telephone_and_cable.md

Telephone and cable

Telephone network

A local loop is a twisted-pair cable that connects a subscriber to the nearest local central office.

Trunks are transmission media that handle the communication between central offices.

Switches are located in a switching office. A switch connects several local loops or trunks and allows establishing a connection between different subscribers.

Traditional telephone lines carry frequencies between $300$ Hz and $3,300$ Hz. The effective bandwidth of a telephone line being used for data transmission is $2,400$ Hz, covering the range from $600$ Hz to $3,000$ Hz.

Dial-up modems

The term modem is a word that refers to the two entities that make up the devices: a signal modulator and a signal de-modulator.

Entity	Role
Modulator	Creates a bandpass analog signal from binary data.
Demodulator	Recovers the binary data from the modulated signal.

V.32

32-QAM with a baud rate of $2,400$. Because only $4$ bits of each $5$-bit pattern represent data, the resulting data rate is $4.2,400 = 9,600 \sf \ Kbps$.

V.32BIS

128-QAM ($7$ bits).

$$2,400.6 = 14,400 \sf \ bps$$

Digital subscriber line

The modulation technique that has become standard for Asymmetric Digital Subscriber Line (ADSL) is called the discrete multitone technique (DMT). It combines QAM with FDM. An available bandwidth of $1,1$ MHz is divided into $256$ channels.

Type	Channel
Voice	$0$
Idle	$1 \to 5$
Upstream data	$\underbrace{6 \to 30}_{25 \sf \ channels}$
Downstream data	$\underbrace{31 \to 255}_{225 \sf \ channels}$