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