(Page 1 of 1 in this chapter)

Appendix G

Echo Cancellation on QX Boards

Introduction

This appendix discusses echo cancellation using QX boards, including default settings for configurations using and not using the analog port.

Hybrid Echo in the Public or Private Network

In the Public Switching Telephonic Network (PSTN) the transit network is built with using four wires. Two wires are used to transmit voice in one direction, and the two others wires are used for the opposite direction. On the contrary the subscriber network is built with using only two wires to transmit voice in the both direction in a full-duplex connection to be cost effective. Figure 30. PSTN Transit Network
Because the length of the subscriber's line varies from 0 to 9 km, the impedance of the line never matches the reference impedance of the hybrid. This non-balanced impedance generates echo on the hybrid.
When the telephone set A is connected to the telephone set B through the telephone network, telephone A will receive a near-end echo from hybrid A and an far-end echo from hybrid B.
If the length of the subscriber A line is up to 9 km, delay will be less than 1 ms. Because the hybrid adds some distortion and multiple reflection, in general the width of the echo is up to 8 ms.
The delay of the far-end echo will depend on the call type (for example, local call, national call, international call). If the network is a full four-wire network only the far-end hybrid will generate the echo. The delay of this echo depends on the group delay of all devices used by the network.
If the network is still using two-wire devices to carry communications, additional echo will be generated.
Because the echo cancellation is using a delay line to store the transmitted signal as a reference, the echo cancellation should be implemented, at the most, near the source of the echo.
Therefore, we identify two types of echo: Near-end echo - 0 to 8 ms Far-end echo - up to 20 ms for local or regional calls - up to 40 ms for national calls - up to 60 ms for international calls - up to 140 ms for submarine transmissions - up to 520 ms for one satellite transmissions

Echo Canceller in the QX 2000

QX 2000 with Analog Ports

If we are using a QX 2000 board (or any analog board), the analog trunk interface of the board adds a local echo superposed to the near-end echo in the same lobe. Figure 31. QX Board Using the Analog Trunk Interface
With this configuration you must set the pre-delay to 0 ms so that the analysis window can match to the local echo and the near-end echo.
If you are using a PBX for a private call, the far-end echo is probably superposed with the local echo and the near-end echo with a delay of less than 20 ms.
If you are connected to the public network, the delay of the far-end echo depends on the equipment used by the operator to make the connection. The connection path could vary at each call. With a 20 ms window, the QX 2000 is able to cancel echo for local and regional calls.
For national and international calls this window's length could be sufficient if the operator network is recent. If not the delay could exceed 20 ms and the QX 2000 is not able to cancel the echo. In general, most of the calls can be managed with a 20 ms window length.
The recommended value is the default setting of the QX 2000 board:
ADI.START.echocancel.mode=1 (Use internal defaults for filter length and pre-delay.)

QX 2000 without Analog Ports

If you are not using the analog port of the QX 2000, but an other interface board through the MVIP bus or H100, you may have to pass the cross time of the board with tuning the pre-delay of the echo canceller. Figure 32. QX Board Not Using the Analog Ports
If the interface board is not an analog port (e.g., E1/T1) you will not find any local echo or near-end echo. You can tune the pre-delay to integrate the cross time of all numeric equipment so that the boundary analysis window could exactly catch the echo of the first hybrid on the network.
The recommended values are the default settings of the QX 2000 board:
ADI.START.echocancel.mode=2 (Use specified value)
ADI.START.echocancel.predelay=xx (Ouput sample delay)
ADI.START.echocancel.filterlength=20 (Length of the analysis window)
The value of the pre-delay depends on the crossing time of the port (e.g., E1/T1) you are using.

(Page 1 of 1 in this chapter)

tech_support@nmss.com