Data Communication over Telephone Lines Using Modems

We can use serial I/O technique to send data over long distance through telephone lines. But, telephone lines are designed to handle some specific range of voice; the bandwidth of telephone lines range from 300 Hz to 3300 Hz. When the time of digital signals will be in nanoseconds then we requires a bandwidth of several megahertz. Therefore, data bits should be converted into audio tones; this is achieved through Modems.

What is Modem ?
A modem is short form of Modulator/Demodulator. Modem is a hardware component/device which can connects computer and other devices such as router and switch to internet. Modems converts or modulates the analog signals coming from telephone wire into digital form i.e in form of 0 s and 1 s. Modems of present time can transfer data at rates of 300-2400 bps (bits per second). Generally, two types of modulation techniques are used: frequency-shift keying(FSK) for low-speed modems and phase shift keying (PSK) for high-speed modems.

The first modems were “dial-up” which means we have to dial a phone number to connect computer to ISP. Their maximum data transfer rate is almost 56 kbps. Modern modems are DSL or cable modems. They have high data transfer rate and wider frequency range as compared to dial-up modems.

Working Process :
Here is the explanation of how data communication over telephone line will takes place through modems. Computer can exchange information over telephone lines by using two modems- one on each side. A calling computer (or a terminal) or an originator, contacts the receiving computer (also known as answering) through a telephone number, and a communication link is established after control signals have been exchange between computers and modems.

A typical process of communication for 300 bps modem is shown in above figure. A parallel word is converted into serial bits; in turn, the originator/terminal modem generates two audio frequencies – 1070 Hz for logic 0 and 1270 Hz for logic. These audio frequencies are transmitted over telephone lines. At the answering end, audio frequencies are converted back into 0 s and 1 s, and serial bits are converted into parallel word that can be read by the computer easily as it is in binary form. When the answering end computer needs to transmit, it transmit on 2025 Hz (Space) and 2225 Hz (Mark).

Clearly, Modem plays an vital role in Data communication as it is mainly converting analog signals to binary form.

Advantages of using Modem :

  • Modem is more useful in connecting LAN with the internet.
  • Modem speed depends on the cost, more is the cost more powerful is the modem.
  • A modem is the most probably used in data communication roadway.
  • A modem converts the digital signals into analog signals.

Disadvantages of using Modem :

  • Modem acts just as an interface between LAN and internet.
  • No traffic maintenance of modem is present.
  • A modem can’t understand the intermediate process.
  • The modem doesn’t know about its own destination path.
  • In modems limited number of devices can be connected min 8 & max 12.
  • Modem has slow speed as compared to the Hub.

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