Modem

• Modem is abbreviation for Modulator – Demodulator. Modems are used for data transfer from one computer network to another computer network through telephone lines.
• The computer network works in digital mode, while analog technology is used for carrying massages across phone lines.
• Modulator converts information from digital mode to analog mode at the transmitting end and demodulator converts the same from analog to digital at receiving end.

Types of Modems

• Directional capacity: half duplex modem and full duplex modem.
• Connection to the line: 2-wire modem and 4-wire modem.
• Transmission mode: asynchronous modem and synchronous modem.

Half duplex and Full duplex modems

Half duplex

• A half duplex modem permits transmission in one direction at a time.

Full duplex

• A full duplex modem allows simultaneous transmission in both directions.
• Therefore, there are two carriers on the line, one outgoing and the other incoming.

2-wire and 4-wire modems

2-wire

• 2-wire modems use the same pair of wires for outgoing and incoming carriers.
• A leased 2-wireconection is usually cheaper than a 4-wire connection as only one pair of wires is extended to the subscriber's premises.
• The data connection established through telephone exchange is also a 2-wire connection.

4-wire

• In a 4-wire connection, one pair of wires is used for the outgoing carrier and the other pair is used for incoming carrier.
• Full duplex and half duplex modes of data transmission are possible on a 4- wire connection.

Synchronous and Asynchronous modems

Synchronous

• Synchronous modems can handle a continuous stream of data bits but requires a clock signal.
• The data bits are always synchronized to the clock signal.
• There are separate clocks for the data bits being transmitted and received.
• For synchronous transmission of data bits, the DTE can use its internal clock and supply the same to the modem.

Asynchronous

• Asynchronous modems can handle data bytes with start and stop bits.
• There is no separate timing signal or clock between the modem and the DTE.
• The internal timing pulses are synchronized repeatedly to the leading edge of the start pulse .