A modem is a device which receives stream of bits that is a digital signal as input and produces a modulated carrier or an analog signal as an output or vice versa. Modem is combination of two words Modulator –demodulator. So, we can say that a modem is a device which performs the function of both modulator (A modulator converts digital information into analog signal) and a demodulator (A demodulator converts an analog signal to digital signal). A modem is positioned between a computer (that’s digital system) and the telephone line (that is analog system).
We know that computers work using digital signal or information and if we need to carry information from one computer to another computer which is physically remotely located then we use telephone lines to transfer messages or information. But telephone lines use analog technology to transfer information. So we use modem at the sender end which converts the digital signal from computer network into analog signal so that it can be transmitted over analog medium and at the receiver end the modem demodulates the received analog signal into digital system and forwards the same to the destination computer network. This process of converting analog signal back into digital form is call digitization. The computers or the digital devices are called Data Terminal equipment or simply DTE.
Function of Modem
Modems have to perform various functions. Some of the functions are as:
The main function of a modem is to produce a signal that is easy to transmit and decode to facilitate digital data to be sent from one network to another while all information is preserved that is without any loss of information. So modems are responsible for modulation of the data signal.
The second important function of modems is decrease the period of time it takes to transmit data and reduce the error possibility in the signal. So, data needs to be compressed. By compressing data the signal to be transmitted is reduced in size.
While information is being transmitted between modems, it is possible that signal gets damaged. That means any part of the data is either lost or altered. To identify the faulty data and correct it (if possible), modems employ error correction techniques.
All modems do not have same speed. That means modems send information at different speeds. So it is mandatory for fast modems to wait or slow down so that modems with less speed can catch up, otherwise the slower modem will receive more data than it cannot process. This issue is resolved by modems by implementing various flow control techniques.
Types of Modems
Modems can be categorized in a number of ways depending on their basic features such as:
- Directional Capacity : Half duplex modem and Full duplex modem
- Line Connection : 2-wire modem and 4-wire modem
- Mode of Transmission: Asynchronous modem and Synchronous modem
Now let us briefly discuss each of these modems
This modem Allows transmission in one direction only at a time. When a carrier is sensed on the transmission line, the modem sends information to the DTE by a control signal about the incoming carrier. While the carrier is being received the modem disables the DTE for transmitting data. If the DTE has to transmit data then it has to wait till the time modem allows it to send data.
Transmission in both the directions is possible using a full duplex modem. That is data can be sent and received by a DTE using full duplex modem simultaneously. So we can say that there are two carriers or signals on the transmission line, an outgoing carrier and an incoming.
To transmit or receive carrier the modems have line interface as transmission medium. Normally either 2-wire modems or 4-wire modems are available. The 2-wire modems two wires interface as transmission medium and the same pair of wires are used for incoming as well as outgoing signals. It is cheaper than a 4-wire connection. It is important to know that telephone exchange establishes a 2-wire connection. Since only two wires are used for sending and receiving carriers so at a time a modem can either send or receive carrier. So we conclude that a 2-wire connection uses half duplex mode of transmission.
In 4-wire modems, 4 physical wires are available. One pair wires for the outgoing carrier or signal and the other pair of wires for incoming carrier. Both full duplex and half duplex modes of data transmission are possible using 4- wire connection. Since the physical transmission path for each direction is separate, so, same carrier frequency can be used for both the directions.
Asynchronous modems process data bytes with start bits to indicate the start of signal and stop bits to indicate end of signal. No timing signal or clock is provided between the modem and the DTE. In asynchronous modems internal timing pulses are synchronized at certain intervals with the leading or trailing edge of the start pulse.
Synchronous modems require a clock signal to handle a continuous stream of data bits and these data bits and clock are always synchronized. Separate clocks signals are used for transmission and reception of the data bits. Usually in synchronous transmission, the DTE use its internal clock and supplies the same to the modem.