FastStart and payload size

Shan Lu slu at
Thu Mar 16 15:59:25 EST 2000

Add my two cents to _maximum_ vs _exact_ framing.

I think it is important that an endpoint take the number of frames in
OLC to mean the MAXIMUM frames that it expects to receive. In other
words, an EP should be able to receive packets that contain anywhere
from 1 to MAX. number of voice frames. The reason is that if I am doing
multiframes per packet and I am using the built-in VAD/CNG of G.723.1 or
G.729, I have no control over where the transition from voice to silence
occurs. The number of voice frames per packet will inevitably change. To
keep the number of frames per packet constant, I'll have to wait till
the next talk spurt, which is not much an option. This actually brings
up another constraint on the packet, i.e., all frames in a packet should
represent consecutive voice frames.


Shan Lu

NexTone Communications

--- Paul Long <plong at SMITHMICRO.COM> wrote:
> Francois,
> (Worries me, too. I don't like Fast Connect.
> "Half-baked" comes to mind.
> Apologies to whoever the authors are...)
> Exactly, the caller has to make a series of educated
> guesses regarding
> outgoing caps, based on the implementor's experience
> and common sense.
> (Wait a minute. Let me get on my high horse... okay,
> I'm on... :-)
> These fields have always indicated _maximum_
> framing. For example, if I
> indicate that I can receive packets containing as
> many as 20 frames per
> packet (for G.711, a frame happens to be 1ms in
> duration), that means you
> can send me packets containing 1, 20, 10, 2, 15, 8,
> etc., frames. Mix and
> match. Put a different number of frames in each
> successive packet if you
> want to. It doesn't matter. The _only_ constraint is
> that no packet may
> contain more than 20 frames. It has never meant that
> you can only send me 20
> frames per packet for each and every packet, which
> for G.711 would be 20ms
> packets all the time. However, several vendors
> apparently have
> "extra-normative" technical constraints that they
> must deal with regarding
> packet size, i.e., DSPs that require being fed
> chunks of fixed-size audio,
> e.g., 20ms packets. But there is no way in H.245 and
> therefore H.323 to
> indicate a fixed-size packet. These vendors should
> either "fix" their DSPs
> or perform re-framing between transport and decoder.
> In our software-only
> implementation, we have a FIFO into which we shove
> all incoming audio
> frames. Once frames are installed in the FIFO, it
> doesn't matter where the
> packet boundary was. The decoder just processes
> frames in the order they
> were received. I'm sure that there are really good
> technical reasons why
> some implementations require fixed-size audio
> packets. Based on our perhaps
> too simplistic implementation, however, this has
> always puzzled me.
> Paul Long
> Smith Micro Software, Inc.
> -----Original Message-----
> From: Francois Audet
> [mailto:audet at NORTELNETWORKS.COM]
> Sent: Friday, March 10, 2000 1:55 PM
> Subject: Re: FastStart and payload size
> Hum... That kind of worries me.
> It means that the burden is on the originating side
> to "guess" what
> the terminator is likely to support.
> I also agree that practically speaking,
> implementations normally
> support multiples of 10 ms as the payload size (with
> the notable
> exeption of Netmeeting).
> However, technically speaking, if you advertize 30
> ms as the maximum
> payload size you can support, it implies that in
> order to comply with
> the H.323 specification, you would support any value
> between 0 ms
> (whatever it means) and 30 ms. I don't think many
> implementations could
> do this. But that is a separate issue.
> In any case, if we all understand that the
> originator is responsible
> for accurately describing a decent subset of the
> capabilities it can
> support, we probably should describe this in the
> spec because I
> am convinced that most people will simply put the
> maximum payload
> size they can support...

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