For the last ten years the PCI Bus has been the PC connectivity stalwart, but in 1997 the AGP port brought into view its many limitations.
The AGP port brought with it increased bus speed / bandwidth and its point to point protocol meant AGP used its own pathway to communicate with the central processing unit of your PC and your computer’s memory. This was in contrast to the PCI bus where all the devices connected to your computer share the 133 MBs of bandwidth available to it. AGP and PCI Bus are based on 32 bit bus but AGP at 66 Mhz had double the bandwidth of PCI at 33Mhz. AGP presently has the ability to transfer data at a maximum of 8x, has ~2100MB/s of bandwidth). There are several high speed I/O devices available today like SATA / ATA150 (150 MB/s), Gigabyte Ethernet (125 MB/s), 1394B (100 MB/s) and any one of them can easily outdo the PCI Bus. However, with the introduction of PCI Express this is about to change.
So what is PCI Express?
PCI Express is the replacement for both the current version of PCI and the AGP system, but how is PCI Express different from what has gone before and why is it superior?
* Classic PCI Bus technology was based upon a parallel architecture while the new PCI Express is serial based. This feature drastically reduces pin count.
* PCI Express is a point to point protocol much like AGP, so the devices on your PC do not have to share bandwidth
PCI Express – Serial vs. Parallel Connections
Why do serial systems make such a difference? As an example take the Serial ATA controller cards and drives that recently emerged in the market. SATA was supposed to provide performance benefits that never really materialized but its interface is far superior.
When compared to the old parallel one, a serial ATA cable uses 8 wires while the older IDE cable required an 80 pin connection (40 for transmission and another 40 as ground). Parallel connections present problems when trying to upgrade their speed. Signals degrade and interference become troublesome (due to the 40 ground wires required in an IDE cable).
Serial connections reduce the complexity of the communications protocol thus reducing the pin count. Due to this the transmission speed may be significantly increased.
Through its transition from a parallel to a serial based architecture PCI Express aims to achieve a similar aim. PCI Express features a lower implementation cost, higher bandwidth per pin and scalable performance -all this, in an attempt to avoid the pitfalls of the current version.
The PCI Express is different in two ways- 2.5 Gb/s per direction translates into about 250 MB/s per direction, (twice the bandwidth of the current version) and this speed is available to each device. Secondly, the devices and slots that require high bandwidth can have extra lanes added to them (x2, x4, x8, x12, x16 and x32 lanes are possible).
Switching from AGP / PCI to PCI Express
If you are an avid gamer there is a downside to PCI Express. There will not be an AGP slot on PCI Express motherboards so this means that the new 9800XTs and 5950 Ultras will probably not work when after you upgrade your motherboard. It may be a good idea to avoid upgrading to a high end part, at this stage.
PCI Express is catching on rather slowly at present since the vast majority of current peripherals like SATA, IDE and Gigabyte LAN are integrated into the motherboard directly. On the i875, Intel has a Communications Streaming Architecture (CSA) bus dedicated for Gigabit LAN, in an attempt to avoid saturating the PCI bus. PCI Express will do away with the need for such additions.
Nforce 3 250 has Gigabit Ethernet built into its southbridge featuring a high speed connection to the processor but the PCI Express seems targeted towards the enterprise sector of the market (it makes good competition for PCI-X in terms of bandwidth and is likely to prove more cost effective.
RAID and SCSI cards will be appearing on the PCI Express bus. Look for SATA controller cards and 1394B cards on the PCI Express front.
HyperTransport Compatibility
Chipsets including the K8T800 and Nforce 3 rely on the HyperTransport 1 spec as a chip to chip interconnectivity protocol. The newer HyperTransport 2 specifications will provide mappings to PCI Express. HyperTransport and PCI Express promoters tell us that HyperTransport and PCI Express are not competing technologies (PCI Express features backplane connectivity protocols while HyperTransport is a chip to chip connectivity solution). HyperTransport is also and not a serial based connection.
Why Switch to PCI Express
Advancing from AGP 4x to 8x showed no real performance benefits to video cards for gaming, so perhaps the problem with video cards in the gaming environment is not due to its I/O interface. Increasing the bandwidth available to the videocard to double its speed isn’t deemed necessary right now but may prove to become so in future (when enough geometry and textures have been passed through the bus to saturate an AGP 8X bus). Low latency was also a design feature goal of PCI Express.
Having said this, the real benefits of PCI Express aren’t really concrete at present. Right now its main benefit seems to be the likelihood that buying a video card like this will guarantee that it will work in future mainboards where as AGP will have a limited life.
Editing HD Video may see some immediate bonuses from PCI Express. Programs like Adobe Premiere, see hardware stressed when trying to navigate a large file and being able to manipulate video directly from the video card will be a definite plus.
PCI Express will be an invaluable step in bringing convergence to the PC industry. It will do this by modernizing and standardizing the I/O standard. Manufacturers will probably benefit more (at least initially) than the end user. It is slanted to be more affordable to implement than both the classic PCI Bus and the AGP slot. It may avoid the need for custom buses like the CSA which the Intel 875 chipset uses for Gigabit Ethernet.
PCI Express is focused towards the graphics segment and the greatly increased in bandwidth offered by the PCI Express x16 slot but beware, the problem is that current AGP cards will not work which means costly upgrades on the graphics front. The extra bandwidth offered by PCI Express is slated as translating into better performance but may not hold up in reality. Its extra bandwidth probably won’t be required initially nor for some time as the AGP bus is not the limiting factor on latest generation video cards. Improved latency may help, but users may not notice much difference between AGP and PCI Express cards at first.