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  • 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




Modem/Digital FAX

  • 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

Analog FAX

  • 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

Key Systems

A "line" is the combination of physical circuitry and a number by which equipment might be addressed and billed.


Signaling within the PSTN is accomplished via the use of tones. Within the North American Numbering Plan (NANP), these are specified by the Precise Tone Plan:

  • 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

Address Resolution

ISDN record

  • 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

   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 [8] and E.163 [7], 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

   CCITT recommendation E.166 [9] 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)

See also