In networking and computing fields, every invention is aimed at solving problems left in previous stages. The emergence of the PCIe (peripheral component interconnect express) standard is to meet the needs for higher bandwidth, greater flexibility, and better performance of networking devices. Since its debut in 2001, PCIe has been rapidly developed and widely applied in many network devices, especially for the PCIe card (aka PCI Express card, PCIe-based card). The definition, working principle, advantages, types, and selection of PCI Express cards are all included in this comprehensive introduction.
PCIe card refers to a kind of network adapter with a PCIe interface, used in motherboard-level connections as an expansion card interface. Specifically, PCIe-based expansion cards are designed to fit into PCIe-based slots in the motherboard of devices like host, server, and network switch. Most recent PC's motherboards have PCIe slots just for PCIe cards. The PCI Express card can be installed in the corresponding PCIe slot. Normally speaking, the slot is as wide as or wider than the card.
Instead of performing like a bus that handles data from multiple sources, a PCI Express card can achieve a series of point-to-point connections via switches, in order to control where the data needs to go. After inserting a PCIe network card, a logical connection will be formed between the slot and it to communicate with each other. This connection, called an interconnect or link, enables a point-to-point communication channel between two PCIe ports, and allows both of them to send and receive ordinary PCI requests and interrupts. As the following diagram illustrates, the PCIe slot contains one or more lanes. For the x2 link, each lane is composed of two different data transferring pairs, one pair for transmitting data and the other for receiving data. Therefore, each lane consists of four wires or signal traces.
Figure 1: How PCIe Card Works?
Before the introduction of the PCIe card, there are two main network adapters available in the market: PCI and PCI-X card. With a completely different connector and electrical design, the newly-launched PCI-E card makes an improvement based on the former two and becomes the most commonly-used one among them. The article PCI vs PCI-X vs PCI-E, Why Choose PCI-E further explains why we use PCI-E most often.
A variety of formats have been developed for PCI Express cards. In the following part, PCI Express card types will be uncovered in terms of sizes and versions.
The size of any PCI Express card is normally indicated by the number of lanes. Generally speaking, there are five physical sizes of PCIe cards: x1, x4, x8, x16, and x32. (FYI, PCIe x32 does exist with a maximum of 32 lanes, but it's ultra-rare and not mainstream.) The number after the "x" refers to the number of lanes in the PCIe slot. For example, a PCIe x4 card means the card has four lanes.
Figure 2: Comparison of Different PCIe Card Sizes
In applications, you will need to insert the PCIe card into a PCIe slot of a host or server with the same size and configurations of that card. However, confronted with cases like slot shortage, the PCIe card also can be fitted into a wider slot. For instance, you can place a PCIe x8 card into a PCIe x16 slot when the PCIe x8 slot has been occupied, but that card will always run in PCIe x8 mode. For more details about the PCIe card sizes, you can refer to the following table.
|PCIe Slot Width||Numbers of Pins||Length|
|PCIe x1||18||25 mm|
|PCIe x4||21||39 mm|
|PCIe x8||49||56 mm|
|PCIe x16||82||89 mm|
PCI Express is a high-speed serial computer expansion bus standard for connecting high-speed components, which has replaced the older AGP (Accelerated Graphics Port), PCI, and PCI-X bus standards. PCIe standard has undergone numerous revisions through improving performance and other features. PCIe 1.0 was initially launched in 2002. In order to meet the growing demands for higher bandwidth, successive versions have been invented and introduced to the market. At present, there are five different generations of PCIe standards: PCIe 1.0, PCIe 2.0, PCIe 3.0, PCIe 4.0, and PCIe 5.0. The PCIe 6.0 will come in the not-so-distant future. Note that, transfer rate doubles with each generation.
Five versions of PCIe cards are applied accordingly: PCIe 1.x, PCIe 2.x, PCIe 3.x, PCIe 4.x, and PCIe 5.x are all available currently. And PCIe 6.x is to be issued in around 2021. The PCIe 4.x card is the most commonly used one among them, and the PCIe 5.x with higher performance is the latest version, just released in 2019.
It's worth mentioning that all PCIe card versions are backward compatible. That is to say, any version of the PCIe card and motherboard can work together in the mode of the lowest version. In the following table, a comparison of transfer rates among five typical PCIe versions (take the original version n.0 for example) has been listed for your better understanding.
|PCIe 1.0||2003||2.5 GT/s (250 MB/s)||40 GT/s (4.0 GB/s)||8b/10b|
|PCIe 2.0||2007||5.0 GT/s (500 MB/s)||80 GT/s (8.0 GB/s)||8b/10b|
|PCIe 3.0||2010||8.0 GT/s (984.6 MB/s)||128 GT/s (15.75 GB/s)||128b/130b|
|PCIe 4.0||2017||16.0 GT/s (1969 MB/s)||256 GT/s (31.51 GB/s)||128b/130b|
|PPCIe 5.0||2019||32.0 GT/s (3938 MB/s)||512 GT/s (63.02 GB/s)||128b/130b|
First and foremost, to use a PCI Express card, your computer must have at least one available PCI Express slot. If you're applying a wired network without purchasing a designed system, then you should look for some PCIe cards. However, it is confusing to pick a suitable PCIe card. Some factors need to be paid attention to when selecting:
PCIe Card Version and Slot Width: make sure that the PCI Express card type is compatible with your current equipment and network environment.
Protocol Standard: whether the card supports standards you need like RDMA, RoCE, iSCSI, and FCoE is necessary to know before buying.
Controller: chips from Intel, Broadcom, Mellanox, and Realtek are the trend.
After nailing down the above mentioned three factors, there are still some variants that influence your choices like transmission speed, port number, connector type, operating system, brand, and price, etc. A complete network adapters' buying guide may help you: How to Choose A Network Card?
The high-end software continues to evolve at breakneck speed to meet more demands, being the impetus for ongoing breakthroughs in PCIe performance. Obeying the de facto PCIe standard, the new invention of PCIe cards like PCIe4.x, PCIe 5.x, and the unborn PCIe 6.x has proven to narrow the development gap between PCIe-based cards and hosts, possessing great potential.
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