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Interactions of Voice Band Data Modems with Network Echo Cancellers. Bob Reeves BT Issue 1 21 April 2010. Overview. This presentation will cover problems encountered with two different types of low speed data modems and their interaction with network echo cancellers:

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Interactions of voice band data modems with network echo cancellers

Interactions of Voice Band Data Modems with Network Echo Cancellers

Bob Reeves

BT

Issue 1

21 April 2010


Overview
Overview Cancellers

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


V 23 telemetry

Network Cancellers

(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


V 23 half duplex modulation used for telemetry
V.23 half duplex modulation used for telemetry Cancellers

  • 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


Example of an unsuccessful v 23 telemetry call
Example of an unsuccessful V.23 telemetry call Cancellers

  • 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


Breakdown of an echo canceller
Breakdown of an Echo Canceller Cancellers

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


What can we do about it
What can we do about it? Cancellers

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





V 22 bis atms epos terminals

Network Cancellers

(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


V 22 bis full duplex modulation atm
V.22 bis full duplex modulation (ATM) Cancellers

  • 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


Example of an unsuccessful v 22 bis atm call
Example of an unsuccessful V.22 bis ATM call Cancellers

  • 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


Failure mechanism
Failure Mechanism Cancellers

  • 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)


What can we do about it1
What can we do about it? Cancellers

  • 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


Summary conclusions
Summary & Conclusions Cancellers

  • 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


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