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VoIP White Paper

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Revised: February 8, 2007

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echo. However, the delays inherent in network echo are typically problematic, even for regular phone

networks, especially with calls that cover long distances. The phone companies have to do something about

this – so they have devices built into long-distance phone networks called Network Echo Cancellers (NEC) to

remove this echo. Now it is generally safe to assume that network echo is not a particular concern for long-

distance calls, even with VoIP systems attached to the PSTN.

5.2 The need for Line Echo Cancellers in VoIP

As we stated above, the PSTN is not concerned with line echo, since it will sound like side-tone. However, if

we attach a LAN VoIP system to a PSTN gateway device (like Allworx), line echo becomes a specific concern

in the system – because the hybrid echo coming back from the line is now delayed by tens of milliseconds in

the IP network and will no longer be acceptable to the VoIP phone station user. VoIP gateway systems employ

a device in their line interfaces called a Line Echo Canceller (LEC) that can cancel up to approximately 16 or

32ms of echo resulting from the hybrid installed on the CO’s local phone line.

5.3 If VoIP systems have an LEC and the PSTN has NECs, why do we still hear

echo at times?

This is a complicated question with several different answers:

• An echo canceller is a very sophisticated device that automatically attempts to dynamically detect, adapt

to, and remove all echo on the fly, while still providing true full-duplex speech performance. Neither LECs

nor NECs are perfect devices – and depending on the design trade-offs of a given implementation, they

will exhibit certain strengths and weaknesses in some operating environments.

• The phone company NECs never perfectly converge down to zero residual echo. When a VoIP system is

introduced at one end of the connection, the increased delay may make the existing residual echo more

perceptible. As described previously, this added delay gives the perception that the echo is worse, even

though the magnitude of the echo signal is actually the same. In a given call, depending on the exact

level of residual echo, this may or may not end up being objectionable to the VoIP system user.

• Regional intra-LATA calls can be very problematic relative to network echo. Because latency in the

network is not significant, the phone company doesn’t usually bother to deploy the relatively expensive

NECs for intra-LATA calls. The delay is relatively short, so the echo is not usually objectionable when

using an ordinary analog phone at each end. However, add a VoIP system to one end and the network

echo can be a real problem. This most often occurs when placing short-haul calls between competitive

local or regional companies, and the called party has a particularly high level of far-end echo coming

back. The inherent latency of the echo falls into the hole between the LECs ability to combat the echo

and the lack of an NEC in the phone network. To be clear, that echo was always there – it just took the

VoIP system to actually hear it.

5.4 Shouldn’t a VoIP gateway then have both an LEC and an NEC?

Deploying both LECs and NECs in VoIP gateways or PBXes has some advantages. In particular, it can help

with regional calling area calls (intra-LATA calls), which are typically the most problematic for VoIP. However,

intra-LATA calls are a small percentage of most users’ calls, and having the NEC operating for other types of

calls – the ones that already sounded good - presents some problems. The fundamental concern is having two

different NECs operating on the same call: the NEC in the VoIP gateway and the one on the phone network. It

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