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Systems Engineering |Системная инженерия

Lesson 9

Read the text: Data transmission

Modem is an electronic device that converts digital data into analog (modulated-wave) signals suitable for transmission over analog telecommunications circuits (e.g., traditional phone lines) and demodulates received analog signals to recover the digital data transmitted. The “modulator/demodulator” thus makes it possible for existing communications channels to support a variety of digital communications, including e-mail, Internet access, and fax transmissions. An ordinary modem, operating over traditional phone lines, has a data transmission speed limit of about 56 kilobits per second. ISDN lines allow communications at over twice that rate, and cable modems and DSL lines have transmission rates of over a million bits per second.

Data transmission is sending and receiving data via cables (e.g., telephone lines or fibre optics) or wireless relay systems. Because ordinary telephone circuits pass signals that fall within the frequency range of voice communication (about 300–3,500 hertz), the high frequencies associated with data transmission suffer a loss of amplitude and transmission speed. Data signals must therefore be translated into a format compatible with the signals used in telephone lines. Digital computers use a modem to transform outgoing digital electronic data; a similar system at the receiving end translates the incoming signal back to the original electronic data. Specialized data-transmission links carry signals at frequencies higher than those used by the public telephone network.

Bulletin-board system is a computerized system used to exchange public messages or files. A BBS is typically reached by using a dial-up modem. Most are dedicated to a special interest, which may be an extremely narrow topic. Any user may “post” his or her own message (so that they appear on the site for all to read). Bulletin boards produce “conversations” between interested participants, who may download or print out messages they desire to keep or pass on to others. BBS sites today number in the tens of thousands.

Streaming is a method of transmitting a media file in a continuous stream of data that can be processed by the receiving computer before the entire file has been completely sent. Streaming, which typically uses data compression, is especially effective for downloading large multimedia files from the Internet; it permits, for example, a video clip to begin playing on a user's computer as soon as it begins to be downloaded from a Web site. Even with improved modems and connection speeds, downloading and playing large audio and video files without the use of streaming techniques still takes an inconveniently long time. To accept streaming data, the receiving computer needs to be running a player, a program that decompresses the incoming data and sends the resulting signals to the display and speakers. The audio and video files may be prerecorded, but streaming can also accommodate a live feed over the Internet.

Data compression is the process of reducing the amount of data needed for storage or transmission of a given piece of information (text, graphics, video, sound, etc.), typically by use of encoding techniques. Data compression is characterized as either lossy or lossless depending on whether some data is discarded or not, respectively. Lossless compression scans the data for repetitive sequences or regions and replaces them with a single “token.” For example, every occurrence of the word the or region with the colour red might be converted to $. ZIP and GIF are the most common lossless formats for text and graphics, respectively. Lossy compression is frequently used for photographs, video, and sound files where the loss of some detail is generally unnoticeable. JPEG and MPEG  are the most common lossy formats.

1. Match the left part with the right:

2. Complete the sentences with the suggested words: scans, compression, sequences, discarded

Data _______ is characterized as either lossy or lossless depending on whether some data is _______ or not, respectively. Lossless compression _______ the data for repetitive _______ or regions and replaces them with a single “token.”