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Unit 6: Data Communication

T3 Line Another most popular and faster 1-carrier line is 13 line. Its data transmission speed is Notes
44 Mbps. It is more expensive than Ti line. The main users of T3 line are telephone companies
and ISPs. The Internet backbone itself also uses T3 lines.

6.2.2.6 Asynchronous Transfer Mode (ATM)
It is very, fast data transmission connection line that can carry data, voice, video, multimedia
etc. Telephone networks, Internet and other network use ATM. In near future, ATM will become
the Internet standard for data transmission instead of 13 lines. Its data transmission speed is from
155 Mbps to 600 Mbps.

6.3 Data Communication with Modems

A modem (modulator-demodulator) is a device that modulates an analog carrier signal to
encode digital information, and also demodulates such a carrier signal to decode the transmitted
information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce
the original digital data. Modems can be used over any means of transmitting analog signals,
from driven diodes to radio.

Figure 6.4: Data Communication with Modem

Modem

The most familiar example is a voice band modem that turns the digital data of a personal
computer into modulated electrical signals in the voice frequency range of a telephone channel.
These signals can be transmitted over telephone lines and demodulated by another modem at
the receiver side to recover the digital data.
Modems are generally classified by the amount of data they can send in a given time unit,
normally measured in bits per second (bit/s, or bps). They can also be classified by the symbol
rate measured in baud, the number of times the modem changes its signal state per second. For
example, the ITU V.21 standard used audio frequency-shift keying, aka tones, to carry 300 bit/s
using 300 baud, whereas the original ITU V.22 standard allowed 1,200 bit/s with 600 baud using
phase shift keying.

(modem) (n.) Short for modulator-demodulator. A modem is a device or
program that enables a computer to transmit data over, for example, telephone
or cable lines.

Fortunately, there is one standard interface for connecting external modems to computers called
RS-232. Consequently, any external modem can be attached to any computer that has an RS-232
port, which almost all personal computers have. There are also modems that come as an expansion
board that you can insert into a vacant expansion slot. These are sometimes called onboard or
internal modems.

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