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The TCP/IP Guide  9  Networking Fundamentals       9  Network Performance Issues and Concepts

Performance Measurements: Speed, Bandwidth, Throughput and Latency 1 2 3 Theoretical and Real-World Throughput, and Factors Affecting Network Performance

Signaling Rate and the Baud

Finally, there's another term that you will encounter frequently in discussions of modems and some other technologies: the baud. Named for telegraphy pioneer Jean Maurice Emile Baudot (1845-1903), this is a unit that measures the number of changes, or transitions, that occur in a signal in each second. So, if the signal changes from a “one” value to a “zero” value (or vice-versa) one hundred times per second, that is a rate of 100 baud.

In the early days of very slow modems, each bit transition encoded a single bit of data. Thus, 300 baud modems sent a theoretical maximum of 300 bits per second of data. This led to people confusing the terms “baud” and “bits per second”—and the terms are still used interchangeably far too often. You'll commonly hear people refer to a 28.8kbps modem, for example, as running at “28,800 baud”.

But the two units are in fact not the same; one measures data (the throughput of a channel) and the other transitions (called the signaling rate). Modern modems use advanced modulation techniques that encode more than one bit of data into each transition. A 28,800 bps modem typically encodes nine bits into each transition; it runs at 3,200 baud, not 28,800 baud (the latter number being the product of 3,200 and 9). In fact, there's no way to operate a modem on a conventional phone line at 28,800 baud—it exceeds the frequency bandwidth of the phone line. That's the reason why advanced modulation is used to encode more data into each transition.

Wow, when I started writing this topic, I never envisioned that I would have to write so much just to explain something that should be fairly simple. Leave it to computer people to complicate the simple, but well, there you have it. At least you should now be able to figure out what all those terms are about, and can impress your friends and relatives with explanations of why their 56k modem doesn't actually run at 56,000 baud. J

Performance Measurements: Speed, Bandwidth, Throughput and Latency 1 2 3 Theoretical and Real-World Throughput, and Factors Affecting Network Performance