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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

People who make networking hardware, or write materials that try to tell you how to operate it, make use of many terms to describe performance, such as throughput and bandwidth. In addition, they also use several different units to measure performance. Unfortunately—and I'm sure you knew this was coming—these units are often used incorrectly, and they are also very similar to each other in name. Worse, they also have overlapping abbreviations, and lots of people use these abbreviations without making clear what the heck they are talking about. Isn't that great?

The first issue is “the infamous letter B”. Or rather, I should say, the matter of the “big B” and the “little b”.By popular convention, the capitalized “B” is supposed to be used for “byte”, and the lower-case “b” for bit—this is the way these abbreviations are always used in this Guide. (A byte is normally eight bits; sometimes the term octet is used instead. If you aren’t familiar with these terms, refer to the primer on binary basics, where you will also find a discussion of the small “controversy” related to bytes and octets.)

Unfortunately, this convention is not followed strictly by everyone. As a result, you may on occasion see “b” being used to refer to bytes, and “B” used for bits. This “b” and “B” business causes a tremendous amount of confusion sometimes, with people mistaking bits for bytes and accidentally thinking that networks are running eight times faster or slower than they really are.

Bear in mind when looking at speed ratings that they are almost always given in terms of bits, not bytes. The “56k” in a “56k modem” means 56,000 bits, not 56,000 bytes, of theoretical transfer speed. (This is true even if someone calls it a “56K” modem.) Similarly, Fast Ethernet operates at 100 megabits per second, not megabytes, and a 1.544 Mbps T-1 link sends a theoretical maximum of 1,544,000 bits each second. This, at least, is usually pretty consistent.

When it comes to throughput measurements, however, both bits and bytes are used, so you have to be careful. Raw throughput values are normally given in bits per second, but many software applications report transfer rates in bytes per second, including many Web browsers and FTP client programs. This often leads to users wondering why they are only getting one eighth of their expected download or transfer speeds.

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