59 lines
3.1 KiB
Plaintext
59 lines
3.1 KiB
Plaintext
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The recent addition of several new CCITT standards has caused
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confusion in the field of data communications. The V.32, V.22bis
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and V.42bis protocols sound like they are related, but they are
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not. V.32 and V.22bis are modulation techniques, which control
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the physical transfer of data between two modems. V.32 defines
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the modulation technique at 9600 bps while V.22bis defines 2400
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bps modulation. Modulation/demodulation (modem) is simply the
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process of changing digital bits of information in your computer
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to analog tones for use on phone lines and vice versa.
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V.42bis and MNP-5, on the other hand, are rival hardware-
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based protocols that handle error correction and data
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compression. Their correction components check for errors,
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retransmitting data if necessary. The compression algorithm
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boosts transmission speed by assigning symbols to common words in
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the file and transferring the symbol instead of the actual word.
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The Ven-Tel 9600 Plus will attempt to connect at V.32
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transfer level and then negotiate down to V.22bis. (Without V.32
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capabilities, 9600 bps modems are limited to 2400 bps transfers
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between different modem manufacturers). Ven-Tel's 9600 Plus will
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communicate with other manufacturer's V.32 modems and can
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reliably swap data at 9600 bps when using the same software-based
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error-correction protocol, such as Xmodem. However, if the 9600
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Plus connects to a V.32 modem that is using the same hardware-
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based correction/compression protocol (V.42bis or MNP-5) it can
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achieve even higher speeds, often beyond 38,000 bps.
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V.32 modems are particularly fast because they provide full-
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duplex (two-way) communications. By sending and receiving data
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simultaneously, one modem can return error-checking data while
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the other sends a file. Ven-Tel's 9600 Plus modem uses echo
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cancellation technology to keep the two signals distinct. By
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subtracting its own signal from the jumble, a modem can
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distinguish incoming data.
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The Ven-Tel 9600 Plus also uses Trellis encoding, which
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corrects most errors caused by noisy phone lines. Trellis
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encoding is called a forward error correction because the
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receiving modem can fix most errors without a retransmission.
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This leaves less error-correcting work for the V.42bis or MNP-5,
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resulting in faster throughput.
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Although error correction can slow transfers on dirty lines,
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both protocols use an efficient synchronous framing technique
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that speeds transfers by 20 percent on average lines. MNP-5
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compression can double the throughput, and the new V.42bis
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compression can even triple throughput.
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V.42 and MNP-5 are compatible only at the correction level,
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not on the compression level. By definition V.42 compliance
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incorporates MNP-4 and LAP-M error correction protocols. V.42
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compatibility is defined as having either MNP or LAP-M. Ven-Tel
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manufactures a complete line of modems that are V.42 compatible
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(MNP-5) at 2400 bps, including PS/2, standard bus, Macintosh and
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external models. The Ven-Tel 9600 Plus and 9600 Plus II are both
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V.32 and V.42bis compatible and carry a five year warranty.
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