The short answer to your question is that the other data rates such as 12k, 14.4k, 19200 and above fall out of a different baud rates than 9600 and below. The longer answer is that bit rates are tied to the bandwidth used by analog modems (historical). In older modem technology no error correction bits were sent with the data. In these products, data rates from 2400 to 9600 used a telephone line bandwidth of 2400Hz. This allows the following data rates to fall out: 2400 bits per second = 1 bit per baud (2 point signal constellation) 4800 bits per second = 2 bits per baud (4 point constellation) 7200 bits per second = 3 bits per baud (8 point constellation) 9600 bits per second = 4 bits per baud (16 point constellation) At the time, due to relatively little horsepower on the processor side, it was sufficient to simply use a slicer to determine what the actual received data point equated to. If the signal-to-noise ratio was good enough, relatively few errors occured. As time progressed, there was a desire to increase the data rate and the SNR performance by adding error correction bits into the transmitted data. Of course this required a huge increase in processing power. (You can search on documents relating to V.32 and trellis coding for more information.) As I recall, a 14400 bps modem used 7 bits per baud (6 bits of data and 1 error correction bit per baud). This would result in a constellation of 2**7 = 128 points, a greatly increased problem to decode. As processing power grew, higher bandwidths were used along with additional error correction techniques up to 28,800 bps line rate. Higher data rates such as 57600 and 115 K were actually done with compression (V.42) and not by increasing the actual line rate). It basically amounts to how much horsepower and how smart we are at any given point in time. 56K modems (line rate) work slightly differently and can't be connected end-to-end and require a digital backbone between them (I'm sort of fuzzy on this and it may be incorrect). Ken ----- Original Message ----- From: "James Newton, Host" To: Sent: Tuesday, January 20, 2004 7:17 PM Subject: [EE] Why don't baud rates just double? > Why is each higher baud rate twice as fast until 14.4K which is only 1.5 > times faster than 9600? > > 115,200 > 57,600 > 28,800 > 14,400 > ? > 9,600 > 4,800 > 2,400 > 1,200 > 600 > 300 > > --- > James Newton: PICList webmaster/Admin > mailto:jamesnewton@piclist.com 1-619-652-0593 phone > http://www.piclist.com/member/JMN-EFP-786 > PIC/PICList FAQ: http://www.piclist.com > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu