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Voice Gateway Call Flow

Hello all,

I've been reading up on call flow through voice gateways as per the URL below.

I'm a little confused as to how the router call flow and the Telephony Interfaces etc work together when dealing with an inbound call from say an FXO port.

Can someone simplify the call flow diagrams for me and explain what SPI's and Telephony interfaces etc are invoked when a router receives a call from the PSTN on an FXO port and then routes it via a VOIP dial-peer to the CUCM? An exact breakdown of all the steps taken by the vocie gateway would be great?

http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a0080094045.shtml#intro

Many thanks

BSOC

2 REPLIES

Re: Voice Gateway Call Flow

The simples way to explain this is, a call comes into the voice GWs FXO or PRI (Telephony SPI) port. The voice GW has a pots dial-peer for incoming calls, which will send the call to an ephone (if you have CME) or an outbound dial-peer. The outbound dial-peer would be a voip dial-peer, which would send the call to an IP address (CUCMs IP address, usually) this is considered a VoIP SPI. CUCM would then route the call to a directory number or translation pattern etc.

Carrier--->VGW--->CUCM--->Phone

HTH

Regards,

Yosh

HTH Regards, Yosh
New Member

Voice Gateway Call Flow

Hi Yosh.

Thanks for the info. Can I take you a bit deeper into this?

Looking at the URL above and more specifically the 'Telephony Interface Architecture'. Is this Architecture used every time an analogue call comes into an FXO port (Telephony SPI)?

For example does the CCAPI speak with the VTSP and the Voice Processor Module and finally the DSP's in order to take the incoming analogue call on the Telephony SPI and convert it into digital packets (coding/packetisation) as well as establish call legs before the CCAPI forwards the call onto the next step ie dial-peer selection and then the VOIP SPI?

There are lots of debug commands around ie debug CCAPI, debug vtsp, debug vpm etc so it looks as though this Telephony process is used with incoming analogue calls?

Regards

BSOC

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