PCI Express (PCIe) standard's emergence is to meet the needs for higher bandwidth, greater flexibility, and better performance of networking devices. PCIe has been fully developed and widely applied in many network devices after debut, especially for the PCIe card. What is PCI express card? How many types of PCIe cards are there? What does PCIe x8, or PCIe x16 mean? Can I use a PCIe 3.0 card in a 2.0 slot? How to find the best PCI express card? Find the answers in this post.
What Is PCIe Card?
PCIe card (aka PCI Express card, PCIe-based 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 to be installed in the corresponding PCIe slot. (The slot is as wide as or wider than the card.)
PCI vs PCI-X vs PCI-E are the three versions of network adapters with different designs and topologies. The latest PCIe card has made improvements and become an inevitable trend when choosing network cards.
Further learning: PCI vs PCI-X vs PCI-E, Why Choose PCI-E Card?
How Does a PCIe Card Work?
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 to control where the data needs to go. After inserting a PCIe network card, a logical connection will be formed between the slot and the card to communicate, which is called an interconnect or link, enabling a point-to-point communication channel between two PCIe ports and allowing 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.
How Many Types of PCIe Cards are there?
Based on PCIe Card Sizes
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.
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 |
Based on PCIe Card Versions
PCIe 1.0 was initially launched in 2003. To meet the growing demands for higher bandwidth, four successive versions have been introduced to the market including 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. Five versions of PCIe cards are applied accordingly to the five standard generations: PCIe 1.x, PCIe 2.x, PCIe 3.x, PCIe 4.x, and PCIe 5.x. PCIe 6.x is to be issued in around 2021.
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.
| Version | Introduced | Transfer rate (throughput, x1) | Transfer rate (througput, x16) | Line code |
|---|---|---|---|---|
| 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 |
How to Select a Desirable PCIe Card?
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. Some factors need to be paid attention to when selecting a suitable PCIe card:
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.
NIC Card Buying Guide: How to Choose A Network Card?
Bottom Line
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 PCIe 4.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.
