Brad Steinka
Director – IP Technology
Telica,inc.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

• Introduction
– Sources of echo
– Echo cancellation basics
• Troubleshooting echo problems
– EC instrumentation
– Diagnostics and statistics
• Customer and Lab experience
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

Two-Wire
Subscriber
Line
Tx
PBX
Four-wire
Trunk
S1’s voice
Rx
Two-Wire
Subscriber
PBX
Line
Rx
PSTN
Rx
Tx Tx
Echo of S1
S1 Hybrid
Transformer
S2
• Hybrid transformers convert four-wire trunk interfaces into two-wire local subscriber lines.
• This conversion process results in a fraction of the four-wire rx signal to be reflected back on the four-wire tx signal causing Hybrid Echo.
• Subscriber S1 hears a delayed and attenuated version of his original voice.
• Output impedance of the Hybrid needs to be balanced with the input impedance of the terminating device. Mismatch in impedance leads to louder echo. Properly balanced, returned echo should be around 20dB.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

Poor isolation between the earpiece and the microphone can result in acoustic echo earpiece echo echo microphone
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

Hybrid
Echo
IP Phone User hears Echo
Caused by Hybrid echo in
PSTN network
IP
Network
IP Phone
VoIP
Access
Networks
PSTN Phone
PSTN Phone
PSTN
PSTN
MG
Voice GW
MG
Voice GW
IP
Network
Internet Access
Device
IAD
PSTN Phone
VoIP Packet Tandem Network
PSTN Phone
Hybrid
Echo
PSTN MG
IP
Network MG PSTN
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com
PSTN Phone

Two-Wire
Subscriber
Line
Tx
Four-wire
Trunk
S1’s voice
Two-Wire
Subscriber
Line
Rx
S1
Rx
PBX PSTN
Tx Tx
Round Trip Delay Echo of S1
Hybrid
Transformer
• Round trip delay is the time it takes a word to go from S1 across the network to the echo source and then back.
• Echo that arrive after short delays, 25-35 ms, is not noticeable and is masked by the phone’s side tone.
• As round-trip delay increases and sufficient echo amplitude exits, echo becomes subjectively more annoying.
S2
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

PSTN
MG
VOIP
MG
PSTN
Long round trip delay , no echo sources
Echo sources
• VOIP round trip network delay is 70+ ms in length depending on codec
(711,723,729) selected, packetization time, and jitter buffer.
• Increased delay causes both subscribers to hear echo introduced in the
PSTN networks.
• Delivering carrier grade voice quality becomes the responsibility of the
Media Gateways and must stop echo from reaching the VOIP network.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

Far-end speech
Near-end speech plus residual echo
VOIP
MG
Echo source
Near-end
Speech from
PSTN phone
PSTN
Phone
PSTN EC
Near-end speech from
PSTN phone
• Echo Canceller (EC) is allocated by the MG to cancel the echo returned from the echo source in the PSTN network.
• Residual echo remaining after echo cancellation should NOT be perceptible to the human ear.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

VoIP
Rin Rout
Inbound speech
Echo estimation
Function
NLP e(t)
Sub()
Sout
X(t)
PSTN
Tail
Circuit
Sin
Y(t)
Returned echo
Hybrid
PSTN
•
EC predicts what the return echo will be once voice signal X(t) enters the PSTN tail circuit and becomes the echo Y(t).
• The Echo prediction is subtracted from the real echo and the resulting signal is ideally zero. EC is continuously adapting its estimate based on error estimate e(t) until it has good estimate for Y(t). This is the convergence time of the EC and is required to be <
200ms.
•
The Non Linear Processor (NLP) replaces any residual echo with comfort noise provided no near-end voice is detected from PSTN phone.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

Rin
Rout
Input speech
VoIP
Echo estimation
Function EPD ERL
PSTN
Tail
Circuit
Hybrid
NLP e(t)
Sub()
PSTN
Returned echo
Sout Sin
• Echo Return Loss (ERL) is level of attenuation provided by the PSTN network and Hybrid. Given an input speech signal at X dB, the returned echo at interface Sin of the echo canceller is (X-ERL) dB.
• Echo Path Delay (EPD) is the delay from the Rout port to the Sin port due to the delays inherent in the echo path transmission facilities through the PSTN tail circuit.
• After EC convergence, changes in EPD require the echo canceller to reconverge on the new echo path delay.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

Rin
Rout
Input speech
VoIP ACOM
Echo estimation
Function
PSTN
Tail
Circuit
Hybrid
NLP e(t)
Sub()
PSTN
Returned echo
Sout
ERLE
Sin
• Echo Return Loss Enhancement (ERLE) refers to the additional echo loss obtained through the operation of the echo canceller . An echo canceller is not a perfect device, and the best it can do is attenuate the level of the returning echo. ERLE is a measure of this echo attenuation through the echo canceller
.
• ACOM is simply the total echo return loss seen across the Rin and Sout terminals of the echo canceller, and is the sum ERL + ERLE + NLP .
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

• Which subscriber on the call hears echo?
• Does the subscriber hear echo all the time? Only to certain phone numbers?
• What type of phone is being used? POTS or VoIP
(SIP/MGCP).
• If POTS, is it connected to an Internet Access Device?
• What number was being dialed (local / long distance)?
• Was the remote party on a speaker phone?
– Maybe acoustic echo, remove the speaker phone
• Whenever someone hears echo, the source of the echo is on the remote end of the call
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

