FreeCalypso > hg > themwi-system-sw

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mgw: implement GSM to PSTN forwarding
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 26 Sep 2022 20:46:19 -0800
parents 97317ede320a
children b7cd66acb123
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1 This Hg repository contains a work-in-progress named Themyscira Wireless system
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2 software. Themyscira Wireless (ThemWi) is an experimental GSM network operated
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3 by Mother Mychaela of FreeCalypso at a semi-urban/semi-rural location in
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4 Southern California, USA; this GSM network is operated with Osmocom CNI software
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5 components, all running on a single Slackware Linux server. ThemWi system sw
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6 is a suite of daemon processes and command line tools that run on the same
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7 machine as all those Osmocom sw components and provide some additional
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8 functionality that is not provided "out of the box" by Osmocom, most important
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9 of which is outside connectivity to USA PSTN.
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11 We are currently experimenting with using bulkvs.com as our USA PSTN
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12 connectivity provider. Like most low-cost PSTN connectivity providers, they
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13 provide the interface to PSTN in the form of a SIP trunk - while I would
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14 absolutely love to get a traditional TDM trunk instead, with SS7 signaling,
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15 such a toy would be far beyond my budget, hence I have to settle for SIP.
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16 Our current status is:
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17
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18 * We have already obtained a block of USA phone numbers (NANP, chosen numbers
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19 from an exchange area local to us) from BulkVS.
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21 * These BulkVS-sourced real NANP numbers have been entered as MSISDNs into
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22 OsmoHLR records for our test SIMs operating on ThemWi GSM.
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23
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24 * We can successfully dial calls from one ThemWi GSM phone to another, with our
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25 themwi-mncc switch understanding all dialing formats that are considered
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26 standard for cellular phone networks in USA (full international, or 11 digits
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27 starting with '1' but no '+', or 10 digits only), as well as our own non-
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28 standard shorthand dialing method with only 4 digits.
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30 * Whenever someone dials one of our NANP numbers from the outside world, BulkVS
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31 servers send UDP SIP INVITE packets to our server. Our inbound call gateway
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32 process is themwi-sip-in; this daemon process listens on UDP port 5060,
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33 accepts SIP calls from BulkVS (ultimately coming from global worldwide PSTN)
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34 and turns them into GSM MT calls in MNCC format, going through themwi-mncc
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35 and ultimately to OsmoMSC.
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36
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37 The following functionality remains to be implemented:
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38
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39 * As a counterpart to themwi-sip-in, there will be another process named
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40 themwi-sip-out that will serve as a gateway for outbound calls, going from
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41 GSM MO MNCC to outside PSTN via SIP. The outbound SIP call functional part
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42 is already implemented in test prototype form in sip-manual-out.
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44 * themwi-mgw will be our transcoding RTP bridge, speaking GSM codecs (FR and
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45 EFR are currently of most interest) on the side toward Osmocom components and
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46 G.711 (PCMU or PCMA) on the PSTN side. Right now themwi-mgw is a working
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47 skeleton that allocates endpoints with RTP & RTCP UDP port pairs, but doesn't
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48 pass any traffic yet.
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49
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50 Differences from osmo-sip-connector
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51 -----------------------------------
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52
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53 In the Osmocom community, the "standard" (or generally accepted) way to connect
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54 a GSM voice network to the outside world is via osmo-sip-connector, an Osmocom
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55 process that connects to OsmoMSC's MNCC socket on one end and talks SIP on the
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56 other end. Our combination of themwi-mncc, themwi-sip-in and themwi-sip-out
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57 effectively takes the place of osmo-sip-connector. Here are the principal ways
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58 in which our solution differs from osmo-sip-connector:
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59
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60 * o-s-c is designed to connect to a local instance of a SIP PBX such as Asterisk
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61 or FreeSWITCH, as opposed to interfacing directly to an outside SIP trunk
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62 from/to a PSTN-via-SIP connectivity provider. themwi-system-sw is different
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63 in this regard: we do NOT use Asterisk or FreeSWITCH or any other similar
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64 software of "spaceship" complexity, instead our themwi-sip-in and
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65 themwi-sip-out processes interface directly to our PSTN-via-SIP connectivity
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66 provider.
