"Play it again Sam."  "Luke, I am your father"  "If you build it they will come".  These are all often quoted misquotes.  Great quotes, but not what the original says.  Looking into the high tech world, we have a list of misquotes of our own.  The one for today is "PCI Express* is 2.5 Gigabits per second".  This is actually a dangerous quote in that it actually creates confusion on the nature of the bus.    Let's look at why.

 

The PCI Express protocol is based on transfers per second.  The listing of the speed is better expressed as 2.5 Giga Transfers per second.  The PCI Express is also an 8bit/10bit symbol based system, so the 2.5 Giga Transfers(GT/s) comes down to just around 2 Gigabits of transactional data.  A functional speed of 250MB/s per lane. When combined with the overhead of the networking device it comes down just a smidgen more.  Now lets talk PCI Express 2.0, also known as PCIe v2.0, AKA Gen 2, AKA Miguel Sanchez.

 

Okay the last one isn't true, and only the "PCI Express® Base Specification Revision 2.0" name is actually embraced by the PCI Special Interest Group.  The base specification for 2.0 includes both a 2.5 Giga Transfer mode and a 5.0 Giga Transfers mode.  You can be PCIe v2.0 compliant and have only 2.5 Giga Transfers.  The first Intel Networking part that supports PCIe v2.0 5.0 GT/s is the Intel® 82599 Ethernet Controller.  Our Intel® 82576 Ethernet Controller is PCIe v2.0 compliant, and runs at 2.5 GT/s.  For a dual 1 Gigabit port (like the 82576), the higher speed is just not necessary.  A dual 1 Gigabit on a x4 would not come close to saturating the bus.  5.0 GT/s isn't needed for 1 Gigabit unless your going to try to do dual port over a x1 lane width.  But the x1 physical connector doesn't hold a card in the slot very well given the physical size required to do a dual port.  That does make x2 a very attractive interface, but x2 is fairly rare so far.  Most vendors (including us) are staying with the x4 and 2.5 GT/s for now.

 

Let's go for the big wrap up.

1)  PCI Express speed is defined in terms of Transfers, not Bytes, or bits.

2)  PCI Express v2.0 includes both 2.5 Giga Transfers a second and 5.0 Giga Transfers modes.

3)  The 82576 does PCIe v2.0 at 2.5 GT/s and the 82599 does v2.0 at 5.0 GT/s (and 2.5 GT/s if that's all you got)

4)  Thanks for using Intel Networking products.