79 lines
4.3 KiB
Plaintext
79 lines
4.3 KiB
Plaintext
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Quoted from a public message from Grant DeLorean 26 Feb 1990.
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Baud != BPS
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The word Baud has a specific meaning, and that meaning is discrete signal
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events per second. The term bits per second (BPS) is similar, it means the
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number of data bits transmitted per second. The two terms, while similar,
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are not synonymous. It is possible to transmit more than one data bit
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per Baud, and that is the way modern modems get the throughputs they
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achieve. In the modems that have seen common useage, the only rate at
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which the BPS and Baud rates have matched has been 300 Baud (there were
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some 450 Baud modems, but they weren't very common).
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Today's dial up telephone lines have an average of 3000 Hz bandwidth
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available (can be as low as 2400 Hz or as high as 3400, but the average is
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around 3000). Modems (shorthand for MOdulator/DEModulator) use this
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to their advantage by encoding (modulating) a signal and sending it along
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the phone line to a modem on the other side (which must be able to use the
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same modulation technique) which will then demodulate it and pass it along
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to its host system. Most modems are capable of sending and recieving at the
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same time because they split the line into two channels. A short explaination
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of the commonly used modulation techniques follows:
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300: This differs from the rest of the common modualtion techniques in that
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only one data bit per Baud is sent. It uses the bandwidth in a slightly
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different way than the rest. It uses a techniques called Frequency Shift
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Keying. In FSK, different frequencies determine of any bit is 'on' or 'off'
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(mark or space in the terminology of FSK). There are two 300 Baud standards,
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Bell 103 (Bell Labs) and CCITT V.21. The Bell 103 is the more commonly used
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standard, V.21 is optional although most good modems also support it. In
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Bell 103, the originating modem uses 2225 Hz as mark and 2025 Hz as space,
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the answering modem uses 1270Hz for Mark and 1070 Hz for space. CCITT V.21
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the originating modem uses 1650 Hz Mark, 1850 Hz space and answering uses
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980 Hz mark, 1180 space.
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1200: At 1200 BPS things become slightly complicated. We are now sending
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600 baud, but we are moving two data bits per baud to achieve 1200 bps.
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This is accomplished by using Differential Phase Shift Keying, which is
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difficult to explain without getting into wave form theory (which is
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beyond the scope of this document). DPSK takes advantage of known waveform
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behaviour to move two data bits per discrete signal event. The bandwidth
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is divided into a 2400 Hz channel and a 1200 Hz channel (the originator
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gets the wider path). The common standard used for 1200 BPS is Bell 212a,
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though CCITT V.22 is an option.
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2400: At 2400 BPS we are still using 600 baud, and the bandwidth is split
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the same way as in 1200 BPS communications. This time we are using a
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technique known as Quadrature Amplitude Modulation (same problem with
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explaining as DPSK, waveform theory) to move 4 data bits (a "quad-bit",
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as they are called "di-bit" for 1200) per discrete signal event. The
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common standard here is V.22bis (not just V.22, which is a 1200 bps
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modulation technique).
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9600 (V.32): V.32 uses a split of 1800 Hz for both channels. This does
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cause problems with cross talk between the two channels, so echo
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cancelation techniques are used to eliminate the problem. Each channel
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sends at 2400 Baud. Trellis Coded Modulation encodes 5 data bits per
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Baud (4 data, one parity), achieving 9600 bps throughput.
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9600 (HST): The original HST split the bandwidth into an 1800 Hz forward
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channel and a 350 Hz backchannel. It sent at 300 Baud on the back channel
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and 2400 Baud using TCM on the forward channel to achieve 9600 BPS through-
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put.
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9600 (HST): The second HST raised the back channel to 375 hz and sent at
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450 Baud along it, otherwise it was the same as the original HST.
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14400 (HST): The 14400 HST uses the same channels as the 450 back channel
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HST. It uses a modification of TCM to send 6 bits per Baud (12000 bps)
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or 7 bits per Baud (14400).
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The addition of such as MNP levels 1-4 and V.42 can change the actual
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throughput, either a slight decrease or a slight increase will be seen.
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The advantage is that the data received will have a better chance of
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being good. MNP level 5, 7 or 9 and V.42bis can greatly improve the
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throughput. If there is a demand, a more full explaination of MNP and
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V.42 will be added here...
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