The Internet

Chapter The Internet

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • alternation between the two tones. In modulation, digital data is changed into ana-log data. It is a type of digital-to-analog (D/A) conversion. Demodulation changesthe analog signals back to digital ones; this is analog-to-digital (A/D) conversion.If you happen to pick up a telephone extension while someone is on-line with thecomputer from another extension, you’ll hear the analog signals from the two modems;it sounds like a hiss or roar. But don’t make a habit of doing this. It can cause the com-puter to be disconnected from the on-line network.Data speedModems work at various speeds, usually measured in bits per second (bps). You willoften hear about kilobits per second (kbps), where 1 kbps 1000 bps, or megabitsper second (Mbps), where 1 Mbps 1000 kbps. Sometimes you’ll hear about speedunits called the baud and kilobaud. (A kilobaud is 1000 baud.) Baud and bps are al-most the same units, but not they are not identical. People often use the term baudwhen they really mean bps.The higher the speed as specified in bps, the faster the data is sent and receivedthrough the modem. Speeds keep increasing as computer communications technologyadvances. Modems are rated according to the highest data speed they can handle, in bitsper second (bps), kilobits per second (kbps), or megabits per second (Mbps). A typical“telephone modem” works at about 56 kbps. Digital-subscriber-line (DSL) modems worksomewhat faster, around 128 kbps. Television cable modems can work upward of 1 Mbps.Basic components of a modemFigure 33-4 is a block diagram of a modem suitable for interfacing a home or businesscomputer with the telephone line. The modulator, or D/A converter, changes the dig-ital computer data into audio tones. The demodulator, or A/D converter, changes theincoming audio tones into digital signals for the computer. The audio tones fallwithin the frequency range, or band, of approximately 300 Hz to 3 kHz. This is theband needed to clearly transmit a human voice.It’s amazing how much computer data can race over a single telephone or radio cir-cuit having such a narrow bandwidth. Even pictures can be sent and received in brilliantcolor and in quite good detail (high resolution). As you might imagine, color images takelonger than gray-scale images to send and receive; also, the more detail an image con-tains, the longer it takes to be transferred at any given data speed.The InternetThe Internet is a worldwide system, or network, of computers. It got started in thelate 1960s, originally conceived as a network that could survive nuclear war. Backthen it was called ARPAnet, named after the Advanced Research Project Agency(ARPA) of the United States federal government.Protocol and packetsWhen people began to connect their computers into ARPAnet, the need becameclear for a universal set of standards, called a protocol, to ensure that all the 636 Computers and the Internet