1. Troubleshooting Echo in VoIP Network Deployments
Brad Steinka
Director – IP Technology
Telica,inc.

2. Agenda Introduction Troubleshooting echo problems
Sources of echo
Echo cancellation basics
Troubleshooting echo problems
EC instrumentation
Diagnostics and statistics
Customer and Lab experience

3. Introduction

4. Sources of Echo: Hybrid Echo
Four-wire
Trunk
Two-Wire
Subscriber
Line
Two-Wire
Subscriber
Line
PBX
PBX
S1’s voice Tx Rx Rx
PSTN Rx Tx Tx S1
Echo of 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.

5. Sources of Echo: Acoustic Echo
earpiece
Poor isolation between the earpiece and the microphone can result in acoustic echo
echo
echo
microphone

6. Network Topologies IP Phone User hears Echo Hybrid Echo
Caused by Hybrid echo in
PSTN network IP
Network
PSTN
IP Phone MG
VoIP
Access
Networks
Voice GW
PSTN Phone
Internet Access
Device IP
Network
PSTN MG
IAD
Voice GW
PSTN Phone
PSTN Phone
VoIP Packet Tandem Network
Hybrid
Echo IP
Network MG MG
PSTN
PSTN
PSTN Phone
PSTN Phone

7. Round Trip Delay PSTN Four-wire Trunk Two-Wire Subscriber Line
S1’s voice Tx Rx
PBX S1
PSTN S2 Rx 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, 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.

8. VoIP impact on Echo VOIP PSTN PSTN Echo sourcesMG 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.

9. Removing Echo Near-end Speech from PSTN phone MG Echo source PSTN EC
Far-end
speech
VOIP EC
PSTN
Near-end
speech plus
residual echo
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.

10. EC Operation Rin Rout Inbound speech X(t) PSTN Tail Circuit
Echo estimation
Function
VoIP
Hybrid
e(t)
PSTN
Y(t)
NLP
Sub()
Returned echo
Sout
Sin
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.

11. ECAN Terminology Rin Rout Input speech PSTN Tail Circuit
Echo estimation
Function
Hybrid
EPD
ERL
VoIP
e(t)
PSTN
NLP
Sub()
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 re-converge on the new echo path delay.

12. ECAN Terminology Rin Rout Input speech PSTN Tail Circuit
Echo estimation
Function
Hybrid
VoIP
ACOM
e(t)
PSTN
NLP
Sub()
Returned echo
Sout
Sin
ERLE
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.

13. Troubleshooting echo problems

14. Identify the problem 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

15. Echo Canceller Instrumentation
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
PSTN
Tail
Circuit
Echo estimation
Function
ERL
Hybrid
EPD
VoIP
e(t)
PSTN
NLP
Sub()
Returned echo
Sout
Sin
ERLE

16. Echo Canceller Parameters
Echo Tail Length
PSTN
Rin
Rout
Output Gain
Echo estimation
Function
ERL
Hybrid
VoIP
e(t)
Input Gain
PSTN
NLP
Sub()
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)

17. Measuring Echo Return Loss (ERL)MG
PSTN
Rin
Rout EC
Test
Tone
1004 hz
@ -10dB
ERL
Hybrid
VoIP
IP Phone
PSTN
Sout
Sin
Over an established phone call, a test tone is injected at a deterministic level, ex dB
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.

18. Customer and Lab Experience

19. No Echo Cancellation EPD PSTN IP Hybrid Network Voice GW
Phone
VoIP
Phone
PSTN
EPD IP
Network
Hybrid MG
Voice GW
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.

20. No Echo Cancellation: Low ERL
T1 interface
(4-wire)
Analog
(2-wire)
VoIP
Phone
PSTN
ERL T1
Channel
Bank IP
Network MG
Voice GW
PSTN
Phone
Echo Source
Problem Reported
Customer reported hearing strong and continuous echo
Analysis
Injected 1004 hz test -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.

21. Mid-Call Echo – EPD Changes
T1 interface
(4-wire)
Analog
(2-wire)
VoIP
Phone
PSTN
EPD T1
Channel
Bank IP
Network MG
Voice GW
PSTN
Phone
Echo Source
LAB Experience
Random echo during the call.
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.

22. Mid-Call Echo – IAD Impact
PSTN
Phone
Etherreal
PSTN IP
Network
IAD MG
Voice GW
PSTN
Phone
Echo Source
Problem Reported
Customer reported random echo during the call.
Analysis
Captured voice conversation using Etherreal.
Analyzed Rin signal from IAD, severe clipping noted by cool Edit, 0dB transmit levels.
Rin 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).

23. Summary
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.

24. Thank You Brad Steinka Director, IP Technology
Tel: