FreeCalypso > hg > gsm-codec-lib
view doc/RTP-analysis @ 169:0c965c3c5e6e
doc/Utils-overview: new Calypso TCH DL utils
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Sun, 25 Dec 2022 09:53:04 +0000 |
parents | 8eb0e7a39409 |
children | aaa0380f9958 |
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The present package includes a utility named rtp-gsmfr-extr; this utility extracts a single RTP stream in either FR1 or EFR codec format from a pcap file, presumably captured with tcpdump on an IP network serving either an IP-based BTS or a gateway from an E1-based BTS to RTP - the intent is to extract a GSM call uplink that has been rendered into an RTP stream by a BTS. The RTP stream being extracted must be fully continuous without any gaps, using Themyscira RTP-BFI-extension BFI marker packets in those 20 ms windows where no good traffic frame has been received. rtp-gsmfr-extr verifies continuity of the RTP stream being extracted: any detected discontinuity (either a sequence number jump indicating packet loss or a timestamp jump indicating an intentional gap generated at the source) will be reported, and the extraction will stop there. To run rtp-gsmfr-extr, you need to have a pcap file (obviously), and you need to identify the RTP stream to be extracted by either source or destination IP:port. rtp-gsmfr-extr will look at every UDP packet that matches this src-IP-port or dest-IP-port filter, and then check for valid RTP, verify the expected increment in sequence and timestamp numbers, check for either FR1 or EFR payload (or a Themyscira BFI marker for FR & EFR), and finally write the extracted frame stream to a gsmx file. This gsmx output can then be analyzed with gsmrec-dump, or decoded to playable WAV with gsmfr-decode or gsmefr-decode.