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How Modem is Worked. Its types and functionality

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The term “modem” is an acronym for “Modulator-Demodulator.” It is a physical device that enables a computer or another device to connect to the Internet, such as a router or switch.

In this tutorial you will learn working of modem, their functionality and its various types of modem

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Working of Modem

It transforms or “modulates” an analogue signal from a telephone or cable connection to computer-readable digital data (1s and 0s). Likewise, it transforms digital data from a computer or other device to an analogue signal capable of being sent over standard telephone lines.

At the transmitting end, the modulator transforms data from digital to analogue mode, while the demodulator turns it back to digital at the receiving end. Digitizing is the act of transforming analogue signals from one computer network to digital signals from another computer network for them to be processed by a receiving computer.

When an analogue facility is used to transmit data between two digital devices referred to as Data Terminal Equipment (DTE), modems are utilized on both ends. DTE may take the form of a terminal or a computer.

At the transmitting end, the modem modulates a carrier to transform the digital signal created by DTE to an analogue signal. This modem demodulates the carrier at the receiving end and transmits the demodulated digital signal to the DTE.

Different Modulation Technique Used By 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).

Function of Modem

Modems provide the following functionality

Error Correction:

When data is transferred across modems, it can sometimes be damaged, resulting in portions of the data being changed or destroyed. Modems utilise error correction to get around this. Information is organised into batches known as frames.

Each frame has a checksum, which is a tiny piece of data produced from the information in the frame. A checksum may be thought of as a type of fingerprint that is unique to the data in a certain frame. The modem that receives the information calculates its own checksum from the frame that was delivered and then compares it to the checksum supplied by the sending modem. If the checksums match, the data is unaltered. If they don’t match, the data was damaged during transmission, and the receiving modem sends it back while waiting for the transmitting modem to re-send that frame.

Flow Control:

Individual modems transmit data at varying rates. Faster modems must slow down in order for slower modems to catch up; otherwise, the slower modem would receive more data than it can handle. When this occurs, the slower modem sends a character to the quicker modem. This character indicates to the fast modem that it should halt delivering data until the slow modem catches up. When the slow modem is ready to communicate further data, it sends a new character to the fast modem indicating that it may resume transmission. This manner, the two modems’ speeds may be matched.

Modem Speed

A modem’s speed is generally measured in terms of the quantity of data it can transmit in a given amount of time. This is commonly represented as bits per second (bps). A different metric for defining modem speed is the change in state of the signal per unit time, or the number of times a modem transmits a new signal in a given period of time. This is referred to as the symbol rate, and it is expressed in baud units (Bd)

Data Compression:

Modems must use data compression to decrease the amount of time it takes to transfer data and to reduce the amount of error in the signal.

This was especially important in the early days of modem technology, when data had to be sent via traditional phone lines. Phone lines, not being built for digital information, imposed severe limits on the amount and speed of signals conveyed through them. Data compression methods minimise the amount of the signal necessary to convey the data.

Modulating Signals:

The most important purpose of a modem is to generate a signal that can be readily sent and decoded, allowing digital data to be transported from one location to another without losing any information in the process.

Modems are most commonly associated with the transmission of information over a telephone network, although they may also be used to relay data over any system that supports the transmission of analogue signals, such as radio and optical networks.

Types of Modems

Modem can be classified into the following types.

  • Dial Up
  • Digital modem
  • Half duplex Modem
  • Full Duplex
  • 2-Wire and 4-wire Modems
  • Asynchronous Modem:
  • Synchronous
Types of Modem

Half Duplex Modem


A half-duplex modem is capable of transmitting data in just one way at a time. If the modem detects a carrier on the line.

It informs the DTE of the arriving carrier through a control signal on its digital interface. 

Full duplex Modem

Simultaneous transmission in both directions is possible with a full duplex modem. As a result, the line has two carriers, one outgoing and one incoming.

4-wire Modems:


The modem’s line interface can connect to the transmission medium through two or four wires. Modem with four wires One pair of wires is used for the outgoing carrier and the other pair is used for the incoming carrier in a four-wire connection.

On a four-wire connection, both full duplex and half duplex data transfer are feasible.

Asynchronous

Asynchronous modems are capable of processing data bytes that contain start and stop bits. Between the modem and the DTE, there is no independent timing signal or clock. Internal timing pulses are synced to the leading edge of the start pulse on a recurring basis.

Synchronous Modem:

Synchronous modems, on the other hand, can handle a constant stream of data bits but require a clock signal to operate. At all times, the data bits are synced with the clock signal.

There are distinct clocks for transmitting and receiving data bits. The DTE can use its internal clock and provide it to the modem to ensure synchronized transmission of data bits.

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