Re: [codec] #16: Multicast?

["Raymond (Juin-Hwey) Chen" <[e-mail]>]

Hi Stephen,

Sorry, I was too busy to respond yesterday.

You wrote:
> Generally the need to buffer the current frame is treated as part of the 
> algorithmic delay.  At least I believe that is what the ITU-T does.
> So maybe we need a list of what all these components are?  

[Raymond]: Sure, my previous analysis was an attempt to do just that, but perhaps my list was not complete enough.

> I'd suggest keeping the gateway out of it for the first pass.

[Raymond]: May I ask why? 

> I've worked with Gateways\MCUs where the packet size had to be increased 
> because packet loading in the product became too high.  Also, if you 
> have QOS features enabled in many routers, the routers themselves have 
> to start using a "software path", which creates a similar throughput 
> problem in the routers.  Too many packets per second can overwhelm these 
> devices, creating both capacity issues and excessive queuing delays. 
 
[Raymond]: OK, now I see what you meant when you said "it is totally possible that reducing the frame size might actually increase the latency". This is probably more likely to happen many years ago but less of a problem now, as I was told by networking guys that nowadays networking gears can handle 5 ms packets without problems.  In fact, the VoIP gateway I talked about, which has a 12 to 17 ms codec-dependent one-way delay for a 5 ms frame/packet size, was done 6 or 7 years ago.  Even back then the gateway can handle it without problems. 

> I don't think the group has an agreed-upon model which names these 
> components consistently, and describes are appropriately in-scope and 
> which are out-of-scope.  Perhaps that is one reason why Koen is saying 
> multiplier the number is 1x.  

> Also, there are real-world negative consequences to higher packet rates, 
> and we have not yet considered them. 

[Raymond]: Yes, higher packet rates means higher packet header overhead bit-rates, more burden on networking gears in I/O bandwidth and throughput, etc.  However, that's the price to pay if we need low latency, just like if we want to avoid all these, the price to pay is higher latency.  It's all a matter of trade-off and the best choice depends on the application at hand.  

In Section 2 of Jean-Marc's Internet Draft draft-ietf-codec-requirements-00, 6 specific applications for the IETF codec were listed.  Fully 5 of these 6 applications list less than 10 ms of codec delay as either a requirement or a desirable feature. (The only exception is point-to-point calls.)  The only way to achieve this less than 10 ms codec delay is with a codec frame size of less than 10 ms, and to get the kind of low latency that these 5 applications desire, each packet had better contain only one codec frame as payload (rather than multiple frames). 

So, yeah, there is negative consequences of the resulting higher packet rates, but hey, if we want to get low latency as desired or required by these 5 applications, that's the price we will need to be prepared to pay.  There is no free lunch.  If we want to use a 20 ms frame/packet size to avoid those consequences, then we need pay the price of not achieving the low latency that these 5 applications desire or require.

Raymond