In a more expensive computer, each device has direct lane access to the processors and ram. This means that each device has direct communication, and therefore can take full advantage of speed.
In order to save costs, manufacturers combine traffic on the same lane. In this example the onboard devices such as USB, Firewire and network share with Slot 1. This means if you have a device in slot 1 that requires a lot of communication with the processors, such as a video card. Then using the onboard ethernet would probably affect the quality of your video on your monitors. This is an extreme example.
This dramatically affects the performance of your system. If you had a video capture card in slot 3, and a drives connected to a card in slot 4, you would have issues getting enough information through the bus, and therefore drop frames, or have unexplained video playback issues. This is not a reliable solution. If you put the video capture card in slot 2, and the drive controller card in slot 3, then they do not cross talk.
Traditionally the more expensive the computer, the more bus segments (or in the example, Lanes). Most manufacturers poorly document the amount of bus segments in their computers. Thus making it more difficult to make a generic white box computer reliable and guaranteed solution. Also, computer manufacturers are always changing and updating to the latest chip and revisions, so one model of computer may vary in bus segments from another, even if they appear to be the same.
Many Video and Audio software companies support specific machines to make the choice easier and the opportunity for success greater. Major manufacturers, like HP and Dell guarantee a specific model will be unchanged. This way once the model is tested and document, it is available for people to purchase.
As computers get faster, and architechture continues to change, eventually everything will have it's own bus segment.
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