Think of VCS-C as a H.323 Gatekeeper and SIP registrar - whereas an MCU is a Multipoint Control Unit also referred to as a "bridge".The MCU is what's used for multi-party video calls.
This ranges from stand-alone units capable of hosting very large multi-party calls, to end-points with "on-board" MCU functionality capable of hosting calls involving on average a maximum of three additional sites.
The VCS-C is a very powerful tool and apart from being a Gatekeeper/SIP registrar performing call control related "tasks", can also, among other things, translate calls between H.323 and SIP as well as IPv4 and IPv6, and, in combination with a VCS-E, will assist in secure firewall traversal.
Other than the secure firewall traversal when teamed up with a VCS-E, a VCS-C sounds like it does the job of a Call Manager. I think all video endpoints can register to a CM, and a MCU is leveraged when trying to achieve an ad-hoc multi-party video call. So how does a VCS-C and a CM compare?
There is much more to the VCS-C than "just" providing secure firewall traversal when teamed with a VCS-E, but yes, the CUCM and VCS are both "call managers" in a way, and they complement each other - and both may be needed - depending on the complexity, and the requirements of your deployment.
I.e. do you need interworking between H.323 and SIP, interworking between IPv4 and IPv6, Lync support, support for older video end-points, provisioning of JabberVideo aka Movi etc - in which case you'll need that VCS.
Also remember a VCS-C/VCS-E combo does not only provide secure firewall traversal between your organisation and another with a similar set-up, but it also enables secure firewall traversal for external, stand-alone H.460.18/19 compliant end-points (including nat'ed) - and will do the same for old, non-compliant end-points by allowing these old systems to register with the VCS-E - as long as they have a public, non-nated address - and of course depending on whether or not you allow this registration.
As far as MCU calls are concerned, again, depends on your requirements, but the VCS can be configured to use Multiway, depending on type of MCU deployed, which will allow end-points and JabberVideo users for that matter, to escalate a point-point call to a multipoint call.
CUCM and VCS are moving towards each other, and they may eventually turn into the one product one day, but until then you might very well need both.
Also be aware Cisco has implemented native support for Lync 2013 in TC6.2 using H.264 SVC, however, you'll need VCS x8.0 for this to work - which is due out shortly.
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