Modem Definition
Modem is the short form of Modulator-Demodulator. It is a device through which computer send and receive data from telephone lines.
Computer generates discrete data. They cannot be send through telephone lines which are designed for carrying analog signals. Modem accepts the data from computer and convert into analog signals using modulation procedure called ASK (Amplitude Shift Keying), PSK (Phase Shift Keying) or FSK (Frequency Shift Keying) or a variant of these modulation methods. These analog signals are send over telephone lines. The signals that come through telephone lines are reconverted back to binary (discrete) form by demodulator part of the modem. A modem can accept as well as transmit data serially simultaneously. Normal data transfer rates are 56 kbps, 144 kbps or 288 kbps.
Modem Types
Modems are often of several types and that they are often categorized during a number of the way .
• Categorization is typically supported the subsequent 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 and full duplex Modems
Half duplex
1. A half duplex modem permits transmission in one direction at a time.
2. If a carrier is detected on the road by the modem, I gives a sign of the incoming carrier to the DTE through an impact signal of its digital interface.
3. As long as they camel' IS being received; the modem doesn't give permission to the DTE to transmit data.
Full duplex
• A full duplex modem allows simultaneous transmission in both directions.
• Therefore, there are two carriers on the road , one outgoing and therefore the other incoming. Wire and 4-wire Modems
• the road interface of the modem can have a 2-wire or a 4-wire connection to transmission medium. 4-wire Modem
• during a 4-wire connection, one pair of wires is employed for the outgoing carrier and therefore the other pair is employed for incoming carrier.
• Full duplex and half duplex modes of knowledge transmission are possible on a 4- wire connection.
• because the physical transmission path for every direction is separate, an equivalent carrier frequency are often used for both the directions.
2-wire Modem
• 2-wire modems use an equivalent pair of wires for outgoing and incoming carriers.
• A leased 2-wireconrlection is typically cheaper than a 4-wire connection as just one pair of wires is extended to the subscriber's premises.
• the info connection established through central is additionally a 2-wire connection.
• In 2-wire modems, half duplex mode of transmission that uses an equivalent frequency for the incoming and outgoing carriers are often easily implemented.
• For full duplex mode of operation, it's necessary to possess two transmission channels, one for transmit direction and therefore the other for receive direction.
• this is often achieved by frequency division multiplexing of two different carrier frequencies. These carriers are placed within the bandwidth of the speech channel.
Asynchronous & Synchronous Modems
Asynchronous Modem
• Asynchronous modems can handle data bytes with start and stop bits.
• there's no separate timing signal or clock between the modem and therefore the DTE.
• the interior timing pulses are synchronized repeatedly to the vanguard of the beginning pulse .
Synchronous Modem
• Synchronous modems can handle endless stream of knowledge bits but requires a clock signal.
• the info bits are always synchronized to the clock signal.
• There are separate clocks for the info bits being transmitted and received.
• For synchronous transmission of knowledge bits, the DTE can use its internal clock and provide an equivalent to the modem.
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