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How PCI Express Works

by Tracy V. Wilson

Browse the article How PCI Express Works 

Introduction to How PCI Express Works

Peripheral Component Interconnect (PCI) slots are such an integral part of a

computer's architecture that most people take them for granted. For years, PCI has

been a versatile, functional way to connect sound, video and network cards to a

motherboard.

But PCI has some shortcomings. As processors, video cards, sound cards and

networks have gotten faster and more powerful, PCI has stayed the same. It has a

fixed width of 32 bits and can handle only 5 devices at a time. The newer, 64-bit PCI-X bus provides more bandwidth, but its greater width compounds some of PCI's other 

issues.

A new protocol called PCI Express (PCIe) eliminates a lot of these shortcomings,

provides more bandwidth and is compatible with existing operating systems. In this

article, we'll examine what makes PCIe different from PCI. We'll also look at how

PCI Express makes a computer faster, can potentially add graphics performance, and

can replace the AGP slot.

Photo courtesy Consumer Guide Products

Thank YouThanks to Joshua Senecal for his assistance with this article.

High-Speed Serial Connection

In the early days of computing, a vast amount of data moved over  serial connections.

Computers separated data into packets and then moved the packets from one place to

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another one at a time. Serial connections were reliable but slow, so manufacturers

began using parallel connections to send multiple pieces of data simultaneously.

It turns out that parallel connections have their own problems as speeds get higher and

higher -- for example, wires can interfere with each other electromagnetically -- so

now the pendulum is swinging back toward highly-optimized serial connections.Improvements to hardware and to the process of dividing, labeling and reassembling

packets have led to much faster serial connections, such as USB 2.0 and FireWire. 

PCI Express is a serial connection that operates more

like a network than a bus. Instead of one bus that

handles data from multiple sources, PCIe has a switch

that controls several point-to-point serial connections.

(See How LAN Switches Work for details.) These

connections fan out from the switch, leading directly

to the devices where the data needs to go. Every

device has its own dedicated connection, so devicesno longer share bandwidth like they do on a normal bus. We'll look at how this

happens in the next section.

PCI Express Lanes

When the computer starts up, PCIe determines which devices are plugged into the

motherboard. It then identifies the links between the devices, creating a map of where

traffic will go and negotiating the width of each link. This identification of devices

and connections is the same protocol PCI uses, so PCIe does not require any changes

to software or operating systems.

Sizing UpSmaller PCIe cards will fit intolarger PCIe slots. The computer simply ignores the extraconnections. For example, a x4card can plug into a x16 slot. Ax16 card, however, would be toobig for a x4 slot. 

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Each lane of a PCI Express connection contains two pairs of wires -- one to send and

one to receive. Packets of data move across the lane at a rate of one bit per cycle. Ax1 connection, the smallest PCIe connection, has one lane made up of four wires. It

carries one bit per cycle in each direction. A x2 link contains eight wires and transmits

two bits at once, a x4 link transmits four bits, and so on. Other configurations are x12,

x16 and x32.

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Scalable PCI Express slots.

PCI Express is available for desktop and laptop PCs. Its use may lead to lower cost of 

motherboard production, since its connections contain fewer pins than PCI

connections do. It also has the potential to support many devices, including Ethernet cards, USB 2 and video cards.

Two by TwoThe "x" in an "x16" connection stands for "by." PCIe connectionsare scalable by one, by two, by four, and so on. 

But how can one serial connection be faster than the 32 wires of PCI or the 64 wires

of PCIx? In the next section, we'll look at how PCIe is able to provide a vast amount

of bandwidth in a serial format.

PCI Express Connection Speeds

The 32-bit PCI bus has a maximum speed of 33 MHz, which allows a maximum of 

133 MB of data to pass through the bus per second. The 64-bit PCI-X bus has twice

the bus width of PCI. Different PCI-X specifications allow different rates of data

transfer, anywhere from 512 MB to 1 GB of data per second.

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Devices using PCI share a common bus, but each device usingPCI Express has its own dedicated connection to the switch.

A single PCI Express lane, however, can handle 200 MB of traffic in each direction

per second. A x16 PCIe connector can move an amazing 6.4 GB of data per second in

each direction. At these speeds, a x1 connection can easily handle a gigabit Ethernet

connection as well as audio and storage applications. A x16 connection can easily

handle powerful graphics adapters.