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67
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68 * o-s-c has no internal call switching function for calls from one local GSM
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69 phone to another, instead such switching is punted to the required Asterisk
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70 or FreeSWITCH etc. With o-s-c, the calling phone's MO call is converted to
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71 SIP, then Asterisk or other PBX hairpins it back to o-s-c, and then o-s-c
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72 handles the destination call leg as a separate conversion from SIP back to
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73 GSM MNCC. In our solution such local calls are switched internally inside
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74 themwi-mncc, staying native within GSM MNCC land and never turning into SIP.
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76 * o-s-c is based on Sofia-SIP, which in turn uses glib - a very unpleasant
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77 dependency in this Mother's opinion. In contrast, our implementation of SIP
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78 is 100% from scratch, written in plain C in the traditional Falconian coding
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79 style.
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80
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81 The need for RTP voice transcoding
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82 ----------------------------------
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83
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84 In the context of GSM voice codecs, the term "transcoding" is used in two
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85 significantly different meanings:
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86
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87 * Transcoding from one lossy GSM codec to another effectively constitutes two
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88 lossy speech codecs running in tandem, and is a highly undesirable condition.
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89
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90 * Running a single GSM codec (not two in tandem), decoding from GSM to G.711 in
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91 one direction and encoding from G.711 in the other direction, is a standard
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92 required function for traditional voice gateways between GSM and PSTN.
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93
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94 In themwi-system-sw, we need to do transcoding in the second sense above.
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95 BulkVS SIP call interface to PSTN does not support any of GSM codecs, they only
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96 support G.711 and G.729, and the same situation is expected to hold with other
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97 PSTN-via-SIP connectivity providers. We certainly don't want to use G.729 - we
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98 don't want to run two lossy speech codecs in tandem, first GSM and then G.729 -
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99 hence the only codecs we speak on the PSTN-via-SIP side of our gateway are PCMU
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100 and PCMA. Therefore, we need to perform RTP transcoding in our themwi-mgw,
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101 very similar to a traditional GSM TRAU.
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102
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103 On the GSM side, the two codecs of most interest to us at the present time are
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104 the original FR and EFR - hence they will be the first to be supported. Note
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105 the big difference from other Osmocom-using GSM community networks which
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106 typically prefer or even strictly require AMR instead! Our reasons for focusing
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107 on FR and EFR instead of AMR are:
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108
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109 * Our OsmoBSC time slot configuration is full rate channels only, no half rate
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110 channels. HR channels are needed only for greater capacity of simultaneous
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111 calls, but with the total number of people *on the planet* who actively want
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112 GSM/2G as opposed to LTE or 5G being no more than maybe 10, the thought of
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113 exceeding the limit of 6 simultaneous call legs per cell (meaning 6 separate
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114 GSM phone handsets talking *at the same time*) is preposterous.
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115
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116 * Without any HR channels in OsmoBSC config, AMR means AMR-FR specifically, not
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117 AMR-HR. The highest level of AMR-FR is identical with EFR - thus if we
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118 support EFR, do we really need AMR?
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119
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120 * The whole point of Themyscira Wireless is to provide service to *vintage*
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121 mobile phones. Our current collection of vintage phones includes models that
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122 only support FR1 and EFR (Ericsson I888, Nokia 5190) and Calypso C05 which
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123 supports FR1, EFR and HR1, but not AMR.
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124
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125 * EFR is desirable because it gives better voice quality than FR1, but we must
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126 support FR1 too, so we can serve the very oldest of phones which support only
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127 FR1 and nothing else.
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128
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129 Voice codec restriction (forcing GSM phones to use EFR instead of AMR, or
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130 forcing all the way down to FR1) is done in OsmoBSC config, with codec-list
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131 setting under 'msc 0'.