Voice Over IP – DTX During Silence Periods Visualized

Voice DTX 2 14 kbyte per seconds one directionWhen I recently made an IMS call from a mobile phone I noticed an interesting output of a throughput graph I was running at the time that shows Discontinuous Transmission (DTX) in action during silence periods.

The screenshot on the left shows two voice calls. Have a look at the throughput bar on the left that stops around halfway through the diagram. Green represents incoming data, blue represents outgoing data. If no DTX had been applied the graph would be level with the same amount of blue and green. The graph, however, is anything but level as during silence periods only little data leaves the device and there is little blue data marked in the graph. In the downlink direction, no DTX is applied as the green area is always pretty high.

To confirm the behavior I made a second call and muted the uplink right from the start. This is shown on the right in the screenshot. The graph there mostly consists of green downlink traffic. About two thirds through that call I unmuted myself. Immediately blue uplink data is shown. After muting myself again only green downlink data can be observed.