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

Understanding Performance Measurement Units 1 2 3 Simplex, Full-Duplex and Half-Duplex Operation

When assessing the performance of networks, keep in mind that there is always a difference between theoretical speed ratings, and “real-world” throughput. If you are lucky—rather, if your network is set up well—then this difference is relatively small but still significant. Otherwise, the difference can be extremely large. Notice that there is no option for the difference between theoretical and practical performance being “negligible”!

The reasons for the difference between what a network or communications method is supposed to be able to do and what it can actually do are many. I consider them as generally falling into three categories: normal network overhead, external performance limiters, and network configuration problems.

Every network has some degree of normal network overhead, which guarantees that you will never be able to use all of the bandwidth of any connection for data. Take as an example 10 Mbit/s Ethernet. Sure, the line may be able to transmit 10,000,000 bits every second, but not all of those bits are data! Some are used to package and address the data—data can't just be thrown onto the network in raw form. Also, many of those bits are used for general overhead activities, dealing with collisions on transmissions, and so on. There are natural inefficiencies in any networking technology.

Even beyond this, there are other overhead issues. Any network transaction involves a number of different hardware and software layers, and overhead exists at each of them, from the application and operating system down to the hardware. These overheads mean that you generally lose at least 20% of the “rated” speed of a LAN technology “off the top”, and sometimes even more. For example, 7 Mbits/s user data throughput on a regular 10 Mbit/s Ethernet network is actually very good.

Understanding Performance Measurement Units 1 2 3 Simplex, Full-Duplex and Half-Duplex Operation