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DTMF Digit Collection Failure

Publication Date:  2015-07-14 Views:  576 Downloads:  0
Issue Description
Version: eSpace U1911 V100R001C01SPC300

Network: IP phones are connected to the eSpace U1911, which connects to the PSTN through four AT0 lines.

When an outer-office user dials the VU switchboard through the AT0 trunk, and enters a number as prompted, the system fails to collect digits and continue to play announcements.
Handling Process
Step 1 Dial the VU switchboard as an intra-office user. The call can be connected properly. Therefore, the VU script runs properly.

Step 2 Adjust the impedance from 0 to 15. The fault is rectified on three lines. Digit collection still fails on one line no matter what impedance is set.
  • To adjust the impedance at an FXO port through the web:
             Log in to the web management system of the eSpace U1900 series unified gateway, choose Trunk > Trunk Configuration > AT0, click the blue icon or link representing the peer office, and click Line Parameter Mode in the window that is displayed.
  • To adjust the impedance at an FXO port through the command line:
             Viewing command: show osu information slot slot
             Configuration command: config modify osu slot slot impedance impedance [startportno startportno number number]

The impedance level ranges from 0 to 15. The impedance for each level is as follows:

0: 600 ohms
1: 900 ohms
2: 270 ohms + (750 ohms||150 nanofarads) or 275 ohms + (780 ohms||150 nanofarads)
3: 220 ohms + (820 ohms||120 nanofarads) or 220 ohms + (820 ohms||115 nanofarads)
4: 370 ohms + (620 ohms||310 nanofarads)
5:320 ohms + (1050 ohms||230 nanofarads)
6: 370 ohms + (820 ohms||110 nanofarads)
7: 275 ohms + (780 ohms||115 nanofarads)
8: 120 ohms + (820 ohms||110 nanofarads)
9: 350 ohms + (1000 ohms||210 nanofarads)
10: 200 ohms + (680 ohms||100 nanofarads)
11: 600 ohms + 2.16 microfarads
12: 900 ohms + 1 microfarad
13: 900 ohms + 2.16 microfarads
14: 600 ohms + 1 microfarad
15: Global Impedance

Step 3 Capture packets on the eSpace U1911. Based on analysis, the captured data contains DTMF signals. When the data is played, the DTMF tone can be heard. However, the noise is loud, making DTMF signals difficult to detect.

The following figure shows the normal data, in which DTMF signals are clear. The slight sound in the front section is the echo of VU voice playback. The office has only one level of VU menu. When the first digit is detected, voice playback stops. Therefore, echoes do not exist after the first DTMF digit.

Step 4 According to troubleshooting, the AT0 line and a network cable are connected to the same hub, resulting in high noise and DTMF digit collection failure. Separate the AT0 line from the network cable. The fault is rectified.

Root Cause
Possible Cause:
  1. Incorrect script
  2. Unmatched impedance
  3. Environment factor