FreeCalypso > hg > gsm-codec-lib
comparison doc/RTP-analysis @ 136:8eb0e7a39409
doc: document command line utilities
| author | Mychaela Falconia <falcon@freecalypso.org> |
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| date | Sun, 11 Dec 2022 22:20:36 +0000 |
| parents | |
| children | aaa0380f9958 |
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| 135:22601ae99434 | 136:8eb0e7a39409 |
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| 1 The present package includes a utility named rtp-gsmfr-extr; this utility | |
| 2 extracts a single RTP stream in either FR1 or EFR codec format from a pcap file, | |
| 3 presumably captured with tcpdump on an IP network serving either an IP-based BTS | |
| 4 or a gateway from an E1-based BTS to RTP - the intent is to extract a GSM call | |
| 5 uplink that has been rendered into an RTP stream by a BTS. The RTP stream being | |
| 6 extracted must be fully continuous without any gaps, using Themyscira | |
| 7 RTP-BFI-extension BFI marker packets in those 20 ms windows where no good | |
| 8 traffic frame has been received. rtp-gsmfr-extr verifies continuity of the RTP | |
| 9 stream being extracted: any detected discontinuity (either a sequence number | |
| 10 jump indicating packet loss or a timestamp jump indicating an intentional gap | |
| 11 generated at the source) will be reported, and the extraction will stop there. | |
| 12 | |
| 13 To run rtp-gsmfr-extr, you need to have a pcap file (obviously), and you need to | |
| 14 identify the RTP stream to be extracted by either source or destination IP:port. | |
| 15 rtp-gsmfr-extr will look at every UDP packet that matches this src-IP-port or | |
| 16 dest-IP-port filter, and then check for valid RTP, verify the expected increment | |
| 17 in sequence and timestamp numbers, check for either FR1 or EFR payload (or a | |
| 18 Themyscira BFI marker for FR & EFR), and finally write the extracted frame | |
| 19 stream to a gsmx file. This gsmx output can then be analyzed with gsmrec-dump, | |
| 20 or decoded to playable WAV with gsmfr-decode or gsmefr-decode. |