CME – CCM Ringback tone generation

Posted: May 6, 2006 by cciestudy in Callmanager Express

When an IP Phone hosted by Callmanager places an outbound call to the PSTN, the ringback tone heard by the caller is usually generated as an audio signal that's passed through end-to-end from the PSTN trunk (in-band tone). This means the audio path from the gateway to the IP Phone is opened and active before the call is connected.

When a call is placed from a Callmanager IP Phone to a CME IP Phone, the ringback tone is provided as an out-of-band H323 indication from CME (via H323 control channel). This means that the CME system signals to the Callmanager, which in turn instructs the Callmanager's IP Phone to generate the ringback tone locally.

To get Callmanager to provide local ringback tone generation for outbound calls from the Callmanager, CME delays negotiation of the media path until the call connects.  Callmanager 3.3 and 4.0 H323 implementations assume that in-band ringing tone is provided by the H323 device on all calls that negotiate the media path (using H.245) before the call is connected. This delayed negotiation can lead to a minor delay in establishing the audio path when the call actually connects. This delay is called voice cut-through delay.

CME only uses this delayed H245 media negotiation on calls that go to or from a Cisco Callmanager. CME can explicitly identify Callmanager calls based on special nonstandard H323 IE messages that Cisco Callmanager attaches to its H323 signaling messages.

  1. Pushkar Bhatkoti says:

    Excellent explanation….
    thanks for sharing …

  2. samir says:

    I am facing a problem that the ip phone registered with callmanager 7.1 is not hearing a call back when calling an ip phone registered with callmanager express.

    could you please tell me the configuration needs to be done to resolve this problem.

  3. RM says:

    I am facing an issue with a ccme connected to NGN network thru an ISDN BRI. When calling an IP phone on the CCME from any PSTN phone I cannot hear any ring back tone.

    Same test was done with the CCME connected to the NGN network thru an ISDN PRI this time, the ring back tone is working.

    Please advise on the above 2 comments as I need a solution for the first one, thanks.

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