Telephone Modems

A modem is an abbreviation of "modulator demodulator". It is a network device that modulates digital information or bits into analog signals for transmission at the sending end, and demodulates the analog signals back to bits at the receiving end. Telephone modems enable data communication between two computers over voice-grade telephone lines.

Purpose and Usage

Computers are digital devices that need to communicate over the analog local loops of telephone networks. There is a need to convert digital bits to analog signals for transmission over physical telephone channels, and conversely convert analog signals back to computer-understandable bits. A modem is placed at the interface between the computer and the local telephone loop to perform this conversion.

Modulation Technique

Data communication speed over telephone lines is restricted by limited bandwidth. Trellis Coded Modulation (TCM) is commonly used to enhance transmission speed while reducing errors. TCM combines error correction coding with modulation, allowing more efficient use of available bandwidth and improving signal quality over noisy telephone lines.

Standards for Modems

The ITU-T defines standards and specifications for modems used in data transmission across telephone lines. Key standards include:

• V.32 − Operates at data rates of 4.8 Kbps or 9.6 Kbps using 32 constellation points to transmit 4 data bits per symbol. Functions as full-duplex on 4-wire circuits and half-duplex on 2-wire circuits.

• V.34 − Full-duplex modem with data rates of 28.8 Kbps, transmitting 12 data bits per symbol. Enhanced V.34 modems can transmit 14 bits per symbol, achieving data rates up to 33.6 Kbps.

• V.90 and V.92 − Used for asynchronous full-duplex transmissions. V.90 achieves download speeds of 56 Kbps and upload speeds of 33.6 Kbps. V.92 improves upload speeds to 48 Kbps and adds features like quick connect and modem-on-hold.

Conclusion

Telephone modems serve as essential interfaces between digital computers and analog telephone networks, converting digital data to analog signals and vice versa. Various ITU-T standards like V.32, V.34, and V.90/V.92 define different performance levels and capabilities for modem communication over voice-grade telephone lines.