How Analog Phones are working in Cisco VOIP Infra?

[techToddler]

Hello Friends,

I had a question a while which requires your help in understanding the concept.

PSTN ----GW----CUCM----IP PHONE

In the above,both incoming and outgoing calls are converted to VOIP as per the process which I understood. But,

PSTN -----GW----CUCM----VG224----Analog Phone.

How this is working? And

Analog Phone -----VG224----CUCM ------GW----PSTN ---(Outgoing call) -> How the conversion happens.

Please help.

Regards

Sanjay. 

[Rajan]

Hi Sanjay,

Your understanding is right. In the below call flow, its the voice gateway which converts the PSTN signaling to VOIP and communicate with CUCM and vice versa.

PSTN ----GW----CUCM----IP PHONE

Similarly in the second call flow, its the VG224 (analog gateway) which does this. Lets say you have configured VG224 with SCCP, it converts the TDM signaling from analog phone and speaks in SCCP (voip) signaling with CUCM.

PSTN -----GW----CUCM----VG224----Analog Phone

The definition for a voice gateway mentions this as well.

Router PSTN connectivity is generically referred to as voice gatewayfunctionality, offering a gateway for voice over IP (VoIP) calls to, and from, traditional analog or digital PSTN or private branch exchange (PBX) calls. So its the voice gateway which makes the calls between a TDM and VOIP world possible.

HTH

Rajan

[Chris Deren]

+5 for Rajan for correct explanation.

In addition the VG224 does not have to be controlled via SCCP from CUCM, it supports SIP, MGCP, H323 as well. And depending on types of devices and features you require connected to it, i.e. fax vs. analog phone with supplemetary features you select appropriate protocol.

[techToddler]

Thank you Rajan and Chris,

I got to understand what you are saying but the Voice gateway which is connected to PSTN uses DSP to do all conversion and transcoding or whatever. 

Do we have any mechanism like this in VG224 or how that happens?

For example,

Fax Machine ---> VG224 --->CUCM----SIP Trunk-----> ITSP.

How things are working here in this direction. 

Analog to digital - where is it happening and vice versa....

Regards

Sanjay. 

[Vivek Batra]

Whenever TDM to IP conversion is required, you eventually need DSP resources. In your case, that is what needed by VG224 to convert and give connectivity between analog ports viz FXS and IP.

-Vivek

[techToddler]

Vivek - the reason why I asked that question is when I happen to check a VG224 in my enviornment, I saw the the below,

voice-card 0

this one just stays and no function is listed below or in this mode.

When I give sh inventory the below only appears,

NAME: "VG224 chassis", DESCR: "VG224 chassis"
PID: VG224 , VID: V02, SN: XXXXXXXXX

But when I do this in Cisco 2921 it gives me more info about the PVDM.

So the conversion is happening without this or I don't know how it is happening...

Regards

Sanjay.

[Vivek Batra · Accepted solution]

You get detail info in ISR because PVDMs are basically external interface to router and can be used for many purpose like mtp, xcode, conference etc. Whereas in VG, you don't connect PVDMs and that comes by default only for one purpose viz TDM connectivity. Those DSPs doesn't need to be configured the way PVDMs are configured in ISR.

-Vivek

[techToddler]

Oh that's awesome Vivek. Thanks for making me clear. Appreciate. 

5+ for you all time.

Regards

Sanjay.