How Gigabit Ethernet Works-Performance Issues

Nowadays several motherboards come with one on-board Gigabit Ethernet port. Some very high-end motherboards can even provide two Gigabit Ethernet ports. Depending on motherboard architecture, however, the Gigabit Ethernet may not achieve its 1.000 Mbps transfer rate.

The problem is how the Gigabit Ethernet chip is connected to the system. If it is connected to the standard PCI bus, it probably won’t achieve its full speed. PCI bus works with a maximum transfer rate of 133 MB/s, while Gigabit Ethernet runs up to 125 MB/s (1.000 Mbps / 8 = 125 MB/s). By just observing these two numbers you could say that Gigabit Ethernet “fits” PCI bus, but the problem is that PCI bus is shared with several other components of your system, thus lowering the available bandwidth. So, even though in theory Gigabit Ethernet can run fine on PCI bus, it is just to close to the bandwidth limit of the bus.

PCI Express, on the other hand, has a maximum transfer rate of up to 250 MB/s and is a point-to-point connection, which means that it doesn’t share this 250 MB/s bandwidth with any other device, thus allowing Gigabit Ethernet to achieve its full speed.

How can one tell which bus Gigabit Ethernet chip is connected to? There are three basic ways. The easiest way is to see if your motherboard is based on PCI Express bus. If it doesn’t, the Gigabit Ethernet chip can only be connected to the standard PCI bus.

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The second way is to take a look at the motherboard manual or the motherboard specs page on the manufacturer's website and look for this information there. Usually on the main specs page it is written “PCI” or “PCI Express” besides the name of the Gigabit Ethernet controller, telling you which bus is used.

The third way is to go to the Gigabit Ethernet controller manufacturer website (VIA, Marvell, 3Com, etc) and look for the main specs for the model used on your motherboard. The bus type should be discriminated there.

Although Gigabit Ethernet original standard was developed for using standard Cat 5 cables, several companies recommend that Cat 5e cables to be used on Gigabit Ethernet networks for performance issues. Cat 5e cables have the same maximum transfer rate as Cat 5 cables, but have a better crosstalk and return loss specs, i.e., they are less susceptible to noise.

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