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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

Throughput Measurement Units and the Kilo, Mega and Giga Multipliers

The standard unit for bit throughput is the “bit per second”, commonly abbreviated “bit/s”, “bps” or “b/s”. The byte unit is “byte per second”, abbreviated “bytes/s”, “Bps” or “B/s”—unless some cruel author decides to use a lower-case “b” just to confuse you. This means that the maximum theoretical throughput of 100BASE-TX (100 Mbps) Ethernet is about 12 MB/s. Where the context is unclear, it is better to spell out the unit as “100 Mbits/s” or “12 Mbytes/s”, which of course, I try to do in this Guide.

You will also occasionally, especially when dealing in the realm of communications, see throughput measured in characters per second, or “cps”. In most computer systems (including PCs) each character takes up one byte, so “cps” is equivalent to “bytes/s”, “B/s” or “Bps”.

Of course, most networking technologies don't move just a few bits and bytes around every second; they move, thousands, millions, or even billions. Thus, most speed ratings are not in bits per second, but rather kilobits (kb), megabits (Mb), or gigabits (Gb) per second, and the same thing can be done for bytes. Thus, we find terms such as “100 Mbps Ethernet” or “700 kb/s ADSL”.

Here we run into another problem: the existence of both decimal and binary versions of the terms “kilo”, “mega” and “giga”. For example, the decimal form of the prefix for a million (“mega”) is 10⁶ or 1,000,000, while the binary form is 2²⁰ or 1,048,576. This differential of about 5% leads to all sorts of confusion. When you see these abbreviations, bear in mind that in networking, they almost always refer to the decimal form. Thus, 100 Mbps Ethernet is rated at 100,000,000 bits per second, not 104,857,600 bits per second.

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