Does TDM switching work with MGCP?

Answered Question
Dec 27th, 2008

If we have 2 PRI's coming into a router that supports TDM switching, and the 2 PRI's on that router are MGCP controlled, can we TDM switch between those 2 PRI's?

I have this problem too.
0 votes
Correct Answer by Nicholas Matthews about 7 years 8 months ago

Hi Jordan,

Yes - with CUCM controlled MGCP you will not be able to do pure TDM hairpinning. There will be an internal RTP stream and it will require DSP resources.



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jordan.bean Sat, 12/27/2008 - 05:59

To clarify, we currently have a carrier provided PRI that is terminated into a Cisco 3745. Right now we have 'isdn bind-l3 ccm-manager' enabled on that PRI, so signalling is backhauled to CUCM.

Let's say we want to connect a 3rd party PRI/analog gateway to break out analog lines. We want to TDM switch between the carrier provided PRI and the PRI connecting to our PRI/analog gateway so IP isn't involved.

Rather than backhaul the signalling to CUCM, I assume we can disable that feature. Then, use dial-plans to forward any analog calls back out the PRI to the PRI/analog gateway and then default the remaining calls to CUCM for control using 'service MGCPAPP' for the default POTS dial peer?

Nicholas Matthews Mon, 12/29/2008 - 09:01

Hi Jordan,

If you want every call that comes in from one of the PRIs to go out of the other PRI, you can do with this partitions and setting the calling search space of the PRI's in CUCM.

It may be easier to use H323 and just use dial peers on the router. When you use MGCP you will always have a VoIP call leg, even if the call is hairpinned through two analog circuits. With H323 you can go under 'voice card 0' and type 'local bypass' (this is the default). This will take the DSPs out of the call and will switch the audio directly from one circuit to another without the router getting involved. That's one implementation of switching.

If you're more concerned about the routing of calls, you can do it with H323 or MGCP. With the way that MGCP works with CUCM, backhauling is required. You will not be able to disable the backhauling and have the circuit still work. If you want the signaling to stay local to the router, you would need to implement a SIP/H323 dial peer plan on the router. The way you would do this is dependent on the dial plan.

What you can do is this:

-Set up the dial plans between the two PRIs with dial peers with the correct destination pattern and preferences

-Configure the gateway as a H323 gateway or SIP trunk in CUCM.

-Add a wildcard dial peer that points to CUCM via H323 or SIP for calls that are not supposed to be switched.

-Configure 'voice card 0' 'local bypass' to prevent altering the voice stream and using unnecessary DSPs



jordan.bean Mon, 12/29/2008 - 15:30

Thanks. What we're trying to do is connect a 3rd party TDM MUX to the router via a PRI so that for any FAX calls that come in over a PRI, we can TDM switch them to the TDM MUX, which then breaks them out to analog lines.

It sounds like the above will work for us. The thing we're battling next are CDR records so we can track LD usage of the calls that the router TDM switches. I know we can setup gateway CDR reporting, but I don't think that's going to give us all of the info that we need. Any thoughts?

Nicholas Matthews Tue, 12/30/2008 - 09:52

If you use CUCM, you can use the CDR reporting there, otherwise you would want to implement gateway call accounting with RADIUS.



jordan.bean Tue, 12/30/2008 - 13:23

So just to be clear:

We're using a Cisco 2811 to terminate a PSTN PRI. We will have another PRI on the same NM-HDV2 module go to a channel bank (PRI as well). Only H.323 will be configured on the router so we can hairpin FAX calls directly to the channel bank. Remaining calls will go to CUCM.

We will see CDR's for the hairpinned calls in CUCM?

Nicholas Matthews Tue, 12/30/2008 - 17:03

Hi Jordan,

You can configure for any call that goes through CUCM to be reported in the CDR. This shouldn't be an issue. With H323, the only accounting you would need to configure any differently would be TDM switched calls that don't go through any CUCM routing.


jordan.bean Fri, 02/06/2009 - 09:00


I have another question regarding this that I couldn't figure out from the above:

If we have an MGCP gateway with signalling backhauled to CUCM, can we TDM hairpin the call? You mentioned there will be a VoIP call leg with MGCP. Does this mean that the call coming into 1 channel on a PRI and going out a channel on another PRI on the same NM-HDV2 card will or will not be TDM switched when MGCP is used?

Correct Answer
Nicholas Matthews Fri, 02/06/2009 - 09:59

Hi Jordan,

Yes - with CUCM controlled MGCP you will not be able to do pure TDM hairpinning. There will be an internal RTP stream and it will require DSP resources.




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