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TW-TS-005 reader: fix maximum line length bug TW-TS-005 section 4.1 states: The maximum allowed length of each line is 80 characters, not including the OS-specific newline encoding. The implementation of this line length limit in the TW-TS-005 hex file reader function in the present suite was wrong, such that lines of the full maximum length could not be read. Fix it. Note that this bug affects comment lines too, not just actual RTP payloads. Neither Annex A nor Annex B features an RTP payload format that goes to the maximum of 40 bytes, but if a comment line goes to the maximum allowed length of 80 characters not including the terminating newline, the bug will be triggered, necessitating the present fix.
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 25 Feb 2025 07:49:28 +0000
parents cd1f0fa936cc
children
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1 Performance of libgsmefr and libtwamr, compared to competition
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2 ==============================================================
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3
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4 Both libgsmefr and libtwamr are based on reference C code from ETSI/3GPP:
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5 libgsmefr is a librified version of GSM 06.53 reference code, and libtwamr is a
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6 librified version of TS 26.073 reference code. Both of those reference sources
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7 were officially presented as simulations, not as production code for running on
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8 general-purpose x86 servers implementing transcoding MGW functionality, and the
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9 code is extremely inefficient.
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10
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11 The problem of poor performance (taking too much CPU percentage per transcoded
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12 call) would not be so acute if Themyscira GSM codec libraries existed in a
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13 vacuum without competition - but we do have a competitor in the form of
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14 libopencore-amrnb, part of the slightly larger opencore-amr package. Just like
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15 libtwamr, that library is a derivative work based on the original AMR C code
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16 from 3GPP, but by a more circuitous route: first PacketVideo created an
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17 implementation of AMR for Android based on 3GPP reference code, then the
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18 creators of opencore-amr took PV's Android implementation (named OpenCORE) and
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19 ported it from Android to standard Unix/Linux userspace.
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20
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21 Aside from its peculiar genealogy, there are practical problems with
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22 libopencore-amrnb that made it unattractive for Themyscira Wireless, and thus
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23 created the impetus for producing the present alternative:
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24
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25 * That library implements only AMR and not EFR, whereas for ThemWi good support
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26 for EFR is a higher priority than good (or any) support for AMR.
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27
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28 * The idea of reshuffling bits in the manner of our gsm-amr2efr and gsm-efr2amr
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29 hack-utilities (see AMR-EFR-conversion article) in order to implement "poor
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30 man's EFR" via libopencore-amrnb was considered, but then rejected on
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31 philosophical and aesthetic grounds: it feels philosophically wrong to
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32 reshuffle bits in the application wrapper only to have internal code within
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33 the library shuffle them back into the natural order of codec parameters.
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34
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35 * Like any AMR-and-not-EFR library, libopencore-amrnb cannot grok EFR SID
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36 frames. Therefore, an implementation of "poor man's EFR" using that library
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37 would have to operate with DTXu disabled, needlessly burning battery capacity
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38 in the MS.
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39
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40 * Even for "pure" AMR and not EFR, libopencore-amrnb exhibits one serious
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41 defect: it omits the in-band homing feature in both the encoder and the
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42 decoder, even though this feature is mandatory per 3GPP specs. (It is not
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43 clear if this removal of homing functions was the work of PacketVideo in the
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44 world of Android, or if it happened at the time of subsequent opencore-amr
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45 port to standard Unix/Linux userspace.) Furthermore, because the interface
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46 wrapper module of libopencore-amrnb does not expose any state reset functions,
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47 it is impossible to implement the homing feature externally to the library.
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48 This lack of homing makes it impossible to test libopencore-amrnb against the
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49 full set of 3GPP test sequences for AMR.
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50
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51 In the present time, as of this release of Themyscira gsm-codec-lib, libgsmefr
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52 is intended for production use while libtwamr is intended for lab use only,
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53 matching test sequences and modeling bit-exact operation of other people's
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54 network elements in reverse engineering workflows. Based on this difference in
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55 applicability to production, more work has been put into improving performance
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56 of libgsmefr compared to libtwamr. Libgsmefr version 1.1.0 exhibits
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57 approximately 2x performance improvement over the original version, but it is
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58 still significantly slower than libopencore-amrnb.
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59
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60 The following performance numbers were obtained on Mother Mychaela's development
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61 laptop, Intel Core2 Duo P8600 CPU @ 2.40 GHz, running 32-bit version of
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62 Slackware, i.e., running only i686 code and not x86_64 despite the CPU having
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63 the 'lm' feature. All tests involve encoding TEST4.INP sequence from GSM 06.54
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64 set (301 frames in total, 6.020 s audio duration), then decoding the output of
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65 the encoder run.
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66
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67 Implementation Encoding time Decoding time
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68 ---------------------------------------------
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69 libgsmefr 183 ms 23 ms
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70 libtwamr 359 ms 52 ms
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71 opencore-amr 53 ms 13 ms
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72 libgsmfr2 (FR) 13 ms 6 ms
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73
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74 FRv1 encoding and decoding of the same TEST4.INP was included in the test in
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75 order to convince ourselves that FRv1 is not in any need of performance
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76 optimization. In a network deployment that generally prefers EFR over FRv1,
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77 almost all calls between a local GSM MS and the outside world (PSTN) will
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78 involve the EFR transcoder rather than FRv1. If on rare occasion the network
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79 selects FRv1 instead of EFR because the subscriber is using a super-old
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80 non-EFR-capable handset (or has artificially restricted the speech version list
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81 to FRv1 only), there will be some CPU cycles saved "for free" by using the much
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82 less computationally expensive FRv1 codec - but CPU capacity planning for the
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83 transcoding MGW has to be based on EFR performance numbers rather than FRv1.
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84
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85 In terms of EFR or MR122 encoder and decoder CPU cycle demand, libtwamr is
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86 currently the worst performer; the first version of libgsmefr exhibited the
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87 same poor performance. The current version of libgsmefr exhibits somewhat
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88 better performance because most saturation arithmetic "basic ops" have been
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89 inlined (eliminating the use of a function call for every elementary operation),
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90 and the implementation code of those newly-inlined functions has been
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91 streamlined for performance, as much as possible with immediately obvious
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92 programming methods. The result is about 2x improvement over the original -
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93 but sadly, still significantly slower than libopencore-amrnb on the same
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94 machine with the same 32-bit execution mode and ABI.
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95
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96 Given that we (Themyscira Wireless) have a ton of other work tasks in our queue
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97 besides the present problem of performance-optimizing libgsmefr to match
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98 opencore-amr, this problem is currently shelved. We are releasing our libraries
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99 as they currently stand, with the ~2x improvement in libgsmefr over the original
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100 but still not matching opencore-amr, and leaving the problem of further
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101 performance improvement to be addressed later, after other work of higher
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102 priority.