Subject: Echo Cancellers

From: "Hari Krishna Vuppaladhadiam" <[email-masked]>
Date: Wed, 26 Apr 2000 11:09:47 -0700

Hi,
  I am interested in knowing what kind of echo cancellers do we require in
typical VoIP applications. Is there a requirement for delays greater than
64msecs?

Regards
Hari

---

From: Scott Keagy <[email-masked]>
Date: Wed, 26 Apr 2000 12:48:08 -0700

I think there is a good application for high-delay echo cancellers.
Consider the following scenarios:

	<user A>----<VoIP network>----<PSTN>----<mobile network>---<userB>
or
	<VoIP network>----<PSTN>-------<VoIP network>
or
	<VoIP network>----<any high delay hop-off to PSTN>

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

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

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From: Jon Crowcroft <[email-masked]>
Date: Thu, 27 Apr 2000 08:20:19 +0100

>>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.
1  cisco (128.16.6.150)  1.290 ms  0.822 ms  0.774 ms
 2  cisco-pb (128.40.14.245)  1.037 ms  1.005 ms  0.931 ms
 3  10.0.121.5 (10.0.121.5)  1.091 ms  1.116 ms  0.943 ms
 4  128.40.255.153 (128.40.255.153)  1.011 ms  0.978 ms  0.961 ms
 5  atmr-ulcc.lmn.net.uk (194.83.100.62)  2.442 ms  2.072 ms  2.861 ms
6  146.97.255.65 (146.97.255.65)  2.888 ms  3.083 ms  3.552 ms
 7  us-gw2.ja.net (128.86.1.242)  2.644 ms  4.118 ms  4.282 ms
 8  193.62.157.10 (193.62.157.10)  77.606 ms  78.305 ms  76.007 ms
 9  ny-pop.i2.ja.net (193.62.157.2)  76.505 ms  77.377 ms  77.356 ms
10  clev-nycm.abilene.ucaid.edu (198.32.8.29)  91.581 ms  91.423 ms
91.912 ms
11  ipls-clev.abilene.ucaid.edu (198.32.8.25)  97.521 ms  97.082 ms
97.209 ms
12  kscy-ipls.abilene.ucaid.edu (198.32.8.5)  106.739 ms  106.723 ms
106.687 ms
13  denv-kscy.abilene.ucaid.edu (198.32.8.13)  117.296 ms  117.093 ms
117.063 m
s
14  scrm-denv.abilene.ucaid.edu (198.32.8.1)  139.592 ms  140.627 ms
138.953 ms
15  BERK--abilene.POS.calren2.net (198.32.249.41)  142.507 ms  142.512
ms  142.5
65 ms
16  pos1-0.inr-000-eva.Berkeley.EDU (128.32.0.89)  142.498 ms  142.269
ms  142.5
63 ms
17  pos5-0-0.inr-002-eva.Berkeley.EDU (128.32.0.74)  139.891 ms
139.231 ms  139
.991 ms
18  fast9-0-0.inr-010-cory.Berkeley.EDU (128.32.0.42)  140.512 ms
141.192 ms  1
40.178 ms
19  f1-0.inr-181-soda.Berkeley.EDU (128.32.120.181)  144.808 ms
143.865 ms  144
.345 ms
20  router1.ICSI.Berkeley.EDU (192.150.186.2)  146.302 ms  145.192 ms
146.140 m
s
21  www.aciri.org (192.150.187.11)  146.306 ms  153.327 ms  146.852 ms

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From: Bart Van Doorselaer <[email-masked]>
Date: Thu, 27 Apr 2000 15:52:26 +0200

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

--
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Bart Van Doorselaer                           \                  /
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