The modem is a device which allows you to connect your computer to any other computer, anywhere else in the world using a telephone line.
This allows you to communicate with the connected person, transfer data, copy software/programs from the connected computer, sent and receive mail using e-mail etc.
Currently, the modem can also be used to sent and receive fax, for this you can use modem as a fax device. To connect to the internet or the WWW (World Wide Web), one requires a modem.
The modem is available as
- Internal Modem
- External Modem
How Modem Works?
Modem is a device which allows us to connect two computer using normal telephone line. The sending computer’s modem modulates the digital signals from the computer into analog signal required for transmission over the telephone line.
At the receiving end, the incoming analog signal is given to the modem. The modem demodulates the analog signal back into digital signal and gives this signals to the computer.
Let us see a how works.
When the computer is switched on and is ready to transmit data it sends a Data Terminal Ready (DTR) signal to the modem. When the modem is ready to receive data or instruction from the computer, Both these signals must be present before the next step can happen.
Next, the computer sends a command to the modem to go “Off Hook”, i.e. to open communication with the phone line. Next, the computer issues a command to modem to dial a phone number.
These commands are given via “Transmit Data TxD” line. The modem acknowledges receipt of these command by replaying to the commuter on the “Receive Data RxD” line.
When the remote modem, the modem at the other end of line respond, the local modem sends a special greeting tone to the remote modem, to inform the remote modem that it is being called by another modem. The remote modem responds with a higher pitched tone.
Once a communication is established between the two modems, local modem sends a “Carrier Detect” CD signal to the computer. Carrier is a steady tone signal of a fixed frequency, which is later modulate by modem to transmit the data.
Next, the two modems enter a process known as “Handshake”. This process is decides how the two modems are going to communicate with each other. Various things decided during this process are
- The number of bits used to represent data,
- Number of bits used to indicate start and end of data,
- Transmission speed, transmission protocol, compression method etc. Being used
If the two modems, local and remote do not use the same settings then they will wind up sending characters that will not make sense, or they may completely refuse to communicate with each other.
Once the handshake is done, the computer sends a Request To Send (RTS) signal to the local modem. This signal asks the modem if the modem is free to receive data from the computer.
If the modem is not busy, then the modem will send a Clear To Send (CTS) signal to the computer. Once the computer receives this CTS signal from the modem, the computer will send the data to the transmitted to the modem.
If the modem cannot transmit the data, as fast as it received from the computer, then the modem will drop the CTS signal to the computer, this will stop any further data from the computer. Computer will stop the data transmission to the modem until it receives CTS signal.
Data received by local modem is transmitted as different frequency sound signal to the remote modem, The remote modem converts these series of tones back into digital signal and gives it to the computer, it is connected to.
Once data transmission is over, modem sends a break communication command. If the connection is broken by remote system, the local modem will drop the “Carrier Detect” (CTS) signal.
This will inform the local computer that communication is broken.
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