Need assistance in determining "max" number of calls before incoming calls would hear busy signal?

Answered Question
Dec 16th, 2009

Looking for assistance with determining the "max" number of calls that system can recieve

before issuing a busy signal to the end user.  This is for external callers.  Primary Voice Engineer is no longer at our company and I'm trying to fill in.  Therefore, only have the basic info.  Have a 2851 with a Voice PRI; and then there is a Voice T1 also connected to this router.  In theory I believe it was suppose to failover to the T1 if the PRI filled out.  Need to verify this.  Running on CCM 7.1.2; code on 2851 is 12.4(7g).  Also using version 8.1 of RTMT.

Thank you,

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Correct Answer by rob.huffman about 7 years 1 month ago

Hi Darcy,

Just to add a little note to the great tips from Art (+5 points Art!)

The theoretical number of Incoming calls on a PRI +T1 combination would be 47

(23 on the PRI and 24 on the T1) then subtract the number of Outgoing

calls at any given time (as these are likely shared between Incoming/Outgoing)

and you have your number. As you can see this number is fluid as the Incoming/Outgoing

call flows continually change

Cheers!

Rob

Correct Answer by asandborgh about 7 years 1 month ago

Darcy,

It will be helpful to know if these are MGCP or H323, or you can cover both by setting up RTMT like the attached scrape.  Also you can run show call active voice on the gateway to see how many active telephony call legs are up, which equals PRI or CAS channels in use.

HTH,

Art

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asandborgh Wed, 12/16/2009 - 10:17

Darcy,

It might be helpful to know whether the gateway is NGCP or H323, or you can set up RTMT for both as shown in the screenscrape attached (use "calls active").  On the gateway you can also do a show call active voice and look at the telephony legs - that will tell you how many channels of the two trunks you are using.

HTH,

Artrtmt.jpg

Correct Answer
asandborgh Wed, 12/16/2009 - 10:20

Darcy,

It will be helpful to know if these are MGCP or H323, or you can cover both by setting up RTMT like the attached scrape.  Also you can run show call active voice on the gateway to see how many active telephony call legs are up, which equals PRI or CAS channels in use.

HTH,

Art

Attachment: 
darcy Wed, 12/16/2009 - 11:39

Art,

I looked on the gateway and found the following:

Telephony call-legs: 12
SIP call-legs: 0
H323 call-legs: 5
Call agent controlled call-legs: 7
SCCP call-legs: 0
Multicast call-legs: 0
Total call-legs: 24

I then did as you suggested and looked in the RTMT for Cisco H323 "Calls Active"  why does it only show the range from 0 - 1?  I believe that we are only running H323 but if there is a way to verify that MGCP isn't on the gateway, let me know.

Thank you for the quick response it was very helpfull.

Darcy

Correct Answer
rob.huffman Wed, 12/16/2009 - 11:12

Hi Darcy,

Just to add a little note to the great tips from Art (+5 points Art!)

The theoretical number of Incoming calls on a PRI +T1 combination would be 47

(23 on the PRI and 24 on the T1) then subtract the number of Outgoing

calls at any given time (as these are likely shared between Incoming/Outgoing)

and you have your number. As you can see this number is fluid as the Incoming/Outgoing

call flows continually change

Cheers!

Rob

darcy Wed, 12/16/2009 - 11:41

Rob,

Thank you for the information.  Question:  How do I verify that my configuration is set to failover to the T1 when the PRI is full?  Also, is there a way to test this.  I'm new to the voice world but have over 25 years in networking.

Thanks,

Darcy

asandborgh Wed, 12/16/2009 - 11:51

Darcy,

Your carrier will be the one to make it actually transition to the next trunk by having both in the same trunk group.  You can force this to happen by shutting down the interface/controller that you think is the first one - but be warned this will likely interupt calls unless you can do it after hours.  If you shut it down the carrier switch should see it go out of service and start sending the calls to the other circuit if they are in  a trunk group..

You might also want to look at your route groups and lists in CCM.  You will likely have a route group for each one of them and then and ordered route list.  NORMALLY it would have the circuits in the opposite search order as the carrier, so that your outbound traffic had a lower likelyhood of butting heads (glare) with the incoming traffic.  Additionally if you have a route list with both of the circuits in it that strongly indicates that the carrier has them in a carrier trunk group (what you have described with the failover), but not guaranteed.

HTH,

Art

Many Thanks Rob!

asandborgh Wed, 12/16/2009 - 12:02

Darcy,

One more clarification - for CCM - a route group is a potential grouping of trunks that have some similarity (destination normally) that will be selected for outbound calls in a round robin manner.  In many cases it will only have one T1, PRI or other type of trunk so that calls will not be round robin, but will be in order which is done at the next level - the route list.  The route list, which will route an outbound call in order, normally from the top down of route groups that are listed in it.  Outbound dialed route patterns are assigned a route list and then they select which route group which selects the gateway and circuit to send the call out.

The CO has a similar selection situation if you have multiple incoming circuits to your facility and they have configured a "carrier hunt" - that hunts for an available circuit/channel when they have a call to deliver to you.

Rather than potentially causing a service interuption you might want to find the contact information for the Telco rep who handles your phone service and give them a call - ask if both circuits are in a carrier hunt.  If you are going to be responsible for the voice infrastructure it doesn't hurt to get to know these folks anyway.

Cheers!

Art

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