How is this possible? A few simple advances have contributed to this massive jump in

serial connection speed:

•

Prioritization of data, which allows the system to move the most importantdata first and helps prevent bottlenecks

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• Time-dependent (real-time) data transfers

• Improvements in the physical materials used to make the connections

• Better handshaking and error detection

• Better methods for breaking data into packets and putting the packets together 

again. Also, since each device has its own dedicated, point-to-point connection

to the switch, signals from multiple sources no longer have to work their waythrough the same bus.

Slowing the BusInterference and signal degradation are common in parallelconnections. Poor materials and crossover signal from nearbywires translate into noise, which slows the connection down. Theadditional bandwidth of the PCI-X bus means it can carry moredata that can generate even more noise. The PCI protocol alsodoes not prioritize data, so more important data can get caught inthe bottleneck. Using the Accelerated Graphics Port (AGP) slotfor video cards removes a substantial amount of traffic, but notenough to compensate for faster processors and I/O devices. 

PCI Express and Advanced Graphics

We've established that PCIe can eliminate the need for an AGP connection. A x16

PCIe slot can accommodate far more data per second than current AGP 8x

connections allow. In addition, a x16 PCIe slot can supply 75 watts of power to the

video card, as opposed to the 25watt/42 watt AGP 8x connection. But PCIe has even

more impressive potential in store for the future of graphics technology.

Photo courtesy Consumer Guide Products

PCI Express video card

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Photo courtesy Consumer Guide Products

AGP 8x video card

With the right hardware, a motherboard with two x16 PCIe connections can support

two graphics adapters at the same time. Several manufacturers are developing and

releasing systems to take advantage of this feature:

• NVIDIA Scalable Link Interface (SLI): With an SLI-certified motherboard,

two SLI graphics cards and an SLI connector, a user can put two video cards

into the same system. The cards work together by splitting the screen in half.

Each card controls half of the screen, and the connector makes sure that

everything stays synchronized.

Photo courtesy NVIDIA

NVIDIA SLI link card

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• ATI CrossFire: Two ATI Radeon® video cards, one with a "compositing

engine" chip, plug into a compatible motherboard. ATI's technology focuses

on image quality and does not require identical video cards, although high-

performance systems must have identical cards. Crossfire divides up the work 

of rendering in one of three ways:

o splitting the screen in half and assigning one half to each card (called"scissoring")

o dividing up the screen into tiles (like a checkerboard) and having one

card render the "white" tiles and the other render the "black" tiles

o having each card render alternate frames

• Alienware Video Array: Two off-the-shelf video cards combine with a Video

Merger Hub and proprietary software. This system will use specialized

cooling and power systems to handle all the extra heat and energy from the

video cards. Alienware's technology may eventually support as many as four 

video cards.

Photo courtesy NVIDIA

Two video cards running parallel

Since PCI, PCI-X and PCI Express are all compatible, all three can coexist

indefinitely. So far, video cards have made the fastest transition to the PCIe format.

Network and sound adapters, as well as other peripherals, have been slower indevelopment. But since PCIe is compatible with current operating systems and can

provide faster speeds, it is likely that it will eventually replace PCI as a PC standard.

Gradually, PCI-based cards will become obsolete.

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Bandwidth

PCI Express in all it's flavors: 1x, 2x, 4x, 8x, 16x and 32x all have much greater bandwidththan basic PCI.

Common Buses and their Max Bandwidth

PCI 132 MB/sAGP 8X 2,100 MB/s

PCI Express 1x 250 [500]* MB/s

PCI Express 2x 500 [1000]* MB/s

PCI Express 4x 1000 [2000]* MB/s

PCI Express 8x 2000 [4000]* MB/s

PCI Express 16x 4000 [8000]* MB/s

PCI Express 32x 8000 [16000]* MB/s

IDE (ATA100) 100 MB/s

IDE (ATA133) 133 MB/s

SATA 150 MB/sGigabit Ethernet 125 MB/s

IEEE1394B [Firewire] 100 MB/s

* Note - Since PCI Express is a serial based technology, data can be sent over the bus in twodirections at once. Normal PCI is Parallel, and as such all data goes in one direction around the loop. Each 1x lane in PCI Express can transmit in both directions at once. In the table thefirst number is the bandwidth in one direction and the second number is the combined bandwidth in both directions. Also please note that in PCI Express bandwidth is not shared the same way as in PCI, so there is less congestion on the bus.

