Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [168]
There are different MPEG standards for different media. MPEG-2 is for high-definition television (HDTV) and for digital video discs (DVDs), also called digital versatile discs. DVDs are the same size as CDs, but they can be recorded on both sides and in two layers per side. On DVDs, video is compressed by a factor of about 50, so a two-hour movie requires only 4 gigabytes, which can fit on one layer of one side. The use of both layers and both sides increases the capacity of DVDs to about 16 gigabytes, which is about 25 times the capacity of a CD. It's expected that DVD-ROM will eventually replace CD-ROM for the distribution of software.
Are CD-ROM and DVD-ROM the modern day realization of Vannevar Bush's Memex? He originally conceived of Memex as using microfilm, but CD-ROM and DVD-ROM make much more sense for such a device. Electronic media have an advantage over physical media by being easily searchable. Unfortunately, few people have simultaneous access to multiple CD or DVD drives. The closest that we've come to Bush's concept doesn't involve storing all the information you'll need at your desk. It involves interconnecting computers to give them the ability to share information and use storage much more efficiently.
The first person to publicly operate a computer from a remote location was George Stibitz, the same man who designed the Bell Labs relay computer in the 1930s. The remote operation of a relay computer occurred at a demonstration at Dartmouth in 1940.
The telephone system is built to transmit sound, not bits, over wires. Sending bits over telephone wires requires that the bits be converted to sound and then back again. A continuous sound wave of a single frequency and a single amplitude (called a carrier) doesn't convey any substantial information at all. But change something about that sound wave—in other words, modulate that sound wave between two different states—and you can represent 0s and 1s. The conversion between bits and sound occurs in a device called the modem (which stands for modulator/demodulator). The modem is a form of serial interface because the individual bits in a byte are sent one after another rather than all at once. (Printers are often connected to computers with a parallel interface: Eight wires allow an entire byte to be transmitted at the same time.)
In early modems, a technique called frequency-shift keying (FSK) was used. A modem operating at 300 bits per second (for example) might convert a 0 bit to a frequency of 1070 Hz and a 1 bit to a frequency of 1270 Hz. Each byte is prefaced by a start bit and concluded with a stop bit, so each byte requires 10 bits. At 300 bits per second, the transmission speed is only 30 bytes per second. More modern modems use more sophisticated techniques to achieve speeds over 100 times that.
An early home computer enthusiast could set up a computer and a modem as a bulletin board system (BBS), to which other computers could call in and download files, which means transferring files from a remote computer to one's own computer. This concept was extended into large information services such as CompuServe. In most cases, communication was entirely in the form of ASCII text.
The Internet is qualitatively different from these early efforts because it's decentralized. The Internet really exists as a collection of protocols for computers to talk to one another. Of major importance is TCP/IP, which consists of the Transmission Control Protocol and the Internet Protocol. Rather than just sending ASCII characters through the wires, TCP/IP-based transmitters divide larger blocks of data into smaller packets, which are sent separately over the transmission line (often a telephone line) and reassembled on the other end.
The popular graphical part of the Internet is the World Wide Web, which makes use of HTTP, the Hypertext Transfer Protocol. The actual data viewed on Web pages is defined by a text format called HTML,