Packet Capture PCM Extraction

Extracting PCM from a packet capture is easily accomplished through software programs such as Wireshark. The main criterion for extracting PCM from a packet capture is that the RTP packets contain the G.711 ^-law or G.711 a-law codec. In addition, passthrough redundancy for these codecs should be disabled before making the packet capture because the RFC 2198 packet format usually causes a poor PCM extraction.

The RTP analysis feature within Wireshark can be used to extract the PCM from a G.711 call. See Figure 12-9 and its surrounding text for more information about the RTP analysis feature. Figure 12-9 displays an RTP analysis screen shot, and on this screen is a Save Payload button. Clicking this button brings up another window that allows you to save the G.711 payload as a raw G.711 file or a regular audio file with a .au file extension. Most audio analysis programs play either format, but most of the time the raw G.711 format is preferable.

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