Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!rutgers!ames!ucbcad!ucbvax!decvax!decwrl!sun!hoptoad!pozar From: pozar@hoptoad.uucp (Tim Pozar) Newsgroups: comp.dcom.modems Subject: Re: baud vs. bit/sec. Message-ID: <2035@hoptoad.uucp> Date: Wed, 22-Apr-87 17:07:06 EST Article-I.D.: hoptoad.2035 Posted: Wed Apr 22 17:07:06 1987 Date-Received: Fri, 24-Apr-87 23:45:10 EST References: <8704220426.AA03311@ucbvax.Berkeley.EDU> Reply-To: pozar@hoptoad.UUCP (Tim Pozar) Distribution: world Organization: Syncstream (San Francisco) Lines: 56 If you want a very good definintion of how modems work (>300 b/s), get the technical Manual for the Advanced Micro Devices Am7910 WORLD-CHIP FSK modem. Section 2 of the manual has a tutorial for modems. One analogy they draw is: "Think of the information being transmitted as a stream of automobiles with people in them moving down a highway (the phone line). Consider each care a baud (symbol). Consider each person a bit. "Bit Rate = Baud Rate * No. of Bits/Baud or people/sec = cars/sec * people/car. "Bit Rate >= Baud Rate. A "baud" is the speed at which the carrier is Frequency Shifted. If it is moved from, say 1000cps to 2000cps 100 times a second, it would be delivering 100 "baud". If this shift shows is representive of a change in state of data (1 to 0, or hight to low, or the other way around), then it would be sending 100 bits per second or 100 b/s. In order to put more that one person per car for FSK modems you can phase shift the frequency. A phase shift of 90 degrees can have 4 people in a car, or 2 bits per baud. Since 00b = 0, 01b = 1, 10b = 2, and 11b = 3. This is how 2400b/s modems work. By spliting the phase "cycle" into eigths, we can send 3 bits per baud, thus sending 4800b/s. This is about the max we can send. What we are talking about is 45 degree phase angle shifts. We start getting into distortion with the phone line that makes getting smaller incraments pretty rough. So, there is one other way to get more bang for our buck. Quadrature Amplitude Modulation or QAM. QAM phase shifts the frequency and amplitude shifts is as well. Also the pattern of the bits are layed out differntly. Instead of a circle, with each bit being a different point on the edge (the size of the circle would represent the amplitude of the signal), the pattern is broken up into squares: 0 1000 1100 | 0100 0000 | 1001 1101 | 0101 0001 270------------+------------90 1011 1111 | 0111 0011 | 1010 1110 | 0110 0010 180 Now we can send 4 bit/baud or 9600 b/s. According to the tech manual: "QAM is even more sensitive to line distortions than PSK (Phase Shift Keying). Automatic equalizers are required to compensate for these distortions. This type of modem is the most complex and expensive of all the modems to implement. -- Tim Pozar UUCP pozar@hoptoad.UUCP Fido 125/406 USNail KLOK-FM 77 Maiden Lane San Francisco CA 94108