What are lanes in PCIE

[sridhars]

My motherboard states that it has

EXPANSION SLOTS
PCI Slots 3
PCIe x1 Slots, Generation 2, 2.0
PCIe x16 Slots, Generation 3.0 and 1, 2.0, 1

What does it mean? What can I install in these? What are lanes in PCIE which graphic cards ask for?

 

[rhysiam]

The PCI slots are legacy (old) slots that are included in case you want to connect some old (or new but cheap devices). There's not a lot that uses PCI anymore (some cheap network cards still do), so you can probably just ignore those.

The vast majority of add-in cards, including all modern Graphics cards use PCIe.
We're currently up to the third generation of PCIe, though everything is backwards compatible (a PCIe Gen 3 video card will run happily on a Gen 2 slot, and vice versa).
Each PCIe slot has a number of "lanes" allocated to it: between 1 and 16. Basically the more lanes, the more bandwidth the device has to communicate with the rest of the system.

PCI Gen 1 has 2Gb per lane
PCI Gen 2 has 4Gb per lane
PCI Gen 3 has (a little under) 8Gb per lane.

A graphics card will run off a single lane... but it will be hamstrung by the lack of bandwidth.

It's really only an issue if you're trying to run multiple cards with high bandwidth requirements. That basically means multiple Graphics Cards or high end RAID Controller cards or crazy high end network cards... everything else (sound cards, Wifi Cards, etc) will usually run just fine of a single PCIe lane.

Basically: If you have a single graphics card, make sure you put it in the slot that offers the full 16 lanes.
If you're trying to run multiple graphics cards... you're not going to do it well with that motherboard.

 

[rhysiam]

OK, I'll have another go...

There are two things that determine the speed at which a card in a PCIe slot can communicate with the rest of the system:

1) The number of PCIe lanes it can utilise (usually denoted by x1, x4, x8, or x16)
2) The speed at which each each of the lanes runs at (determined by the generation... i.e. PCIe Gen 1, Gen 2 or Gen 3) - see the list in my first post above.

The lanes are actually similar to the lanes a road... the more lanes, the more traffic can be sent and received.

So, a PCIe 3 x16 slot provides 16 lanes with (a little under) 8Gbits per lane: 16x8 = 128Gbit per second of bandwidth (or a little under to be precise): Provided that the card that is plugged into it supports PCI Gen 3. If the Card only supports PCIe Gen 2, everything will still work fine, but just at the PCIe2 data rate... (4Gb per lane x 16 = 64Gbps)

Alternatively, a PCIe 2.0 x 4 slot will provide just 4Gb per lane, with 4 lanes: 4 x 4Gbps = 16Gbps... or roughly an 8th of the bandwidth of a PCIe3.0 x 16 slot.

As a rule of thumb for Graphics Cards: for most cards in most situations 32Gbps is enough to make sure the card isn't ever waiting for data over the PCIe interface. That means if a card has 4 PCIe Gen 3, or 8 PCIe Gen 2 lanes, it'll run just fine. Go lower than that and you will start seeing lower frame rates as a result of the card sitting idle while it waits for data. If you're running ulta-high resolutions, or ultra-high end cards, you may need more than 32Gbps, but that's unusual.

Hopefully the above makes sense and allows you to understand what it means when a particular slot denotes the lanes and PCI gen that it runs at.

Things can get a little more complicated, so let me just add a little more to the picture.
Most Intel CPU & Chipsets only provide the following:
- a total of 16 PCIe 3.0 lanes from the CPU AND
- a further 4 PCIe 2.0 lanes from the chipset.
It's then up to the motherboard manufacturers to decide how to allocate those lanes.

Most motherboards provide several PCIe slots, but some will only have a single lane (they're the really short ones - fine for a WiFi card or a sound card, but would seriously hamsting a graphics card). Some motherboards have 2 long PCIe slots, that look like they have 16 lanes... however often the second of these slots only has access to the 4 PCIe 2.0 lanes that come from the chipset... which would probably have an affect on a Graphics Card.
The better motherboards have 2 PCIe slots which will operate at Gen 3.0 with 16 lanes when only one card is installed, but will split these lanes across both slots if they detect a card in each, providing 8 PCIe Gen 3 lanes to each slot, which is ample bandwidth for just about any use-case.

There are more extreme (and expensive) solutions to enable additional PCIe lanes... but that's only relevant in extreme builds.

Hopefully that answers your question?

Do you want specific advice? If so... what do you want to put in the slots?