We run a Call Manager cluster with centralized call processing. Branch offices (connected via T1) are currently in the same city and have the phones at the branch offices register to the centralized CCM cluster over the T1. This works great.
We are beginning to add branch offices across the nation. We added one office with a site-to-site VPN over the Internet and we are having issues with dropped calls (not sure offhand if its signalling or voice packet related, but I think it's signalling related). 2 different Internet providers are currently used.
The nation-wide telecom vendor that we are consolidating to has an all fiber MPLS network with very low latency, jitter and virtually no packet loss. The metrics seem ideal for voice. They claim that if we peer directly to them for Internet access on both sides that we should able to use an Internet VPN (with QoS implemented on our borders) for voice until we get to the point where we need to utilize VPLS for multipoint between our geographic sites. We will only be traversing their all fiber MPLS network.
What is the best way to configure the branch offices? Should we do a site-to-site VPN and let the phones register to the centralized CUCM cluster and use SRST for fallback? Or, should we use CME with a SIP trunk so the phones can register locally? We'd prefer the first method so everything can be centrally managed.
I've found docs online regarding centralized processing with branch office configurations that show the phones register over the "WAN", but can't find any documentation that outlines the latency and jitter metrics for voice signalling and traffic over the "WAN". We don't want to commit to a 12, 24 or 36 month T1 and continue to have issues.
From my experience QoS is limited if calls are routed over the Internet using VPN. I would recommend you getting a MPLS cloud or a point to point T1 as desired. The round trip delay should be less than 150ms for voice to work fine. My personal recommendation would be to do CME with SIP trunk if you are going to do the Internet VPN route.
I would recommend you to go with your 1st idea that is CUCM in the HQ as centralized call processing engine & all branch offices will have SRST feature inbuilt.
Getting a MPLS cloud would be the suitable solution so that rtr delay can be maintained esser or equal to 150 ms.
In CUCM you can configure different regions like Inter-Region & Intra-Region where as with Intra-Region codec is being used as g.711 & within Inter-Region g.729a. So nation-wide branches can be a part of Inter-Region so that all calls between branches & from Branch-HQ & vice-versa can take 25.6 Kbps using g.729. In g.711 a established voice call will have 85.6 Kbps of B/W.
All branch Voice gatewys can be configured as RSVP agent so that a minimum b/w % of b/w is available for making outbound calls.
The vendor we use claims that going the Internet T1 route vs. MPLS will be the same because the Internet T1 traffic hits their MPLS backbone and there is no difference in traffic prioritization between the two. So, the IPSEC connection will be primary, not a backup.
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