• Per call information from ECs used in analyzing echo problems
– ERL – Echo Return Loss
– ERLE – Echo Return Loss Enhancement
– Echo Path Delay (ms)
– Number of times echo path changed during the life of the call.
– Rout signal level (dB)
– Sin signal level (dB)
Rin Rout
Input speech
VoIP
Echo estimation
Function
NLP e(t)
Sub()
EPD
ERL
PSTN
Tail
Circuit
Hybrid
Returned echo
Sout Sin
ERLE
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com
PSTN

Rin
Echo Tail Length
Rout
PSTN
VoIP
Echo estimation
Function
NLP e(t)
Sub()
Output Gain
Input Gain
ERL
Hybrid
PSTN
Sout Sin
• Output gain control enables the EC to attenuate or boost the signal level
AFTER the echo canceller has ‘seen’ the original output signal.
•
Input Gain control enables the EC to attenuate or boost the signal level
BEFORE the echo canceller ‘sees’ the echo.
• ERL can be enhanced through changes in Output / Input gain control.
• Echo Tail Length (ETL) is the amount of time the Echo Canceller will
‘Remember’ a signal that has been output. ETL must cover the Echo Path
Delay (EPD)
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

MG
PSTN
Rin Rout
IP Phone
VoIP
Test
Tone
1004 hz
@ -10dB
EC
ERL
Hybrid
PSTN
Sout Sin
• Over an established phone call, a test tone is injected at a deterministic level, ex. 1004 hz @ -10dB
• Analyze the EC statistics
– Echo Return Loss (ERL) < 6dB? Insufficient ERL.
– Changes in Echo Path Delay (EPD)? EC is periodically reconverging
– EPD > Echo Tail Length. Insufficient coverage to cancel echo.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

VoIP
Phone
PSTN
Phone
IP
Network
MG
Voice GW
EPD PST
N
Hybrid
• Problem Reported
– Customer reported hearing strong and continuous echo.
• Analysis
– Customer upgraded his service offering to include VoIP access.
– Customer didn’t account for the additional delay introduced by VoIP.
– Local Area trunk groups were not configured for Echo cancellation.
– In the past, no need for echo cancellation coverage for local-local calls. Round-trip delay < 30ms.
• Resolution
– Provisioned Echo Cancellation on local trunk groups for calls originating / destined for
VoIP devices.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

VoIP
Phone
IP
Network
MG
ERL PST
N
T1 interface
(4-wire)
T1
Channel
Bank
Analog
(2-wire)
Voice GW
• Problem Reported
– Customer reported hearing strong and continuous echo
Echo Source
PSTN
Phone
• Analysis
– Injected 1004 hz test tone @ -10db, ERL from channel bank was 3dB.
– EC was unable to differentiate the returned echo from the PSTN near end speech.
– Increasing ERL was required to achieve G.168 minimum of 6dB.
• Resolution
– Increased the output gain on the EC for this subscriber by 4dB to achieve an effective
ERL of 7dB to meet G.168.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

VoIP
Phone
IP
Network
MG
EPD PST
N
T1 interface
(4-wire)
T1
Channel
Bank
Analog
(2-wire)
Voice GW
• LAB Experience
– Random echo during the call.
Echo Source
PSTN
Phone
• Analysis
– Changes in Echo Path Delay was causing the EC to reconverge and as a result echo was leaking to the VoIP subscriber.
– Identified that the T1 interface on the channel bank was set for internal timing and
NOT network / loop timing.
– This caused T1 frame slips which caused the EC to detect changes in the echo path.
• Resolution
– Changed T1 interface timing configuration for network / loop timing.
– All equipment must be timed from a common reference to prevent frame slips / errors otherwise EC will detect changes in echo path delay.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

PSTN
Phone
Etherreal
IAD
IP
Network
MG
PST
N
Voice GW
• Problem Reported
– Customer reported random echo during the call.
Echo Source
PSTN
Phone
• Analysis
– Captured voice conversation using Etherreal.
– Analyzed Rin signal from IAD, severe clipping noted by cool Edit, 0dB transmit levels.
– Rin signal @ 0dB was causing non-linear behavior in the Hybrid.
– Returned echo signal level > PSTN near end speech, so EC was fooled into thinking echo was PSTN speech and didn’t cancel echo.
– IAD was configured for 0db of attenuation to PSTN tx/rx signals
• Resolution
– Added 6dB of attenuation to the PSTN rx signal to the IAD (EC Rin) signal.
– Resulted in an additional 20dB of attenuation of the returned echo (ACOM).
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

• EC vendors must provide necessary instrumentation to assist in troubleshooting.
• MG vendors must make this information available for EC troubleshooting.
• Add EC statistics to call detail records to assist in post mortem analysis of escalated issues.
• Remember that these EC problems already existed prior to introduction of VoIP. They are now noticeable due to increase in network round trip delay.
• When subscribers report Echo problems, majority of the time the problem is at the far end.
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com

Brad Steinka
Director, IP Technology
Email: bsteinka@telica.com
Tel: 508-804-8154
August 3-4, 2004 • San Jose, CA • www.voipdeveloper.com