Types of Modems
• Modems can be of several types and they can be categorized in a number of ways.
• Categorization is usually based on the following basic modem features:
1. Directional capacity: half duplex modem and full duplex modem.
2. Connection to the line: 2-wire modem and 4-wire modem.
3. Transmission mode: asynchronous modem and synchronous modem.
Half duplex
1. A half duplex modem permits transmission in one direction at a time.
2. If a carrier is detected on the line by the modem, I gives an indication of the incoming carrier to the DTE through a control signal of its digital interface.
3. As long as they camel’ IS being received; the modem does not give permission to the DTE to transmit data.
• A full duplex modem allows simultaneous transmission in both directions.
• Therefore, there are two carriers on the line, one outgoing and the other incoming. Wire and 4-wire Modems
• The line interface of the modem can have a 2-wire or a 4-wire connection to transmission medium. 4-wire Modem
• 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.
• As the physical transmission path for each direction is separate, the same carrier frequency can be used for both the directions.
2-wire Modem
• 2-wire modems use the same pair of wires for outgoing and incoming carriers.
• A leased 2-wireconrlection 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.
• In 2-wire modems, half duplex mode of transmission that uses the same frequency for the incoming and outgoing carriers can be easily implemented.
• For full duplex mode of operation, it is necessary to have two transmission channels, one for transmit direction and the other for receive direction.
• This is achieved by frequency division multiplexing of two different carrier frequencies. These carriers are placed within the bandwidth of the speech channel.
Asynchronous Modem
• 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
Synchronous Modem
• 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.
The basic modulation techniques used by a modem to convert digital data to analog signals are :
• Amplitude shift keying (ASK).
• Frequency shift keying (FSK).
• Phase shift keying (PSK).
• Differential PSK (DPSK).
These techniques are known as the binary continuous wave (CW) modulation.
• Modems are always used in pairs. Any system whether simplex, half duplex or full duplex requires a modem at the transmitting as well as the receiving end.
• Thus a modem acts as the electronic bridge between two worlds – the world of purely digital signals and the established analog world.