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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

External Performance Limiters

There are external factors that limit the performance of a network. Important issues here include the ability of the hardware to process the data, and also any bandwidth limitations that exist in the chain of data transmission between two nodes. Hardware issues most often show up with very fast networking technologies. Consider a Gigabit (1000 Mbps) Ethernet connection between two regular PCs. In “theory”, this connection should allow the transmission of 1 gigabit of data every second. Well, even beyond the matter of overhead mentioned above, no regular PC is capable of pumping this much data per second if its life depended on it. Only high-end servers have this capacity—and even they would have problems sustaining this unless they were doing nothing else. An older PC's hard disk probably can't even stream data fast enough to keep a 100 Mbit/s Ethernet connection busy. Thus, upgrading a 100 Mbps Ethernet card in an older machine to Gigabit is not likely to help as much as you might expect.

Bandwidth limitations cause network throughput issues because the entire network can only run as fast as its slowest link. These bottlenecks create reduced performance. As a common example, suppose you have a cable modem connection to the Internet that is rated at 1 Mbps for downloads. It may be very fast most of the time, but if the Web site you are accessing is totally bogged down, or it is having connectivity problems itself, you are not going to download from that site at 1Mbps. In fact, probably not even close.

Finally, it’s also important to remember that there are many technologies that simply do not always operate at a constant fixed speed; they may change speeds based on physical network characteristics. A good example is an analog modem, which can vary greatly in performance depending on the quality of the line over which it operates.

The issues I mentioned above are usually ones that you cannot do anything about; they are just the nature of the networking beast. The third category of performance limiters, misconfiguration, is different. This refers to network slowdowns that occur because hardware or software have not been set up correctly. Poor cabling, misconfigured interface cards, or bad drivers can seriously reduce the performance of a network—by 90% or even more.

These problems can usually be corrected, but only if you are looking for them. Driver problems are particularly insidious because the natural tendency is for people to blame hardware when slowdowns occur. However, you cannot get the most of your hardware devices without proper software to run it. These issues are much more significant with “bleeding edge” hardware than with established products, incidentally.

Also included in this category of issues are problems that occur due to poor design. For example, putting 30 busy workstations on a shared 10 Mbit/s Ethernet segment is likely to result in poor performance—using a network switch would be much better, since this would create multiple, independent segments for higher performance. Another common mistake is not providing a “fatter pipe” (higher bandwidth connection) to servers in a client/server setup. These issues can be avoided or ameliorated by reconfiguring the network—or even better, designing it properly in the first place, right?

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