Increased bandwidth can be equated into increased system performance. We've long knownthat to get the most out of your processor you need to get as much information into it aspossible, as quickly as possible. Chipset designers have consistently addressed this byincreasing Front Side Bus speeds. The problem with this is that front side bus speedincreases the speed of transfer between the memory and CPU but often you've got data that'scoming from other sources that needs to get to the memory or CPU like drives, networktraffic, video, etc. PCI Express addresses this problem head on by making it much faster andeasier for data to get around the system.

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Physical Differences: PCI Express [ PCI-E ] vs AGP vs PCI

Currently, the most common use for PCI Express is Video. On thegraphic at right you can see the

physical differences between thecards.

The connector on PCI Expressvideo cards will always start out witha small piece not directly joined withthe rest of the slot, and then anotched piece not directly joinedwith the rest of the slot. This notchgoes into the 1x portion of the 16xslot. The characteristic notch makesit easy to tell the difference betweenPCI-E (PCI Express) and an AGP

Video card. As you can imagine, aPCI Express Video card will not fitinto an AGP card slot, and a AGPVideo card will not fit into a 16x PCIExpress card slot.

Another physical difference betweenPCI Express Cards, AGP, and PCIis the distance between the card'sbracket and the start of theconnector. On PCI Express cards,there is very little distance betweenthe metal bracket and the start of 

the connector. On both PCI andAGP the distance is much longer.

PCI Express 1x / 4x cards also havethe physical difference in regard tobracket distance. They are both agood deal smaller than standardPCI. At this time this type of card isstill quite rare so at this point there islittle chance of confusing them withsomething else. The PCI Express 1xconnector does however bare someminor resemblance to an AMR slot,so it is important not to confuse thetwo. No motherboard on the markettoday currently has both the older AMR slot and PCI Express.

Q&A Common Questions aboutPCI Express

Q:Is PCI Express Faster ThanPCI? A:PCI Express is much faster thanPCI. For 1x Cards it at least 118%faster . When you compare PCI

Express video to PCI Video thedifference is enormous: PCI Express16x video is over 29x faster than

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PCI Video.

Q:Is PCI Express Video Faster than AGP Video?A:Yes and No. A 16x PCI Express connection is at least 190% Faster than AGP 8x but this isthe connection between the system and the video card. You use the connection the mostwhen your video card is low on memory or when the game you are using uses a Direct X or 

Open GL feature that isn't supported in hardware.

So, what this means is that in terms of real world performance there may not be a hugedifference between AGP and PCI Express if you are talking about identical chipsets.Unfortunately this is very hard to prove because graphics chipsets are designed either for PCIExpress or AGP. If you have a card that is available in both forms then you have a graphicschipset that was designed for PCI Express and has a special bridge chip installed to let itcommunicate with the AGP bus. The short of this is: if two cards of the same chipset areavailable in AGP and PCI-E then the PCI-E one will always be faster . On PCI-E you don'thave the overhead of the bridge chip so it's faster, and you have the better bandwidth so inintense situations such as high resolution gaming you'll come out on top every time.

The main point here is: If you have a system with AGP on it, it doesn't make sense to upgrade

just to get PCI-E video right now. The fastest AGP card to ever come out is likely to be thenVidia 6800GT. If you are at a point where that is too slow then by all means it makes senseto make a complete switch. If your happy with you're AGP graphics options, wait until you areready to upgrade the processor or other components before making the PCI-E switch. For more information on AGP and PCI please see the general FSB guide.

Q:What is SLI?A:SLI or Scalable Link Interface is a technology that lets you take two identical nVidia basedgraphics cards *that support SLI* and a motherboard *that supports SLI* to achieve a veryhigh level of video performance. SLI works by splitting the rendering of the screen betweenthe two cards- one card renders half, the other card renders the other half. This technique isextremely effective. For instance two 6600GT cards in SLI can do vastly better than a6800GT or X800 card even though the price is lower for two 6600GT Cards. The downside to

this is that SLI is still new and is limited to systems based on AMD 64 / AMD FX Socket 939processors. Needless to say though, there are some good solutions like this one from Asus.

Q:Do I need a special power supply for PCI-E [PCI Express].A:Yes and no. Although the PCI-E spec calls for a PCI Express power connector, most PCI-Ecards don't currently use it. This means that you should only probably worry about this if youare buying bleeding edge PCI Express parts. Card based on the ATI X600, ATI X700, ATIX300, ATI X1300, Nvidia 6600, Nvidia 7600 or Nvidia 7300 series graphic chipsets rarely usethe connector. If you are in a situation where you need a PCI Express power connector butthe power supply doesn't have one you can always just use a PCI Express Power Adapter  that converts a 4-pin molex connector to PCI-E Power.

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