Subject: Re: Echo Cancellers

From: [email-masked]
Date: Thu, 27 Apr 2000 10:51:56 -0400

Bart:

As far as I know, ETSI TIPHON, VoIP Forum ( IMTC) and the new ITU G.799 require
the use of echo canceller on gateways.  All the gateway vendors that I am aware
of have network echo cancellers in them.  If you sell a gateway without it,
nobody in their right mind should buy it.

In general - I believe that if an application needs a 20 msec echo canceller
they either have a wireless application or something else is wrong.  For
wireless applications, the longest that I have heard as a requirement is 60
msecs.  People are over-spec'ing echo canceller out of ignorance, not out of
real need.

Do you agree?

Kevin,

Bart Van Doorselaer <[email-masked]> on 04/27/2000 09:52:26 AM

To:   Jon Crowcroft <[email-masked]>
cc:   Scott Keagy <[email-masked]>, Hari Krishna Vuppaladhadiam
      <[email-masked]>, [email-masked] (bcc: Kevin Bruemmer/Natural
      MicroSystems/US)
Subject:  Re: Echo Cancellers

Let us not confuse near-end and far-end echo canceling.

Assume a PSTN-GW-IP-GW-PSTN scenario.

If you only provide near-end echo cancelling you only have to take into account
low delays (i.e. mainly the transmission delay in the PSTN between the hybrid
and the gateway - which is about 5 microseconds per km).
In that case, it is of course a prerequisite to provide near-end echo
cancelling on both gateways (and how do you guarantee that?)

If you want to provide far-end echo cancelling as well, the more-than-100-ms
assumption will be realistic indeed.

Bart

Jon Crowcroft wrote:

> In message <[email-masked]>, Scott Keagy typed:
>
>  >>Typical echo cancellers at the edge of the VoIP network (e.g. Cisco
>  >>routers) only provide 32 msec of coverage. If a hop-off tail circuit has
>  >>a round-trip delay that exceeds the coverage of the echo canceller (on
>  >>the outbound edge of the VoIP network), then the additional delay of the
>  >>VoIP network (100 msec or more usually) exacerbates the echo problem.
>
>  >>This is an intractable problem without the use of echo cancellers that
>  >>have longer coverage ability.
>
>  Scott
>
> well a well engineered path from London to San Francisco consistently
> has a delay of around 150ms.... (see below) - i dont see why someone
> cant think a bit about echo cancellors for this.....references would
> be interesting on why it is "intractable" - is it only if the delay is
> also highly variable (the path below has low delay variance - probably
> lower than a lot of paths with lower e2e delays by about an order of
> magnitude)
>
> j.

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---

From: Scott Keagy <[email-masked]>
Date: Thu, 27 Apr 2000 13:13:21 -0700

Distance between New York, NY and :
San Diego, CA = 3914 km          ====> 39.14 msec round trip
San Francisco, CA = 4156 km      ====> 41.56 msec   "     "
Seattle, WA = 3884 km            ====> 38.84 msec   "     "

These times assume fiber spread "as the crow flies"
and no switching/regeneration delays.

An international company with a single U.S. office in one of these
cities (or any city on either coast) will have problems with VoIP
tail-end hop-off to other major population centers in the U.S.

The engineering solution says, "why don't they just centrally locate
their office in Chicago?" Echo cancellers seems like a funny reason to
relocate a business though.

-Scott

-- 
=======================================================
 Scott Keagy, CCIE #3985
 Senior Networking Consultant  
 SK Networks, Inc.         mailto:[email-masked]
=======================================================