Interactions of Voice Band Data Modems with Network Echo Cancellers
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Transcript of Interactions of Voice Band Data Modems with Network Echo Cancellers
Interactions of Voice Band Data Modems with Network Echo CancellersBob Reeves
BT
Issue 1
21 April 2010
© British Telecommunications plc
Overview
This presentation will cover problems encountered with two different types of low speed data modems and their interaction with network echo cancellers:
•V.23 telemetry modems used by the UK Water Industry to monitor lakes, reservoirs and inland waterways
•V.22 bis modems used in Automatic Teller Machines (ATMs) and Electronic Point of Sale (EPOS) terminals
•Both of these problems are caused by the echo canceller’s Non-Linear Processor (NLP)
•Good opportunity to encourage EC designers to follow guidance in ITU-T Recommendations with respect to NLP design
© British Telecommunications plc
V.23 Telemetry
Network(PSTN)
Modem Bank
In-station Modem
EC
Out-station Modem
EC
EC
EC
Out-station Modem
Out-station Modem
Out-station Modem
Out-station Modem
Out-station Modem
Out-station Modem
Used by UK Water Industry to monitor water levels in lakes and reservoirs. Remote out-stations report information to central in-station over dial-up connections
Remote out-stations often on long lines
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V.23 half duplex modulation used for telemetry
In-station Request
Network Delay
Out-stationResponse
Top Trace recorded at In-station (2-wire)
Bottom Trace
recorded at Out-station
(2-wire)
• Half duplex V.23 (FSK) at 1200 bit/s
• In-station modem sends a request to the outstation modem
• Out-station modem responds very quickly (in the order of 15 to 30 ms)
• 2100 Hz answer tone may be present at the start of the call, but plenty of silence to allow NLP to re-enable
• Example V.23 telemetry call
Turnaround = 15 ms
Wave Sound
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Example of an unsuccessful V.23 telemetry call
Network Delay
In-stationRequest
Out-stationResponse
• In-station modem sends a request to the outstation modem
• Out-station modem responds very quickly (in the order of 15 to 30 ms)
• Out-station response is truncated or clipped as it passes through network echo canceller
• Dependent on line length (long lines cause failures)
• Problem isolated to NLP by manually disabling the NLP. This resulted in successful calls
Truncation of signal by NLP 30-40 ms
Turnaround = 15 ms
Top Trace recorded at In-station (2-wire)
Bottom Trace
recorded at Out-station
(2-wire)
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Breakdown of an Echo Canceller
CNGIn-station modem side
Out-station modem side
Adaptive Filter
Echo Canceller
-HP
Filter+NLP
Echo
Comfort Noise Generator inserts noise in place of background noise when NLP is active
Non-Linear Processor removes any “residual” echo after cancellation. Acts as a suppressor.
High Pass Filter removes any DC component from the echo path
Adaptive filter forms model of echo path to “cancel” echo. Note that in this example ONLY the reflection from the out-station side is cancelled.
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What can we do about it?
Network:
• Re-design NLP with faster de-activation time (transitions 2 and 3 in G.168 Figure 39) – preferred longer term solution
• Use separate V.23 detector - turn off NLP before it gets the chance to clip the waveform – acceptable work-around
• Increase line card gain to remote sites where failures occur (although many sites so remote that they are already at their highest gain setting) – not an option in most cases and would result in “special” treatment for particular lines with associated long term overheads
Protocol:
• Add redundancy (null characters) to initial out-station response so that clipping has no effect – not an option in practice since it places the burden on the customer to modify in some cases 1000s of remote units
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G.168 Figure 39 – NLP operating regions
G.168(07)_F 34
N L P in activ e
N L P activ eL eve l o f s ig n a la t send -in p o rt
(
L S in
d B m 0 )
L eve l o f s ig n a l a t receive -in p o rt (d B m 0 )L R i n
W
Z
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G.168 Annex B – Reference NLPTable B.1 – NLP hangover times
Boundary
Initial signal Final signal Recommended
value (ms)
Test No. [ITU-T G.164]
Excursion (see
Figure 39)
Test circuit, Figure:
Oscilloscope trace Send LSin
(dBm0) Receive LRin
(dBm0) Send LSin (dBm0)
Receive LRin (dBm0)
Fixed –25 –10 –25 –30 15-64
Z/W Adaptive
–55 –40 –30
–20 –15 –5
–55 –40 –30
–40 –40 –30
