Thanks Priscilla for the clear and excellent presentation. The answer to
my question
("Do I fragment large data packets or not?), lies in the implementation of
the dejitter
buffer at the receiving end. You pointed me in the right direction.
dj
Priscilla Oppenheimer wrote:
> At 01:35 PM 4/25/02, dj wrote:
> >on "slower speed" WAN links, does one have to fragment all large data
> >packets if the serialization delay introduced by the WAN link exceeds
> >the VoIP codec packet update interval?
> >
> >As an example, a 1500 byte data packet has roughly about a 23 msec
> >serialization delay thru a 512 kbits/sec WAN link. If my Voice codec is
> >sending VoIP packets at 10 or at 20msec intervals, am I forced to
> >fragment all large data packets over the WAN link?
>
> You should check with Cisco, but I think in this case you are safe not
> fragmenting.
>
> I hate to introduce another variable, but you should consider that the
> recipient has a dejitter buffer. The dejitter buffer is probably big enough
> to handle the 23 millisecond delay caused by a 1500-byte frame, and that
> would be approximately your worst case.
>
> You must make sure that your queuing is set up to always send a voice
> packet when one is available. Your worst case, then, is just a single
> 1500-byte serialization delay. Sure, if your CODEC sends every 10
> milliseconds instead of every 30 milliseconds, more voice packets may get
> queued, but this isn't a big deal if voice packets are always prioritized.
>
> Basically you are combining constant-bit rate voice traffic with
> variable-bit rate data traffic. The voice traffic arrives in a predictable,
> synchronous fashion. The data traffic arrives asynchronously, in a
> unpredictable fashion.
>
> Consider the case where your CODEC sends a voice packet at a constant rate
> of every 23 milliseconds. There could be some unpredictable jitter because
> of the asynchronous nature of data traffic, but because of your queuing,
> even the worst case is not a major problem:
>
> 1) The interface sends a voice packet. Another one won't arrive for 23
> milliseconds.
>
> 2) 22 milliseconds go by. A data packet arrives!
>
> 3) The queuing algorithm allows the data packet to get transmitted, because
> no voice packet has arrived. It will take 23 milliseconds to send the data
> packet.
>
> 4) The voice packet arrives. It must wait.
>
> But that's the worst that can happen. The voice packet is delayed by 22
> milliseconds. The recipient must be able to handle this. The receivers have
> dejitter buffers that are at least this big.
>
> Now let's say that voice packets arrive every 30 milliseconds. 29
> milliseconds go by and a data packet sneaks in. It's still just 23
> milliseconds of delay. So you're cool.
>
> Now let's say that voice packets arrive every 10 milliseconds. The worst
> case would be that 9 milliseconds go by and a data packet sneaks in. It's
> still just 23 milliseconds of delay. But in that time three voice packets
> would have gotten queued up. But they will go out right away after the data
> packet is done because of your queuing algorithm that always prioritizes
> voice, so you're probably still OK.
>
> Now, let's say you don't really have a 512-Kbps link, but a 256-Kbps link,
> in which case the time to output a 1500 byte packet is 46 milliseconds. Now
> you have more of a problem due to the longer time being more noticeable to
> the human ear (depending on the size of the recipient's dejitter buffer)
> and the fact that more voice packets can get queued up during that time.
> Queuing delay isn't nearly as much of an issue as serialization delay, but
> at some point it can become relevant.
>
> ARGH. It's making my brain hurt. I think you better ask Cisco. They have
> brilliant engineers who work this kind of stuff out better than I can. But
> I hope my brainstorming helped some. ;-) And once again, in the particular
> case that you bring up, I think you are OK not to fragment as long as the
> data packets aren't bigger than 1500 bytes.
>
> Priscilla
>
> >What if my Voice codec is sending VoIP packets at 30msec intervals; am I
> >fundamentally OK over a 512 kbits/sec WAN link assuming queuing is set
> >correctly for properly marked qos VoIP packets??
> >
> >Thanks for the quick feedback
> >dj
> ________________________
>
> Priscilla Oppenheimer
> http://www.priscilla.com
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