I thought PVDMs were used for transcoding and voice conferencing sessions between different phones registered in CME / CUCM, but I think I'm wrong. I set up Cisco IP Communicator on two separate Windows computers and both registered witch CUCM 8.6 successfully. I then did a test call between the two phones and it worked, but the router that I used for CUCM did not have any PVDMs installed on it.
I did not configure any commands on the router starting with "sccp", "mgcp", or "voice register."
1. When are PVDMs necessary? I looked in "Settings" and then "Model Information" in IP Communicator and it says "SCCP" under "Call Control Protocol."
2. Also, is it possible to register with CUCM and place calls between a SIP, an MGCP, and an SCCP Cisco 7960G physical phone?
Because as you mentioned, that's a phone call, point to point, NOT a conference.
I'd recommend you yo start by doing some reading before further questions.
1 You're looking in the wrong place, nothing to do there with DSPs or PVDMs
They're used for TDM conversion, XCODING, conferencing and MTP
2 yes, just configure the endpoints and the dial plan as needed.
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I get a 403 forbidden error when I try to visit that link, but I found a document with the same title on Cisco's site that I was able to access at
Assuming that link is correct, it states that PVDMs are used for "voice sessions, transcoding sessions, conference sessions, and video," which I would think would included point-to-point phone calls? But apparently my test shows otherwise and you are stating this as well.
Also, I'm pretty sure that transcoding is used when two phones cannot support a common protocol, but is transcoding also needed when phones use a different protocol (SCCP, SIP, or MGCP)?
no, SCCP, MGCP and SIP are signalling protocols. This means if phone A speaks SIP only and phone B speaks SCCP only there's no way they're going to agree or even talk to each other - you'll have to setup a proxy server, in other words, an entity, that speaks both "languages". Like CUCM (aka CallManager), which can translate between signalling protocols.
PVDM's translate media streams. This means if a Phone A can only speak G.711uLaw and Phone B only G.729 then you'll have to use a transcoder of some kind. A PVDM sits on a router and is invoked when necessary, it will terminate the G.711uLaw stream coming from Phone A and will translate it to G.729 and send this new audio stream to Phone B.
PVMD's are also used when conferencing, when you need to "mix" phone calls.
Ok that makes sense.
When you say "mix" phone calls, are you saying that the PVDMs are used for conferencing only when the codecs used by each party in a voice / video conference are different? Does this mean that if we have a 4 way voice call and everyone is using G.711u, we don't need to use any PVDMs?
Also, do you know how I might have achieved my test call between the two IP Communicators without configuring any SCCP on the router? I did a "show sccp" on the router and it stated that the SCCP was administratively down.
No, you need PVDM's even if you are using the same codec, when doing audio conferencing. There must be something that mixes up all the "point-to-point" streams and "broadcasts" them to all parties so everyone can hear all parties.
About your test call: if you've got two phones, they can negotiate the codec. In this case, two IP Communicators can talk to each other without invoking any transcoding chips, as they can agree on the codec to be used.
Oh I actually meant to ask how the IP Communicator apps were able to do a test call even though I didn't configure any signaling protocol (SCCP, SIP, MGCP) on the router? The Cisco IP phone said that it was using under "Call Control Protocol" in the settings menu.
You MUST have something configured, they do not work unless they're registered to CME/CUCM.
SCCP devices are dumb endpoints, CUCM/CME is the one that handles everything related to routing, signaling and media.
Same for MGCP, SIP does provide some intelligence, but still relies on CUCM/CME.
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IP Communicators use SCCP (Skinny).
You don't need to set anything on the router, it's transparent. By the way, if both IP Communicators sit on the same subnet, chances are the signalling and the media stream between the two endpoints do not even go through the router.
Of course there are situations when you need to enable the SCCP capabilities (or SIP or MGCP etc) on the router, but it's a different story. Then the router sort of becomes an endpoint. For instance, if you enable the MGCP capabilities, the Voice over IP stream terminates at the router and then the voice is being transmitted over a TDM link.