Centralised CCM Deployment with SRST Design Guidance

Answered Question
Dec 17th, 2009

Greetings, ive worked with CCME for a few years now and am starting to get involved with CCM, hopefully ill start my CCVP course in the new year.


One thing that has been on my mind though is how incomming calls are handled by CCM in a centralised CCM/SRST environment.


Scenario:


  • Three sites, A, B, C
  • All sites connected by way of MPLS network
  • A is the HQ with CCM and ISDN Pri Circuit
  • B is running a 2900 series ISR w/ CCME/SRST and an ISDN PRI Circuit
  • C is running a 2900 series ISR w/ CCME/SRST and an ISDN PRI Circuit
  • All incomming and outgoing PSTN calls should be handled by each sites respective ISDN connection.
  • All intersite telephony traffic will go via the MPLS network, IE: site to sites calls, transfers etc


Now this is where i dont fully understand how calls are handled, i agree that all phones would be registered with CCM using SCCP and that SRST would be configured on each ISR as a fall back to local PSTN in the event that the IP link is lost.


However in the above situation what is the recommended solution for handling incomming and outgoing calls for sites B and C bearing in mind that all incomming and outgoing PSTN traffic with the exception for intersite calls is sent via the local ISDN PRI circuit.


Would i be correct in saying that you configure the ISR gateway at each site to teminate the initial ISDN connection which then passes a signalling message to CCM using SCCP resulting in the appropriate phone ringing for that particular site based on the called number?


Again would a similar methodology be used for placing outgoing calls, a user goes off hook at Site B and dials an external PSTN number, CCM now knows the phone has gone off hook and see's the dialled number, does CCM then signal the the local ISR gateway using SCCP to setup the call, following on from this does CCM also then back off and hand the connection off to the local gateway?


I appreciate this seems straight forward but would like to have a clear understanding.


Regards

Mark Rigby


I have this problem too.
0 votes
Correct Answer by iptuser55 about 7 years 1 month ago

You are correct in your thinking with regards to the call flow, Ideally you should have locally terminated ISDN`s on the remote sites for incoming and outgoing calls. The remote site GW is "registered" MGCP, H323 to CUCM and is configured to pass the incoming digits back to CUCM which is then used to route the call. The routing of the call will normally be back to the remote site again as with incoming DDI so when a call comes in via the remote site, the GW signals back to CUCM, At the same time the call is anchored at the remote GW waiting for CUCM to signal where to send the call. CUCM looks for the destination DN and identifies it being back at the remote site and signals the handset to apply ringing. If the user picks the call up at the remote site then the RTP is local, if the call is forwarded no answer to say another user not at the same site or to a central VM system then again SCCP signal is apply to the called party and if answered RTP is back to the central, other site consuming bandwidth- it is this part you need to configure locations, regions and Codecs. Other consideration is where do you keep the MOH , again if you require MOH at the SRST site then this should be local if WAN bandwidth is an issue

if you have all the incoming ISDN, DDI at the central site then the opposite happens in that the majority of answered calls will be at the remote site consuming bandwidth. In some ways you need to think of remote SRST sites as small telephony systems by themselves in that if the WAN goes down then they have the local resources- Music on Hold, Conference, ISDN etc to operate by themselves - too much centralising then the SRST site becomes ineffective as it can not operate without the CUCM . What you save with SRST is local PABX maintain costs, local support as with MACS and reduce 3rd party applications through centralising Voice Mail, Call Logging equipment

The main concern is the impact of losing incoming DDI, outgoing calls not an issue as any ISDN circuit can be used at the remtoe site. It is how you manage the loss of incoming DDI for each site

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Correct Answer
iptuser55 Fri, 12/18/2009 - 00:45

You are correct in your thinking with regards to the call flow, Ideally you should have locally terminated ISDN`s on the remote sites for incoming and outgoing calls. The remote site GW is "registered" MGCP, H323 to CUCM and is configured to pass the incoming digits back to CUCM which is then used to route the call. The routing of the call will normally be back to the remote site again as with incoming DDI so when a call comes in via the remote site, the GW signals back to CUCM, At the same time the call is anchored at the remote GW waiting for CUCM to signal where to send the call. CUCM looks for the destination DN and identifies it being back at the remote site and signals the handset to apply ringing. If the user picks the call up at the remote site then the RTP is local, if the call is forwarded no answer to say another user not at the same site or to a central VM system then again SCCP signal is apply to the called party and if answered RTP is back to the central, other site consuming bandwidth- it is this part you need to configure locations, regions and Codecs. Other consideration is where do you keep the MOH , again if you require MOH at the SRST site then this should be local if WAN bandwidth is an issue

if you have all the incoming ISDN, DDI at the central site then the opposite happens in that the majority of answered calls will be at the remote site consuming bandwidth. In some ways you need to think of remote SRST sites as small telephony systems by themselves in that if the WAN goes down then they have the local resources- Music on Hold, Conference, ISDN etc to operate by themselves - too much centralising then the SRST site becomes ineffective as it can not operate without the CUCM . What you save with SRST is local PABX maintain costs, local support as with MACS and reduce 3rd party applications through centralising Voice Mail, Call Logging equipment

The main concern is the impact of losing incoming DDI, outgoing calls not an issue as any ISDN circuit can be used at the remtoe site. It is how you manage the loss of incoming DDI for each site

Mark Rigby Tue, 12/22/2009 - 01:54

Excellent explanation, thank you for your time it has helped greatly.


Regards
Mark Rigby

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