Andrew Mercer
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From the 1960s to the 1990s the phone network quietly turned into a digital computer network. By the end, the only analog part left in most places was the copper pair running to each home: the last mile.

Digitizing voice: PCM and T1

Pulse-code modulation (PCM) turns an analog voice signal into numbers:

  • Sample the signal 8,000 times per second. That is twice the roughly 4 kHz voice band, as the Nyquist theorem requires.
  • Store each sample as an 8-bit value, using a logarithmic scale (μ-law in North America, A-law elsewhere) that devotes more resolution to quiet sounds.
  • 8,000 × 8 = 64 kbps per voice channel. This rate, called DS0, is the basic unit of the digital phone network.

In 1962 the Bell System introduced the T1 carrier, which interleaves 24 DS0 channels (time-division multiplexing) onto two copper pairs at 1.544 Mbps. Digital signals can be regenerated perfectly at each repeater instead of picking up more noise with distance, so call quality improved dramatically. T1 lines later became a common way for businesses to connect to the internet.

The 64 kbps DS0 number is also the reason dial-up modems topped out at 56k (see Dial-up modems).

Touch-Tone (DTMF)

In 1963 the Bell System introduced Touch-Tone dialing, known technically as dual-tone multi-frequency (DTMF) signalling. Each key sends two tones at once, one for its row and one for its column:

1209 Hz 1336 Hz 1477 Hz 1633 Hz
697 Hz 1 2 3 A
770 Hz 4 5 6 B
852 Hz 7 8 9 C
941 Hz * 0 # D

The A–D column was used mainly on military and network equipment. Ordinary phones left it out.

DTMF had two big advantages over pulse dialing:

  1. Speed: a tone pair takes a fraction of a second, while a rotary "0" takes a full second.
  2. It travels through a connected call. Pulses only reach the local switch. Tones are audio, so they pass through the whole call path. That is what made automated phone menus, voicemail, and phone banking possible.

Phone companies, Bell Canada included, charged a monthly fee for Touch-Tone service for decades. Some customers kept pulse-dialing phones, or set pushbutton phones to "pulse" mode, to avoid paying it. Bell Canada removed the Touch-Tone fee in 2015.

Electronic and digital switches

  • 1965: 1ESS (Bell System). This was the first stored-program control switch: call handling was done by software running on a computer, while the voice path itself was still analog. New features like call waiting and call forwarding could be added with software instead of rewiring.
  • 1976: 4ESS, a digital toll (long-distance) switch.
  • 1979: DMS-100 (Northern Telecom, now Nortel). This was a fully digital local switch, developed in Canada with Bell-Northern Research. It sold well throughout North America and around the world, and became one of the most widely deployed central office switches ever made. Nortel went bankrupt in 2009. Much of the switching equipment being retired today comes from that generation.
  • 1982: 5ESS (AT&T), the DMS-100's main U.S. competitor.

Phreaking and the move to out-of-band signalling

Long-distance trunks originally signalled in band, using tones on the same audio path as the voice. A 2600 Hz tone told a trunk that it was idle and ready for new routing instructions. A toy whistle that came free in boxes of Cap'n Crunch cereal happened to produce exactly that frequency.

Phone phreaks used whistles and homemade blue boxes, which generated the trunk-control tones, to take control of trunks and make free long-distance calls. Steve Wozniak and Steve Jobs built and sold blue boxes before founding Apple.

The permanent fix was to move signalling out of band, onto a separate data network that subscribers could not reach. Signalling System No. 7 (SS7), rolled out through the 1980s, still handles call setup, caller ID, number portability, and SMS routing on the traditional network. The idea of separating the control plane from the data plane appears all over modern networking.

Deregulation

For most of the 20th century the phone company owned everything, including the phone in your home, which was rented.

  • 1968 (U.S.): the Carterfone decision allowed customers to attach third-party equipment to the network if it did no harm. That opened the market for answering machines, fax machines, and modems.
  • 1984 (U.S.): the AT&T monopoly was broken up into a long-distance company and seven regional "Baby Bells."
  • Canada: the CRTC gradually allowed customer-owned equipment, then opened long distance to competition in 1992 and local service in 1997.

Sources and further reading