What is PCIe Lane Segmentation and Why It Has Different Lanes

The PCI-Express interface, also called PCIe or Peripheral Component Interconnect Express, can be quite a complicated specification. When your PC boots, PCIe is what determines the devices that are connected to the motherboard, identifies the links between each one of them creating a traffic map, and negotiates the bandwidth of each link. For this reason there are PCIe lanes , through a process called PCIe lane segmentation , and in this article we are going to explain why it has been designed this way.

A PCIe connection consists of one or more data transmission lanes (up to 16 at this time) connected in series. Each of the lanes consists of two pairs of cables, one pair to transmit and the other to receive data, and in a single PCI-Express socket there can be 1, 4, 8 or 16 lanes (that’s why we always talk about PCIe x16 , for example).

PCIe Lane Segmentation

How do PCIe lanes work?

PCIe is a multi-layered protocol: there is a transaction layer, a data link layer, and then a connecting physical layer. The data link layer is subdivided to incorporate yet another layer called MAC, which is that of media access control. Each lane consists of two unidirectional differential pairs that operate at 2.5, 5, 8 or 16 Gbit / s according to the negotiated capacities, while on the other hand the transmission and reception of data are separate differential pairs, which adds up to a total four data cables per lane.

Each lane is a separate connection between the processor chipset PCI controller (Southbridge) or the processor itself (which is almost always the graphics card slot) and the attached expansion card. Bandwidth scales linearly, so a four-lane connection will have twice the bandwidth of a two-lane connection. Depending on the bandwidth requirements of the connected expansion card, the slot may need to be sized accordingly, which is why you will see that there are motherboards with different sizes in their PCIe sockets.

Carriles PCIe

Therefore, a physical PCIe x16 socket can work with expansion cards that need x1, x4, x8 and x16 bandwidth, and can actually make a connected expansion card work at x16, x8, x4, and x1. as needed. In the same way, if a socket is PCIe x8 you cannot operate an expansion card that requires PCIe x16. To make this all even more confusing, the PCI-Express interface has different versions (currently only PCIe 3.0 and PCIe 4.0).

Buses and theoretical bandwidth

In the following table you can see what the theoretical maximum bandwidth has each of the buses that we can find on a PC:

BUS Bandwidth
PCI 1056 MBps
AGP 8x 2.1 GBps
PCIe 1.0 / x4 1 GBps
PCIe 1.0 / x8 2 GBps
PCIe 1.0 / x16 4 GBps
PCIe 2.0 / x4 2 GBps
PCIe 2.0 / x8 4 GBps
PCIe 2.0 / x16 8 GBps
PCIe 3.0 / x1 1.97 GBps
PCIe 3.0 / x4 3.94 GBps
PCIe 3.0 / x8 7.88 GBps
PCIe 3.0 / x16 15.75 GBps
PCIe 4.0 / x1 3.94 GBps
PCIe 4.0 / x4 7.88 GBps
PCIe 4.0 / x8 15.75 GBps
PCIe 4.0 / x16 31.5 GBps
PCIe 5.0 / x16 63 GBps
Firewire 400/800 400/800 Mbps
USB 1.0 12 Mbps
USB 2.0 480 Mbps
USB 3.0 4.8 Gbps
USB 3.1 10 Gbps
Gigabit Ethernet 1 Gbps
IDE (ATA 100) 800 MBps
IDE (ATA 133) 1064 MBps
SATA 1.5 Gbps
SATA 6 6 Gbps

Why do PCIe lanes matter and what are they used for?

As we have seen, the bandwidth of a motherboard PCI-Express socket depends on its physical connection, since each lane has four pairs of data transmission cables. Through the concept of PCIe lane segmentation, this literally serves so that the controllers can segment, organize and based on this assign bandwidth to the devices that we connect to the PC in a way that is not only orderly, but also adequate.

Thus, if we connect a graphics card to the PC that uses a PCI-Express 3.0 x16 interface, the controller will automatically know what bandwidth and resources it will have to allocate for that device to function properly.

Some examples that we can put are, for example, the functions of the PCIe lanes that the processor manages: integrated graphics, PCIe socket for the graphics card, U.2 sockets for storage devices or even the network card in some category motherboards enthusiastic. For its part, the PCIe lanes managed by the motherboard chipset are those related to the SATA ports, integrated sound card, integrated RAID controller, integrated network card, all other PCIe sockets except the first one, Thunderbolt ports, ports USB, etc.