I/O channels are buses that connect peripherals with main memory. These peripherals will range from disk and network controllers to video controllers, and USB and firewire. Machines will have several of these buses, each connected by a bridge (also referred to as the PCI chipset) into main memory. Because I/O is one of the most common tasks on computers, this subsystem is an integral part of any system.
The most common I/O channel in commodity hardware is the PCI bus. Every machine sold today has at least one; many have multiples of these buses. Many flavors of PCI exist; these buses have been included in commodity hardware since 1994. Earlier versions of the PCI bus were 32-bit, 33 MHz buses. The theoretical maximum rate of data transmission on these buses is 132 MB/s. Good implementations of the PCI chipset are able to provide nearly this rate; maximum observed bus rates greater than 125 MB/s are not uncommon.
Newer revisions of PCI buses are 64-bit buses, running at 66 MHz or higher. These buses have become quite common over the last three to four years. The theoretical maximum rate for these is upwards of 500 MB/s. Good implementations of this PCI chipset provide between 400 and 500 MB/s of read and write bandwidth. Good PCI-X implementations, running at 133 MHz, provide upwards on 900 MB/s of read and write bandwidth.
AGP is a port used for high-speed graphics adapters. It is connected closely with main memory, providing better peak bandwidth than that offered by PCI or PCI-X. AGP devices are able to directly use data out of main memory. AGP is not a bus, like PCI. It is only able to support one device, and systems only have one port. AGP 2.0 provided a peak bandwidth over 1 GB/s to main memory. The successor to this, AGP 3.0, provides upwards of 2.1 GB/s to main memory.
Older machines will also have other buses. The ISA bus is an 8 or 16-bit bus, commonly used in older machines. Vesa local bus is a 24-bit bus, common in some generations of 486 machines. EISA is an extension to ISA that was common in older servers. All of these buses should be avoided if possible: They are slow, and peripheral choice is non-existent.