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comparison NANP-specifics @ 0:6da76097c86e

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initial import from old themwi-system-sw repository
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 25 Dec 2023 07:41:31 +0000
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1 North American Numbering Plan (NANP) specifics in ThemWi system sw
2 ==================================================================
3
4 Themyscira Wireless system software, as currently written, is strongly tied to
5 NANP. More precisely, it thoroughly assumes that all local phone numbers are
6 in +1 country code and follow NANP rules, and that all E.164 telephone numbers
7 in country codes other than +1 are foreign - numbers which we can call and
8 receive calls from, but which can never be local to us.
9
10 Why does our software need to have these assumptions baked into it, why can't
11 it be country-agnostic? The present article answers this question, and this
12 long answer needs to be thoroughly understood before there can be any meaningful
13 discussion of how the software could possibly be adapted to other countries and
14 their respective telephone numbering plans.
15
16 In principle, there exists a standardized dialing format on all GSM phones that
17 remains the same no matter which country you happen to be in. If you always
18 enter phone numbers (dialing, SMS manual entry, phone book entries) in full
19 international format beginning with the '+' symbol (press and hold the '0'
20 button before the number in most phone UIs), the phone handset firmware will
21 capture the number with TON=1, NPI=1 attributes (signifying international number
22 format in GSM call control and SMS protocols) and transmit it as such to the
23 GSM network. Every properly designed GSM network, upon seeing a number with
24 these TON=1, NPI=1 attributes, must address the call or message to the country
25 indicated by the country code at the beginning of the full E.164 number, whether
26 that country is your local one or some other. Thus if everyone were to always
27 use only full E.164 numbers in full international format, network software could
28 hypothetically be written in a country-agnostic way, treating full E.164 phone
29 numbers as opaque strings without parsing. However, there are two practical
30 problems with such country-agnostic approach:
31
32 1) Most human users of mobile phones dial local phone numbers (within their own
33 country) in a way that follows local dialing conventions, rather than in
34 international format beginning with '+' and their own country code. For
35 example, in USA a given 10-digit NANP phone number can be dialed as just the
36 10 digits NPANXXXXXX, as 11-digit 1NPANXXXXXX, or as full international
37 +1NPANXXXXXX - and a proper cellphone network MUST accept all 3 formats as
38 equivalent.
39
40 2) As explained further in this article, a network implementation must be able
41 to look at a telephone number and immediately tell if that number is locally
42 owned ("one of ours") or belongs somewhere outside of the local GSM network.
43 Practical implementation of this distinction requires a database of locally
44 owned phone numbers, and the implementation of that database in turn becomes
45 much easier when the local numbering plan is known and fixed.
46
47 Supported dialing formats in ThemWi
48 ===================================
49
50 When themwi-mncc processes a mobile-originated (MO) call from a GSM subscriber,
51 it supports dialing the following classes of numbers:
52
53 * NANP numbers in any of the 3 standard dialing formats;
54
55 * International numbers in any country - an international number beginning
56 with +1 is enforced to be valid NANP, but E.164 numbers in all other country
57 codes are accepted as-is;
58
59 * Local 4-digit numbers described in Local-short-numbers article;
60
61 * Whatever special numbers are configured in themwi-sip-out, such as 511 and
62 911.
63
64 Only NANP numbers and specially configured 4-digit numbers (see
65 Local-short-numbers) can be local - all E.164 numbers in non-NANP country codes
66 are sent to the outbound call gateway, and all other unrecognized number formats
67 are likewise sent to themwi-sip-out so that the latter process can catch and map
68 special numbers like 511, 911 etc.
69
70 If a dialed number is recognized as NANP, themwi-mncc looks in the database of
71 locally owned numbers to see if the dialed number is one of ours - and the
72 outcome of this look-up determines if the call is handled locally or sent to
73 the outside world via themwi-sip-out.
