- 1942 - SIGSALY system is used for secure voice communications, introducing PCM and spectrum spreading
- 1972 - ITU-T G.711 audio companding standard
- 1977 - RFC 741, the Network Voice Protocol
- 1990 - RFC 1183 introduces the ISDN record
- Linear Predictive (LPC)
- Continuously Variable Slope Data Modulation (CVSDM)
- Pulse Code Modulation (PCM) (used in G.711 and many others)
- Companding mitigates the detrimental effects of a channel with limited dynamic range
- ITU-T V-Series recommendations
- Group 3 FAX -- ITU-T T.4 + ITU-T T.30 (interoperable with Group 2)
- Group 4 FAX -- ISDN-oriented, ITU-T T.6, T.563, T.503, T.521, T.6, T.62, T.70, T.72, T.411 to T.41
- Group 1 -- ITU-T T.2
- Group 2 -- ITU-T T.3 + ITU-T T.30 (interoperable with Group 3)
Plain ol' Telephone Service. At the user level, analog telephones connect to telco Network Interfaces (NIs) and thus the telephone exchanges making up the Public Switched Telephone Network, most commonly via twisted pair cable, this most commonly terminated via Registered Jacks 11, 14 and 25 (handsets and the like use 4P4C terminators, with no Registered Jack identifier). Connection of an analog telephone to a digital network is performed via an Analog Telephony Adapter (ATA), implementing one or more Foreign Exchange Stations (FXS). Devices on the PSTN use E.163/E.164 addresses, also known as '+1 telephone numbers'; these can further reduce to, for instance, [NANP Numbers] (3-3-4-style North American numbers). Unification of the E.163/E.164 and IPv4/IPv6 address spaces is attempted via tElephone NUmber Mapping (ENUM) and NAPTR records.
The PSTN is implemented atop carrier systems such as T-Carrier (North America, Japan, Korea) and E-Carrier (most everywhere else). These carrier systems specify the digital transmission hierarchy in terms of circuits and the multiplexing strategies used to aggregate and disaggregate circuits.
Between the customer and the telco lies the Point of Demarcation (aka MPOE, Minimum Point Of Entry). The customer owns wiring and appliances behind the MPOE; telcos (ILECs) and common carriers (CLECs) own everything above (switches/NIs are housed in CO (when a CLEC rents space from the ILEC, this is a POP)). In the United States, demarcation points are wiring junctions produced under the regulating auspices of FCC 47 C.F.R. Part 68. The customer may freely implement either or both of:
- Extension telephones -- multiple phones wired to the same physical circuit and numbers
- Telephone extensions -- internal lines using a PBX or key system
- Circuits to the telco are "trunk lines", internal circuits are "extension lines"
- Require a Direct Dial Central Office (DDCO) prefix to select an external line
A "line" is the combination of physical circuitry and a number by which equipment might be addressed and billed.
- dial tone, a continuous tone having frequencies of 350 and 440Hz at a level of −13dBm
- ringback tone, frequencies of 440 and 480Hz at a level of −19dBm and a cadence of 2 seconds ON and 4 seconds OFF
- busy tone, frequencies of 480 and 620Hz at a level of −24dBm and a cadence of half a second ON and half a second OFF
- reorder tone (also called "fast busy tone"), a busy tone with a cadence of 0.25 of a second ON and 0.25 of a second OFF
- RFC 1183, "New DNS RR Definitions", defines the ISDN record:
3.2. The ISDN RR The ISDN RR is defined with mnemonic ISDN and type code 20 (decimal). An ISDN (Integrated Service Digital Network) number is simply a telephone number. The intent of the members of the CCITT is to upgrade all telephone and data network service to a common service. The numbering plan (E.163/E.164) is the same as the familiar international plan for POTS (an un-official acronym, meaning Plain Old Telephone Service). In E.166, CCITT says "An E.163/E.164 telephony subscriber may become an ISDN subscriber without a number change." ISDN has the following format: <owner> <ttl> <class> ISDN <ISDN-address> <sa> The <ISDN-address> field is required; <sa> is optional. <ISDN-address> identifies the ISDN number of <owner> and DDI (Direct Dial In) if any, as defined by E.164  and E.163 , the ISDN and PSTN (Public Switched Telephone Network) numbering plan. E.163 defines the country codes, and E.164 the form of the addresses. Its format in master files is a <character-string> syntactically identical to that used in TXT and HINFO. <sa> specifies the subaddress (SA). The format of <sa> in master files is a <character-string> syntactically identical to that used in TXT and HINFO. The format of ISDN is class insensitive. ISDN RRs cause no additional section processing. The <ISDN-address> is a string of characters, normally decimal digits, beginning with the E.163 country code and ending with the DDI if any. Note that ISDN, in Q.931, permits any IA5 character in the general case. The <sa> is a string of hexadecimal digits. For digits 0-9, the concrete encoding in the Q.931 call setup information element is identical to BCD. For example: Relay.Prime.COM. IN ISDN 150862028003217 sh.Prime.COM. IN ISDN 150862028003217 004 (Note: "1" is the country code for the North American Integrated Numbering Area, i.e., the system of "area codes" familiar to people in those countries.) The RR data is the ASCII representation of the digits. It is encoded as one or two <character-string>s, i.e., count followed by characters. CCITT recommendation E.166  defines prefix escape codes for the representation of ISDN (E.163/E.164) addresses in X.121, and PSDN (X.121) addresses in E.164. It specifies that the exact codes are a "national matter", i.e., different on different networks. A host connected to the ISDN may be able to use both the X25 and ISDN addresses, with the local prefix added.
- Station == Phone
- Switch == Exchange
- Network Interface == Terminal Attachment == MPOE
- Local Loop == Subscriber Line == Last Mile (wrt PSTN)
- E.164 specifications at the International Telecommunications Union (ITU), aka "Numbering Plan for the ISDN Era"
- Federal Standard 1037C Telecommunications: Glossary of Telecommunication Terms (superseded by 2001's ANSI T1.523-2001)
- A good glossary of Registered Jacks 11 through 48
- Tone Frequencies from tech-faq.com
- NANPA, the North American Numbering Plan Administration