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

Balancing Network Performance with Key Non-Performance Characteristics 1 2 3 Understanding Performance Measurement Units

There are a number of terms that are commonly used to refer to various aspects of network performance. Some of them are quite similar to each other, and you will often see them used—and in many cases, misused or even abused. J It's a good idea for us to take a look at each of them, therefore, discuss how they are commonly used and what they really mean.

More than just the issue of different terms related to performance, however, is the more important reality that there are multiple facets to performance. Depending on the application, the manner in which data is sent across the network may be more important than the raw speed at which it is transported. In particular, many multimedia applications require real-time performance; they need data sent in such a manner that it will be delivered steadily. For these purposes, raw speed isn't as important as consistent speed, and this is an issue that is often not properly recognized.

Let's take a look at the most common performance measurement terms and see what they are all about.

This is the most generic performance term used in networking. As such, it can mean just about anything. Most commonly, however, it refers to the rated or nominal speed of a particular networking technology. For example, Fast Ethernet has a nominal speed of 100 megabits per second; it is for that reason often called 100 Mbit Ethernet, or given a designation such as “100BASE-TX”.

Rated speed is the biggest “performance magic number” in networking—you see it used to label hardware devices, and many people bandy the numbers about as if they actually were the real “speed of the network”. The problem with using nominal speed ratings is that they are theoretical only, and as such, tell an incomplete story. No networking technology can run at its full rated speed, and many run substantially below it, due to real-world performance factors.

Speed ratings such as “100 Mbps Ethernet” are also often referred to as the “throughput” of a technology, even though the maximum theoretical speed of a technology is more analogous to bandwidth than throughput, and the two are not identical. More on this in the next two bullet points.

Bandwidth is a widely-used term that usually refers to the data-carrying capacity of a network or data transmission medium. It indicates the maximum amount of data that can pass from one point to another in a unit of time. The term comes from the study of electromagnetic radiation, where it refers to the width of a band of frequencies used to carry data. It is usually given in a theoretical context, though not always.

Bandwidth is still used in these two senses: “frequency band width” and data capacity. For example, radio frequencies are used for wireless technologies, and the bandwidth of such technologies can refer to how wide the RF band is. More commonly, though, it refers to how much data can be sent down a network, and is often used in relative terms. For example, for Internet access, a cable or xDSL is considered “high bandwidth” access; using a regular analog modem is “low bandwidth”.

Balancing Network Performance with Key Non-Performance Characteristics 1 2 3 Understanding Performance Measurement Units