DIAL-UP MODEMS

Traditional telephone lines can carry frequencies between 300 and 3300 Hz, giving them a bandwidth of 3000 Hz. All this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility.

Topics discussed in this section:

• Modem Standards

Traditional Telephone line bandwidth

Modern phone line has higher bandwidth

Modem
The term modem is a composite word that refers to the two functional entities that make up the device:

1. A signal modulator and
2. A signal demodulator.

A modulator creates a bandpass analog signal from binary data. A demodulator recovers the binary data from the modulated signal.

Modulation/Demodulation

Digital Data, Analog Signal: Modulation Techniques

Quadrature Amplitude Modulation (QAM)
QAM used on asymmetric digital subscriber line (ADSL) and some wireless
Combination of ASK and PSK
Logical extension of QPSK
Send two different signals simultaneously on same carrier frequency
Use two copies of carrier, one shifted 90°
Each carrier is PSK modulated
Two independent signals over same medium
Demodulate and combine for original binary output

QPSK Illustration

Modem Standards

The V.32 and V.32 bis constellation and bandwidth

Uploading and downloading in 56K modems