Question on Codec Pass-through

[techToddler]

Hello Friends,

Could some one explain me what is called Codec Pass-through is all about. I have tried reading many documents and all it says is about the fax and modem pass-through. But I know this command is inserted under MTP and Transcoder in IOS. Why we need this and What is exactly happening after this command. I don't see any other codec commands under MTP or Transcoder. Appreciate your help.

Thanks.

Sanjay.

[Manish Gogna]

Hi Sanjay,

In the pass-through mode the actual content of the RTP stream is not modified. The SCCP device processes the media stream by using a pure software MTP, regardless of the nature of the stream, which enables video and data streams to be processed in addition to audio streams. When the pass-through mode is set in a transcoding profile, no transcoding is done for the session; the transcoding device performs a pure software MTP function. The pass-through mode can be used for secure Real-Time Transport Protocol (RTP) sessions.

Manish

[techToddler]

Thanks Manish, I have read that in SRND or somewhere. What exactly that I wanted to know is to understand the below configuration.

dspfarm profile 20 mtp
codec pass-through
maximum sessions software 100
associate application SCCP

What is Codec pass-through meant here. What is the codec that will be used. I really appreciate if someone could explain me in plain english. Thanks again. 

Regards

Sanjay

[Kasiraman S]

Hi Sanjay,

Let me try to explain here. Lets say for an example you have configured this dspfarm profile for MTP which might be acting as a RSVP agent which is only needed to reserve the bandwidth but it does not modify the RTP stream in anyway, i.e does not terminate and re-originate the media, where RTP just work in a flow through mode and it does require codec pass-through command.

Another example, we configure codec pass-through for the streams that has some immersive video calls which not only has audio and video but also has some Data Stream, BFCP stream, FECC, where BFCC is not supported for SIP trunks which involves MTP, RSVP, TRP or Transcoder and those streams are passed through without modifying the stream.

Thanks,

Kasi

[techToddler]

Thats sound too technical for me at this stage. Okay I got one point clear on the first paragraph. So no RTP termination when used with codec passthrough. Do you have any example, I mean real time one which I can imagine to understand. 

Thanks 

Sanjay.

[Kasiraman S]

I believe we can consider taking a call which uses RSVP based CAC, which does require MTP for bandwidth reservation but it does not terminate/reoriginate the call.

And codec pass-through is mandatory for such software based MTP which act as a RSVP agent.

At present, I could not create this in a lab environment but I will see if I can try lab it out and get back.

Thanks,

Kasi

[Ayodeji Okanlawon]

+5 Kasiraman,

I believe another example would be for DTMF mismatch..as explained above

[Ayodeji Okanlawon · Accepted solution]

Sanjay,

Another example apart from RSVP/CAC would be this.

DTMF mismatch

When there is a mismatch in DTMF capabilites between two call legs, then CUCM need to insert MTP/transcoder for this ( you can change this behaviour in service parameter though).

When an MTP is imserted all the media between the two call legs is sent to the MTP, hence the reason why its called media termination point. Media is terminated on it. Now in scenarios like this we may not want the MTP/transcoder to be involved in media megotiation ( ie codecs etc), so we put codec pass-through on the it. This informs the MTP not to perform any media negotiation/codec selection and to just pass the codec it receives on one end to the other end..

[Kasiraman S]

+5 Deji. Well explained. I believe your answer explains why only codec pass-through is configured.

[Manish Gogna]

Kasi and Ayodeji,

Thanks for the useful stuff [+5]

Manish

[Ayodeji Okanlawon]

Deleted

[Ayodeji Okanlawon]

+5 Manish.

I think I have answered my own questions..I assume by pure MTP you mean it will  be used for DTMF mismatch and not get involved in media negotiation

[Rolando Valenzuela]

Manish, is right.

In esence, whatever coded you receive, that coded you will uses.

Rolando Valenzuela.

[techToddler]

You guys rock. Now that explained and nailed it. Thank you again...Ayodeji....