Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!ucsd!nosc!logicon.com!trantor.harris-atd.com!charybdis!sonny From: sonny@charybdis.harris-atd.com (Bob Davis) Newsgroups: comp.sys.ibm.pc.hardware Subject: Re: Baud vs. bps. Message-ID: <4188@trantor.harris-atd.com> Date: 24 Aug 90 18:49:37 GMT References: <849@idcapd.idca.tds.philips.nl> Sender: news@trantor.harris-atd.com Reply-To: sonny@trantor.harris-atd.com (Bob Davis) Organization: Advanced Technology Dept., Harris ESS, Melbourne, FL Lines: 62 In article <849@idcapd.idca.tds.philips.nl> lexw@idca.tds.PHILIPS.nl (Lex Wassenberg) writes: > >Okay, in some recent articles there were these hints about the difference >between "baud" and "bits per second". Now I for one have always thought that >baud and bps is the same. So what's the difference? Has it something to do >with bits that don't carry actual data like start and stop bits? Or is it >more subtle? Thanks to anyone who can inform me (and others). > ________________ Baud rate and Bit rate are two different things (although they are sometimes numerically equal). THE LIGHTS DIM AS THE LEGEND BEGINS... Initially, data rate requirements were low enough that folks were happy to send binary data ONE bit at a time. And the modems employed did SIMPLE things -- like: for a 0, send a tone; and for a 1, send a slightly higher tone. Or things like: for a 0, turn the tone off; and for a 1, turn the tone on. The RATE AT WHICH THE TONES ARE SENT IS THE BAUD RATE. In these simple modems, since ONE BIT WAS CARRIED PER TONE INTERVAL, the baud rate equaled the bit rate. The tone interval is called the baud time. Then, with increasing data rate requirements, the modems got more complicated. In order to double the bit rate, people decided to allow 4 different tone states per baud time, thus allowing 2 bits to be carried per baud time. Some folks chose to modulate the tone to one of 4 different phases depending on the 2 bits. Other folks chose to go to 4 different amplitudes. Thus, the 2 BITS PER BAUD modem arrived, and with it ETERNAL CONFUSION over just what the hay *is* the difference between bps and baud rate ANYWAY. For these more complicated modems, the BIT RATE WAS TWICE THE BAUD RATE. THE BIT RATE *ALWAYS* IS EQUAL TO THE BAUD RATE TIMES THE NUMBER OF INFORMATION BITS CARRIED PER BAUD TIME BY THE MODEM. WHY, you might ask, didn't they just double the rate at which tones were sent and still just use two tone states and get double the throughput that way? Good question. It turns out that real, bandwidth-limited channels - like the telephone channel - shut you down quicker as you try to increase the rate at which tones are sent THAN they do if you choose to send more tone states. The way they "shut you down" as you increase the baud rate is by interference between one tone and the next as you try to send them faster. Real channels "smear" the waveform of one tone into the following one. This is called intersymbol interference, or ISI. Once the designer of the modem has increased the baud rate to the point that he is hitting the ISI "stop", he must increase the number of baud states; i.e., he must assign more bits per baud. Modern modems employ combinations of tone amplitude and phase to achieve 8 or more tone states to obtain even higher data throughputs over the fixed bandwidth telephone channel. Since 8 states will carry 3 bits, an 8-state modem has a bit rate equal to 3 times the baud rate. A 16 state modem has a bit rate = 4 times baud rate, and so on. BPS AND BAUD RATE ARE DIFFERENT. ______________________________________________________________________________ Bob Davis \\ INTERNET : sonny@trantor.harris-atd.com | _ _ | Harris Corporation, ESS \\ UUCP : ...!uunet!x102a!trantor!sonny |_| |_| | | Advanced Technology Dept.\\ AETHER : K4VNO |==============|_/\/\/\|_| PO Box 37, MS 3A/1912 \\ VOICE : (407) 727-5886 | I SPEAK ONLY | |_| |_| | Melbourne, FL 32902 \\ FAX : (407) 729-2537 | FOR MYSELF. |_________|