5 Transition 2 14/G.164 Trace 1 and trace 2 of
Figure B.3 ()
Fixed –15 –25 –40 –25 16-120
W/Z Adaptive –40 –40 –25
–50 –30 –15
–55 –55 –40
–50 –30 –15
30-50 6 Transition 4 17/G.164 Trace 1 and trace 2 of
Figure B.2 ()
Table B.2 – NLP operate times
Boundary
Initial signal Final signal Recommended
value (ms)
Test No. [ITU-T G.164]
Excursion (see
Figure 39)
Test circuit, Figure:
Oscilloscope trace Send LSin
(dBm0) Receive LRin
(dBm0) Send LSin
(dBm0) Receive LRin
(dBm0)
Fixed –25 –30 –25 –10 16-120
W/Z Adaptive
–55 –40 –30
–40 –40 –30
–55 –40 –30
–20 –15 –5
15-75 4 Transition 1 14/G.164 Trace 2 of
Figure B.3 ()
Fixed –40 –25 –15 –25 1
Z/W Adaptive –55 –55 –40
–50 –30 –15
–40 –40 –25
–50 –30 –15
5 6 Transition 3 17/G.164 Trace 2 of
Figure B.2 ()
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G.168 Annex B – Reference NLP
Figure B.2 – Traces for NLP operate and hangover time, LRin constant
Figure B.3 – Traces for NLP operate and hangover times, LSin constant
Sout
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V.22 bis ATMs & EPOS Terminals
Network(PSTN)
Modem Bank
Central Modem
EC
EC
EC
EC
Used by cash machines (ATMs) and for Point of Sale transactions in shops, restaurants, etc. The ATM or EPOS terminal uses V.22 bis to complete a transaction over dial-up connections
HELP
ALPHA
SHIFT
ENTERRUN
DG ER FI
AJ BK CL
7M 8N 9O
DG DG DG
DG T 3U
0V .WX Y Z
TAB
% UTILIZATION
HUB/MAU NIC
2BNC4Mb/s
HELP
ALPHA
SHIFT
ENTERRUN
DG ER FI
AJ BK CL
7M 8N 9O
DG DG DG
DG T 3U
0V .WX Y Z
TAB
% UTILIZATION
HUB/MAU NIC
2BNC4Mb/s
HELP
ALPHA
SHIFT
ENTERRUN
DG ER FI
AJ BK CL
7M 8N 9O
DG DG DG
DG T 3U
0V .WX Y Z
TAB
% UTILIZATION
HUB/MAU NIC
2BNC4Mb/s
HELP
ALPHA
SHIFT
ENTERRUN
DG ER FI
AJ BK CL
7M 8N 9O
DG DG DG
DG T 3U
0V .WX Y Z
TAB
% UTILIZATION
HUB/MAU NIC
2BNC4Mb/s
HELP
ALPHA
SHIFT
ENTERRUN
DG ER FI
AJ BK CL
7M 8N 9O
DG DG DG
DG T 3U
0V .WX Y Z
TAB
% UTILIZATION
HUB/MAU NIC
2BNC4Mb/s
EPOSTerminal
EPOSTerminal
EPOSTerminal
EPOSTerminal
EPOSTerminal
ATM
ATM
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V.22 bis full duplex modulation (ATM)
• Full duplex V.22 bis (QAM) at 2400 bit/s
• Analysis performed in frequency domain
• 2100 Hz answer tone not always present at the start of the call
• Example V.22 bis ATM call
Central Modem
(Answer)
ATMModem (Calling)
Captured on 2-wire point at ATM
Unscrambledbinary 1s
S1 Signals
No Answer Tone
Wave Sound
© British Telecommunications plc
Example of an unsuccessful V.22 bis ATM call
Central Modem
(Answer)
ATMModem (Calling)
• 2100 Hz answer not present at the start of the call
• S1 signal is not recognised by Central Modem which tries to connect in V.22?
• ATM cannot fall back to V.22 so call fails
• Dependent on line length
• Problem isolated to NLP by manually disabling the NLP. This resulted in successful calls
• Example unsuccessful V.22 bis ATM call
Captured on 2-wire point at ATM
Unscrambledbinary 1s
S1 Signal
No Answer Tone
No S1 Signal
Wave Sound
© British Telecommunications plc
Failure Mechanism
• Difficult to establish the exact failure mechanism here
• We know that turning the NLP off in the echo canceller facing the ATM or EPOS terminal cures the problem
• Dependent on line length (long lines cause failures)
• Truncation of S1 signal from ATM or EPOS terminal by NLP the suspected failure mechanism (but not proven)
© British Telecommunications plc
What can we do about it?
• Re-design NLP with faster de-activation time (transitions 2 and 3 in G.168 Figure 39) (assuming failures are due to NLP truncation) – preferred longer term solution
• Detect V.22 bis modulation (unscrambled binary 1s) and turn off NLP – acceptable work-around
• Increase line card gain to remote sites where failures occur – would result in “special” treatment for particular lines with associated long term overheads
• Works if Answer Tone present, since echo cancellers turn off NLP on detection of 2100 Hz
© British Telecommunications plc
Summary & Conclusions
• Problems encountered with two different types of low speed data modems and their interaction with network echo cancellers:
– V.23 telemetry modems used by the UK Water Industry to monitor lakes, reservoirs and inland waterways
– V.22 bis modems used in Automatic Teller Machines (ATMs) and Electronic Point of Sale (EPOS) terminals
• Both of these problems are caused by the echo canceller’s Non-Linear Processor (NLP)
• Some echo cancellers do not exhibit the problem so it is possible to design an NLP that does not interfere with these modems
• Echo Canceller designers are encouraged to follow the guidance in ITU-T Recommendations for NLP design, especially G.168 Annex B and the target timings given in Tables B1 and B2
© British Telecommunications plc