Re: [rtcweb] On ptimes and maxptimes

[Magnus Westerlund <[e-mail]>]

Hi,

(As individual)

On 2014-03-05 17:26, Justin Uberti wrote:
> From the session this morning, I recall this consensus:
> 
>   * We will add recommendations on the frame sizes to use for PCMU/A to
>     the rtcweb audio codec I-D (20, 30, 60 ms frames).

The above applies to sending payloads

Yes, but that was given that a receiver will be capable of receiving any
valid number of frames, or samples within a reasonable packetization,
and I think that should be 120 ms.

>   * We will consider adding an a=maxptime attribute that represents the
>     "minimum maxptime" of all the offered codecs. That is, if both Opus
>     (maxptime of 120ms) and PCMU (maxptime of 60) are offered, a
>     maxptime=60 SHOULD be included.
>       o Since maxptime spans all codecs, this means that some modes
>         (e.g. Opus 120ms) will be unavailable, unless only Opus is offered.

I think you base the assumption on maxptime based on the sending
recommendations, not what is capable of receiving. If one is capable of
receiving 200 ms of a-law and Opus, then you would need to say 120 ms as
Opus will limit you.

>   * We will consider adding an a=ptime attribute, set to the receiver's
>     preferred frame size to receive. Again, this value spans all codecs,
>     so the specified value may not be viable for all codecs (e.g. 30 ms
>     works for PCMU but not for Opus)
> 
> After thinking about this some, I'm not sure the maxptime/ptime values
> will lead to any different behavior than if they were not specified.
> Since there is a complexity cost (especially if the values need to
> change based on which codecs are offered), is there a clear upside here?

The maxptime values may in some cases be limited downwards for
interoperability, for example if one like to gateway AMR into a CS
network then the gateway might indicate a maxptime that is less, more
like 20 or 40 ms.

Please remember that these rules do apply to any codec, not only the MTI
ones.

Cheers

Magnus Westerlund

----------------------------------------------------------------------
Services, Media and Network features, Ericsson Research EAB/TXM
----------------------------------------------------------------------
Ericsson AB                 | Phone  [phone]
Färögatan 6                 | Mobile [phone]
SE-164 80 Stockholm, Sweden | mailto: [e-mail]
----------------------------------------------------------------------