Interactions of Voice Band Data Modems with Network Echo Cancellers

1. Interactions of Voice Band Data Modems with Network Echo Cancellers Bob Reeves BT Issue 1 21 April 2010

2. 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

3. Network (PSTN) V.23 Telemetry 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 Out-station Modem Out-station Modem EC Out-station Modem EC EC Out-station Modem In-station Modem EC Out-station Modem Out-station Modem Remote out-stations often on long lines Out-station Modem

4. V.23 half duplex modulation used for telemetry • 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 In-station Request Top Trace recorded at In-station (2-wire) Network Delay Out-station Response Bottom Trace recorded at Out-station (2-wire) Turnaround = 15 ms

5. Example of an unsuccessful V.23 telemetry call • 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 In-station Request Truncation of signal by NLP 30-40 ms Top Trace recorded at In-station (2-wire) Network Delay Out-station Response Bottom Trace recorded at Out-station (2-wire) Turnaround = 15 ms

6. Breakdown of an Echo Canceller 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. Echo Canceller Adaptive Filter Comfort Noise Generator inserts noise in place of background noise when NLP is active Out-station modem side CNG In-station modem side Echo - NLP HP Filter + Non-Linear Processor removes any “residual” echo after cancellation. Acts as a suppressor. High Pass Filter removes any DC component from the echo path

7. 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

8. G.168 Figure 39 – NLP operating regions

9. G.168 Annex B – Reference NLP

10. G.168 Annex B – Reference NLP

11. Network (PSTN) V.22 bis ATMs & EPOS Terminals ATM 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 EPOS Terminal EC EPOS Terminal EC EC ATM Central Modem EC EPOS Terminal EPOS Terminal EPOS Terminal

12. 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 Captured on 2-wire point at ATM No Answer Tone Central Modem (Answer) S1 Signals ATM Modem (Calling) Unscrambled binary 1s

13. Example of an unsuccessful V.22 bis ATM call • 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 No Answer Tone Captured on 2-wire point at ATM No S1 Signal Central Modem (Answer) ATM Modem (Calling) Unscrambled binary 1s S1 Signal

14. 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)

15. 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

16. 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