WHEN IS A BAUD NOT A BIT A DATA COMMUNICATIONS TUTORIAL by JAMES A. WALKER AUGUST 27, 1987 ********************************************************************** Much has been written over the years about the intricacies of data communications. Most of them written by very knowledgeable authors. Still, I question an author's credibility when he makes reference to 9600 baud modems. It has become more and more apparent that people are using the terms bit and baud interchangeably. This usage, more times than not, is incorrect. Additionally, I view BBS messages referencing modem speeds at baud rates, not bits per second (bps). Perhaps it is a pet peeve of mine but I wanted to do something for my new endeavor (hobby wise) and make a contribution which could be considered worthy. So, I decided to draft this tutorial. I hope it is of benefit to those who read it and aides them in understanding the difference between a "bit" and a "baud". A FOUNDATION OF UNDERSTANDING In order to proceed, a common ground of understanding must be established. The purpose and function of a modem is to convert binary information into a signal compatible with today's analog telephone lines; and vise versa. The term "modem" is derived from the major functions performed by a modem. These are, MOdulation and DEModulation. Although this is not a tutorial or text on analog transmission techniques, you must know that modems can modulate a signal in a variety of ways. Common techniques used are: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK). The easiest way to understand modulation, is to picture in your mind, a Sine Wave. Through electronic technology, we are able to change or alter the characteristics of a Sine Wave's shape, frequency, phase and amplitude. Modulation, as it is implemented within a modem, is the creation of a Sine Wave at a certain frequency (called a carrier frequency), then alters or changes it's frequency, phase or amplitude. A modem, when connected to a telephone line, transmits the sine wave which is received at the distant end by another modem. Now to complete the picture. Your PC or terminal sends binary "1's and 0", or "bits" of data to the modem where it is received and then converted to a modulated sine wave, transmitted to the distant modem where the sine wave is demodulated and binary "1's and 0's" are sent to the receiving PC or terminal. Job complete. We all know that a "bit" is a binary 1 or 0 and when 7 or 8 of them are grouped together, we have a byte or character of information. Webster defines a baud as "a variable unit of data transmission speed usu. equal to one bit per second." The two key words are "variable" and "usu." meaning "usually", when Webster uses it. It is, indeed variable but in today's technology and higher transmission speeds, it is not usually equal to one bit per second. A more correct definition for "baud" would be: a variable unit of data transmission speed determined by the rate of change of a modulated signal. WHAT THEN IS A BAUD? As we discussed, data, in it's analog form, is represented by a Sine Wave. If we were to change or alter this Sine Wave in a predetermined and meaningful manner, we can allow the Sine Wave to carry information. A baud is very simply a change in amplitude, frequency or phase of a sine wave. Baud rate is the speed at which we perform this change. Let's look at an example. Today's standard for transmitting at 300 bps was established many years ago by AT&T. We utilize the frequency shift keying schemes developed for the Western Electric manufactured 103J modem. In order to simplify this discussion, I will deal only with half of the full duplex link. When you originate a call to another modem at 300 bps, you transmit to the other modem by shifting the frequency of your sine wave between 1270 Hz and 1070 Hz. A binary "1" is represented by the 1270 Hz signal and a "0" is represented by the 1070 Hz signal. If you were to connect an oscilloscope to the analog line, you would be able to see the sine wave change it's frequency back and forth as data is transmitted. In this example, we are transmitting at 300 bits per second and at 300 baud because each change of the sine wave represented one bit of data. This relationship between a bit and a baud being equal stops very quickly. Why? You may ask. Well it was determined many years ago by two acclaimed scientists and researchers, by the names of Shannon and Nyquist, that the maximum allowable "changes of state" of a sine wave was in direct relationship with the bandwidth of an analog line. They developed, you guessed it, the Shannon/Nyquist theory; which is a mathematical formula proving that the maximum baud rate for a typical unconditioned line having a bandwidth of 2700 Hz is approximately 3000 baud per second. So, only 3000 changes to a sine wave per second can take place before you surpass the capabilities of a typical analog line. How then, do we transmit at speeds in excess of 2400 bits per second (bps)? THE EVOLUTION OF SPEED AND TECHNOLOGY The need to be able to communicate remotely with and between computers dictated advancements in analog data communications. To do so economically, meant that we would have to utilize the existing analog network built by AT&T and it's Operating Companies. To do so efficiently, required that we communicate at high speeds. To do so accurately, meant that we could not exceed the limitations of the analog network. Modem manufacturers rose to the challenge by developing multi-level modems. And here is where we truly delineate between a bit and baud. Multi-level modem means that a single change to the sine wave can represent more than one bit of data. Therefore, one baud can represent two, three or perhaps four bits. Let's take a moment to see how this takes place. At a speed of 1200 bits per second, our modems utilize Phase Shift Keying (PSK) as the method of modulation. The transmit carrier frequency (after handshake) is 1200 Hz for the originator of the call and 2400 Hz for the answerer of the call (for full duplex operation). Our modems are also 4-level. That is, our modems use 4 different shifts in the phase of the sine wave in order to transmit bits of data. The question now is, in the binary world of 1's and 0's and with a 4 level modem, how many bits can each of the 4 different phase shifts carry? The answer of course, is 2. Let's look at all of the possible combinations, 00, 01, 10 and 11. Now to build the correlation between bits and bauds. In one change of the phase of the sine wave, let's say 45 degrees, I am really transmitting 2 bits of data (00). A change of 90 degrees conveys the bits (01). A change of 135 degrees means (10). And last, a change of 180 degrees means (11). So, in this example, we are transmitting 1200 bits per second while actually transmitting at a baud rate of 600. Half the bit rate. By the way, a 4-level modem transmitting two bits of data for each baud is referred to as Di-bit modem. Let's take this another step. Let's look at a tri-bit modem. That is, a modem which transmits 3 data bits for each baud. Most modem manufacturers utilize tri-bit technology for speeds of 4800 bits per second. The modulation technique is usually Phase Shift Keying. Eight different combinations of three bits can be transmitted. Therefore, we have an 8-level modem. The three bit combinations of course are: 000, 001, 010, 011, 100, 101, 110 and 111. The data rate is 4800 bits per second, the baud rate is 1600. One last modulation technique which must be discussed because it is widely utilized in our modems at 2400 bits per second is Quadrature Amplitude Modulation (QAM). This technique combines two modulation techniques together. Phase Shift Keying and Amplitude Shift Keying. In addition, it utilizes both the Sine and Cosine Wave forms. Each of the Waves carries 2 bits of data and when received, are then combined into 4 bit groups. So, at 2400 bits per second, we are actually transmitting at a baud rate of 600. SUMMARY So is a bit and a baud the same? The correct answer is, sometimes. But not really because they each should be used in referring to different aspects of the speed at which data is transmitted. Can they be used interchangeably? Absolutely not! Is there such a thing as a 9600 baud modem? There may be, but it won't work on Ma Bell's lines. Shannon and Nyquist proved their theory in mathematical terms and the industry has proven them to be correct. Last but not least, you might be asking yourself, What is the baud rate of a 9600 bit per second modem? Most 9600 modems are 16-level or 16-point, in which case they are transferring data at a baud rate of 2400. Questions or comments can be left on Compuserve #76266,3455, Wildcat File Cabinet, (214) 351-9859 or Wildcat PC Help, (214) 680-2693. 4) 351-9859 or Wildcat PC Help, (214) 680-2693.