74
75 No 7-digit dialing support
76 ==========================
77
78 In the olden days of land lines, most localities in USA supported 7-digit
79 dialing: to call Jenny, you would merely dial her local 7-digit number 867-5309,
80 without needing to dial the local area code; full 10-digit numbers (or 11 digits
81 with leading '1') had to be dialed only when calling someone in a different
82 area code from your own. However, this 7-digit dialing has now been disabled
83 even for land lines in most localities, including the locality where ThemWi
84 currently operates: per official rules, 7-digit dialing gets disabled (full
85 10-digit numbers become mandatory) whenever an area code overlay is implemented,
86 such as overlay of 760 and 442 area codes in our locality.
87
88 In the case of mobile phones, 7-digit dialing never made much sense to begin
89 with: if you dial only 7 digits, should the implicit area code be taken from
90 your own number, or should it be the area code of the locality you happen to be
91 traveling through at the moment? The latter option is impossible in the case
92 of localities with two or more overlaid NPA codes, and it appears that official
93 rules once again call for simply disabling 7-digit dialing.
94
95 Based on these considerations, ThemWi system sw was written from the outset to
96 not support 7-digit dialing - it is no longer relevant in the current state of
97 telecom culture in USA. We do, however. provide optional support for
98 abbreviated 4-digit local numbers - see Local-short-numbers article.
99
100 Database of locally owned numbers
101 =================================
102
103 The telecom culture in USA features full number portability - end users can take
104 their phone numbers with them anywhere, from one telecom provider to another,
105 and with mobile phones and VoIP services, from one geographic locality to any
106 other, making the entire country effectively "flat" for local/non-local
107 distinction purposes. Therefore, the set of phone numbers "owned" (or rented
108 in reality) by a small network operator such as Themyscira Wireless does not
109 constitute any kind of clean-cut digit range partition in the numbering plan -
110 instead we can have a small set of locally owned numbers (say, on the order of
111 5 to 20 individual numbers), and each of these locally owned numbers can fall
112 anywhere in the whole nationwide 10-digit numbering plan. So how can we tell,
113 by looking at an arbitrary NANP number, whether it is "one of ours" or not?
114
115 The implemented solution is an explicitly maintained database of locally owned
116 phone numbers, described in detail in Number-database article. The format of
117 this database (the way numbers are entered, the way the input format is parsed,
118 and the compiled binary format used for fast look-ups) is specific to NANP -
119 only NANP numbers can be local in the present design.
120
121 Porting to other national telephone numbering plans
122 ===================================================
123
124 If someone wishes to port ThemWi system sw for use in other countries with
125 respective local phone numbers, the following aspects will need to be changed:
126
127 * Based on the structure of your country's national numbering plan, you will
128 need to come up with an appropriate local number database format for your
129 country, or if the range of numbers belonging to your GSM network forms a
130 clean-cut digit range partition, implement that scheme instead.
131
132 * You will need to modify MO call handling to recognize your country code
133 (rather than +1) as the one calling for parsing and closer scrutiny of the
134 dialed number, determining if it is local or not.
135
136 * Handling of non-international dialing formats (numbers dialed without '+')
137 will need to be changed to whatever is appropriate for your country's telecom
138 culture and customs.
139
140 * Handle all secondary fallout (throughout the code base) from the previous
141 essential and necessary changes.
142
143 Using fake NANP numbers
144 =======================
145
146 If someone outside of North America wishes to merely play with ThemWi system sw
147 on a casual basis, without actually interconnecting to your non-USA PSTN with
148 non-NANP real phone numbers, the easiest way to bring the software up is to use
149 fake NANP numbers. There are two types of guaranteed-fake (can't collide with
150 real ones) phone numbers in NANP:
151
152 1) NPA-555-01XX, where NPA is some real area code for some (any) actual locality
153 in USA and XX can be any two digits. This number range is specifically set
154 aside for use in movies etc, with realistic USA settings - the area code can
155 be any real one, but 555-01XX numbers are reserved for fake use in every
156 area code.
157
158 2) Fake area codes of form N9X (290-299, 390-399, ..., 990-999) are also good
159 for guaranteed-fake numbers as the middle digit of NPA is not allowed to be
160 '9' per official NANP rules. This method allows large ranges of fake NANP
161 numbers.
162
163 See Fake-NANP-numbers article for more info.