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Telephony

3 min read

Part of Phase 5: Rebuilding Technology

Telephony takes a working pair of telephones, already built in the Phase 4 communications pages, and scales it into a network: more instruments, an exchange to join them, lines that reach further, several conversations on one pair, and finally data on a voice line. The parent article is Telecommunications, which stays in its original folder.

Telephone Systems, Microphone, and Earpiece/Speaker cover the instrument. A voice needs only 300-3,400 Hz carried faithfully, a carbon microphone controls current supplied by a battery rather than generating one, and an earpiece is the same transduction run backwards, with a magnet that stops it doubling the pitch.

Switching Networks, Manual Exchange, and Automatic Switching cover joining callers. A hundred telephones wired pair to pair need 4,950 pairs, against 100 lines through an exchange, and the exchange is a social institution before it is a machine: a manual operator hears who calls whom and when, and an automatic switch ends that listening ear while moving power to whoever maintains the machine and keeps the directory.

Long-Distance, Wire Line Systems, and Carrier Systems cover going further. A plain loop of 0.5 mm copper wire stops near 8 km on its 1,500 ohm resistance limit, loading coils offset capacitance but block higher frequencies, and carrier channels put several conversations on one pair at the cost of bandwidth.

Signal Processing, Amplification, and Filtering cover the signal itself. Gains and losses add in decibels, a line repeater must match its gain to the section loss or the line sings, and noise builds up along a chain because a voice cannot be re-formed the way a telegraph pulse can.

Network Architecture, Topology Design, and Reliability Design cover the system. Star, ring and hierarchy trade link count against what one cut takes down, trunk groups are sized from measured traffic, and ten repeaters at 99% each leave a route up only about 90% of the time.

Data Communication, Teleprinter Systems, and Modem Concepts cover bits on wires. A five-bit teleprinter code has 32 symbols, start and stop bits let two clocks stay in step, and a modem turns bits into tones inside the voice band.

The Instrument

The transmitter, the receiver, and the idea that makes them a telephone.

Exchanges

Joining any caller to any other, by a person or by a machine.

Long Lines

Carrying a voice beyond a few kilometres, and sharing a pair between conversations.

Signals

Restoring, cleaning and repositioning a voice signal.

Network Design

Layers, layouts, and keeping a network up when something fails.

Data

Text and bits over telegraph and voice lines.

The building blocks that already exist elsewhere in the corpus.

18 items under this folder.