RE: [EFM] Network timing, ATM, ADSL/VDSL and EFM
I agree to what Matt wrote about QoS mechanisms being crucial mostly when
resources get congested. This is why the argument that ATM is inefficient as
it involves the cell tax is not a good one. Over-engineering the link to the
extent that prevents congestion and thus eliminates the need for QoS might
involve a much higher tax.
Moshe
-----Original Message-----
From: Matt Squire [mailto:mattsquire@xxxxxxx]
Sent: Friday, September 28, 2001 3:11 PM
To: Roy Bynum
Cc: stds-802-3-efm@ieee.org
Subject: Re: [EFM] Network timing, ATM, ADSL/VDSL and EFM
I agree as long as I can make a few assumptions...
1) the links aren't heavily loaded, and
2) there aren't a large number of hops (ie Internet scale # of hops)
Once the links start getting loaded, your performance becomes less
predictive, packets get dropped, wait longer, etc., and QoS structures
(queueing, policing, shaping, etc) become important.
As you get into larger networks, there are more merge/congestion points,
and you require some way to control the merged traffic streams else the
jitter can grow unacceptable even at lower congestion levels.
But big fat pipes certainly solve a lot of problems...
- Matt
Roy Bynum wrote:
>
> Matt,
>
> Part of the work that I have been doing in a lab for the last two years to
> characterize Ethernet transmission functionalities. One of the surprising
> things was how stable Ethernet inherently is. Non-routing, switched GbE
> Ethernet will often have a worst case data latency variance of a few
> microseconds. The end to end latency is more a factor of speed of light
> latency than system latency. This is several factors better than what is
> normally expected of standard IP infrastructure. Other than something to
> handle "circuit" overloading, Ethernet does not normally need QOS.
>
> Thank you,
> Roy Bynum
>
> At 02:38 PM 9/28/01 -0400, Matt Squire wrote:
>
> >Not being a billionaire, I tend to pay attention to taxes.
> >
> >There are numerous examples of providing both voice and video over
> >Ethernet networks. The examples require prioritation and QoS at layers
> >2 and/or 3. But its more than possible, its already working.
> >
> >The QoS is not free, and the jitter and wander over variable size packet
> >networks are generally higher, but the end result is cheaper and the
> >quality of the service can be engineered to be acceptable.
> >
> >- Matt
> >