is locally connected via LAN to the IOS CFB and uses G.711 inside the region, both the phone and IOS CFB are part of the same region REG_COLL which permits up to G.711 (64k) intra-region. OK
  • BR3 called BR1 via the PSTN. BR1 ISDN connection will have terminated on the BR1 PSTN GW (same as IOS CFB device) and routed to the BR1 phone using the trunk configuration on CUCM which is in device pool DP_COLL. Therefore G.711 should be used between the GW and the BR1 phone and the GW and itself for ISDN call to CFB on the same device.
    BR1 dialpeer to CUCM did not have a voice class codec and therefore was negotating to G729 with GW, CUCM and BR1 phone. Added voice class codec to prefer G711 then 729r8. 

Now with the conference testing, when BR1 tries to join the two calls together (in the same sequence as above), a double beep is heard on BR1 and both calls stay connected to BR1 but the conference is not created. FAILS.

  • The call from BR1 to HQ is extension dialled (2001000) so is internal and over the WAN where the region settings enforce a max of 8kbps (G729) codec. This is what is seen on the devices.
So this means we require the HQ call leg to be G.711 within REG_COLL but with G.729 between REG_COLL and the HQ region HQ_BIRTLES as per the region settings. This means we need a XCODE somewhere ?
Well we actually need it in BR1 as the codec required between the regions is G.729, once inside REG_COLL, it can be XCODE’D to G.711 and then on to the CFB on BR1. [IS THIS POSSIBLE] – BR1 does not have an XCODE configured and the CFB is only configured to allow G.711.

Fix 1

Enable G.729r8 in BR1 conference bridge, BUT this has the effect of adding a complex codec to the CFB and reducing the max number of permitted CFB sessions (total conferences). Checking this as:

BR1#sh voice dsp group all 
DSP groups on slot 0:
dsp 1:
State: UP, firmware: 28.3.14
Max signal/voice channel: 16/16
Max credits: 240
num_of_sig_chnls_allocated: 0
Transcoding channels allocated: 0
Group: FLEX_GROUP_CONF, complexity: CONFERENCE
Shared credits: 0, reserved credits: 240
Codec: CONF_G729, maximum participants: 8
Sessions per dsp: 2
dsp 2:
State: UP, firmware: 28.3.14
Max signal/voice channel: 16/16
Max credits: 240
num_of_sig_chnls_allocated: 7
Transcoding channels allocated: 0
Group: FLEX_GROUP_VOICE, complexity: FLEX
Shared credits: 225, reserved credits: 0
Signaling channels allocated: 7
Voice channels allocated: 1
Credits used (rounded-up): 15
Voice channels:
Ch01: voice port: 0/2/0:15.1, codec: g711ulaw, credits allocated: 15

0 DSP resource allocation failure
BR1#

Retesting provides the required result of the conference bridge being invoked and the 3 phones being connected to the bridge.

BR1#sh sccp connections
sess_id    conn_id      stype mode     codec   sport rport ripaddr conn_id_tx
16791491   16777908     conf  sendrecv g711u   18208 20608 192.168.3.150
16791491   16777907     conf  sendrecv g729    16770 16408 192.168.3.1
16791491   16777903     conf  sendrecv g711u   19190 17126 192.168.3.1    
16791492   16777906     mtp   sendrecv pass_th 16408 16770 192.168.3.1
16791492   16777902     mtp   sendrecv pass_th 16614 16674 192.168.0.1

The last two sessions for DTMF RFC2833 from HQPhone >> HQGW >> BR1GW >> CFB

Fix 2

Restoring the codec selection for the CFG in BR1 to G.711 only (i.e. removing Fix 1) and troubleshooting. The BR1 phone again keeps both call legs up but briefly displays “cannot create conference”

Looking at RTMT traces it seems we have RSVP errors. Now RSVP was thought to have been removed by setting the inter-Location BW for audio/video to UNLIMITED. This apparently forces CUCM not to use RSVP but it seems it still does. RTMT traces show the following:

  •  resource_rsvp_AllocateRSVPResourceErr Device=MTP_BR1_GW
  • AppInfo  |UnicastBridgeControl::restart0_RSVPRegisterRes – Device Name = CFB_BR1_GW, Party CI = 23333959, RSVP Failure
So in view of this, the first call to be made from BR1 to HQ phone was a direct call, which was allowed by RSVP, call was made on line 2 of BR1 phone. Due to RSVP being used, a session on MTPs on each side (BR1 GW & HQ GW) for the call legs due to RSVP. So..
BR1_Phone >> BR1_GW_MTP >> HQ_GW_MTP >> HQ_Phone
which is taking up resources.
When the conference was initiated the resources remain and the new call leg between the BR1_GW_CFB and HQ phone is attempted using RSVP of course which fails in this case as the RSVP configuration on the GW routers is not sufficient for more than one call.  Fixing the RSVP bandwidth issue on the interfaces of the WAN routers running RSVP leaves same result.
Looking at RTMT traces..

Simon Birtles

I have been in the IT sector for over 20 years with a primary focus on solutions around networking architecture & design in Data Center and WAN. I have held two CCIEs (#20221) for over 12 years with many retired certifications with Cisco and Microsoft. I have worked in demanding and critical sectors such as finance, insurance, health care and government providing solutions for architecture, design and problem analysis. I have been coding for as long as I can remember in C/C++ and Python (for most things nowadays). Locations that I work without additional paperwork (incl. post Brexit) are the UK and the EU including Germany, Netherlands, Spain and Belgium.