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New Member

What is call throttling ?


I'm having problems with my cluster and while checking my out of sync system parameters, I found out that I had a few of them regarding CCM Call Throttling. What is call throttling exactly ?



Re: What is call throttling ?


Are you talking about this parameter ?

H225 ICT call throttle timer -

This parameter defines the length of time that Cisco CallManager will throttle the new call attempt after it detects an unusually high number of call attempts for the H.225 trunk. When Cisco CallManager detects an unusually high number of call attempts for the H.225 trunk, which might be caused by events such as intercluster trunk looping, it triggers a call-throttling mechanism to reject the new call attempt with a switch system congestion cause code value. By doing so, Cisco CallManager can protect itself from high CPU usage or crashing the system. If the timer has the value of 0, Cisco CallManager does not throttle new call attempts. The default value of 30 seconds allows Cisco CallManager to have reasonable time to clean up the buffered messages and, at the same time, to allow the new legitimate call attempt to complete as soon as possible. Valid values specify 0 (disable throttling), 10, 20, 30, 40, 50, and 60.

This is a required field.

Default: 30.

The description above is very self explanatory.



PS: please remember to rate posts!


Re: What is call throttling ?

the concept of call throttling is that a system can 'limit' or 'throttle' the amount of calls it processes at a given time.

usually this is to prevent an overload of the system that is doing the call processing.

in other cases, it can be used to limit the customers amount of calls.

i have yet to find any official definition of 'call throttling' but it is a way to dictate how much can or can't be done at any given time.


a customer makes 10,000 calls per day and you as the provider notice this takes up quite a bit of your resources that need to be shared by all other customers.

if you limited the customer to only 1,000 calls per day, you have in essence 'throttled' their calling capacity.

Hall of Fame Super Red

Re: What is call throttling ?

Hi Sandy,

Just wanted to add to the answers from Sankar and Greg (surely two of the best!!);

CallProcess View

CallProcess view displays information about call activity, gateway activity, trunk activity, and SDL queues. The information displayed under Gateway Activity assists you in evaluating the capacity of the PRI usage and assists you in troubleshooting issues with PRI when the channel is activated but not operating.

Selecting SDL Queue from the CallProcess view displays the number of signals and requests in various SDL queues and the number of processed requests. To understand the importance of the SDL queue parameters, you need to know about the call-throttling feature in CallManager. Call-throttling protects CallManager from high CPU usage caused either by a burst of call attempts that exceeds the threshold that the CallManager can support or by a call routing loop. In addition, a faulty hardware or faulty firmware could send too many events to CallManager, forcing CallManager to process all the faulty requests, thereby reducing the CPU cycles available for other devices.

To protect from such events, CallManager places the incoming messages and signals into four different SDL priority queues:

High priority

Normal priority

Low priority

Lowest priority

CallManager processes messages in the high-priority queue before messages in the normal-priority queue, processes messages in the normal-priority queue before messages in the low-priority queue, and so forth. The types of messages that go into the normal-priority queue are the initial call setup messages. Whenever CallManager receives a new message request, it calculates the expected delay to complete the request. The expected delay depends on the number of messages in the queue and the number of messages processed.

The higher the number of signals waiting in the queue, the higher the delay is. If the delay is more than the configured threshold values (configured via the Cisco CallManager (CCM) Call Throttling service parameters), the call-throttling feature denies the incoming requests.

Do not change the default values of the parameters in CallManager Service ParametersCCM Call Throttling unless directed to do by Cisco support personnel.

Entering 0 for System Throttle Sample Size disables the call-throttling feature. The call-throttling mechanism applies to incoming calls from phones, Cisco IOS gateways, MGCP gateways, and MGCP back-haul PRI gateways. Whenever the call-throttling feature is called for, the IP phone devices making calls display the message "Too Much Traffic Try Again Later."

From this doc;

Hope this helps!


New Member

Re: What is call throttling ?

Wow! thanks for the responses, now I understand!

Thank you.


Re: What is call throttling ?

an extra 5 points for rob....

you've clearly done an outstanding job this month and have earned it!

Hall of Fame Super Red

Re: What is call throttling ?

Thanks Greg! It means alot to get a nice nod from one of your favourite NetPros. You know how highly I think of the people on these forums.

Take care,


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