Ethernet ports are force multipliers. With a single computer you can do neat things, but with a network, one plus one is more than two.  When you get to hundreds of computers you get amazing things, and when you have millions of computers you get magic.  The magic wouldn’t happen if every system had just one port, the number of clients would require an equal number servers.  But we don't have that ratio.  So there are two ways to get the number of clients to balance the smaller number of servers.  One, big pipes. Two, tons of ports. Today the topic is making intelligent purchasing decisions for dual or quad port solutions to maximize your servers.

     On the surface it’s a pretty easy decision.  If you need more ports, buy a card with more ports!  But that isn't always the right answer.  With the current generation of quad port adapters, a switch is required to get the port density right.  This sounds innocuous enough, but there is a hidden cost.  Even something that is as fast as a PCI Express switch will still add latency.  The more devices between the end silicon and memory complex will slow things down.  At its root, performance is all about speed.  You may have enough bandwidth to send the packets, but you can run out of time to send them all.  With small packets, 1.44 million packets a second at 1 Gigabit means you only have a few fleeting moments to get the packet from host memory out onto the wire.  Any delay may cause packets to be missed.  This is where using more dual port cards can help out.  By eliminating one intermediate device, that latency can be saved.  Make sure you check your motherboard's documentation since there may be "hidden" switches on the motherboard that are increasing the slot density.  Putting a quad into a switched port on a motherboard, especially one off of the IOH or ICH will really be worse case for latency.  If you are latency sensitive, try to put duals into those switched slots.

     Another thing to factor is the redundancy factor.  With multiple cards you have a buffer to single points of failure.  On a quad port adapter, if the switch goes out all the other ports on the devices go away.  Intel uses highly reliable switch suppliers and the Mean Time Between Failures (MTBF) is roughly the same on dual and quad port adapter, but if your business criteria require more distributed ports for up time, dual is the way.    Actually, the dual ports have better MTBF due to having less parts but both the dual and quad ports have respectable MTBF numbers.

     The last issue is electro-magnetic interference.  EMI is part of using electronics.  All those electrons zipping around cause a small field that can potentially cause negative side effects in some devices.  When using a quad port card, the density of components is higher, which can change the EMI profile of a machine when compared to just using a dual port card.  Just like with the MTBF, the difference between a dual and a quad can be very small, but if it’s a critical focus of your design, you need to go in eyes open. All our cards are certified to the same Class A or Class B standards but in a particular user setup, there may be incremental value in using 2 cards that may broaden any transmission spectrum and reduce the overall impact to a particular frequency. This setup can also reduce the combined cable effects since 4 ports in a single slot leaves the cables near one another and the multi slot implementation will have cable separation by default.

     One thing you don't have to factor in is software support.  Our drivers support both dual and quad port adapters the same.  The driver for a dual port adapter using the Intel® 82571 Gigabit Controller is the same as for a quad port adapter using the 82571.  So you can qualify one driver and know it will work on either the dual or quad version of the same adapter family.  This can save qualification time, which is money.  This will also allow you qualify and build out with duals today and add-in quad ports for higher densities without having to qualify a new driver.

     Now don't think that I'm talking bad about our quad port adapters.  This article is just providing you a full discussion of the dual versus quad decision.  Quad ports provide great feature sets with port density that maximizes your system investment.  When using 1U machines, for example, the only way to get the density needed for today’s datacenters is via a quad port adapter.  Quads also don't take up as much space internally as two boards, so it can change the air flow characterizations.  Quad ports are big sellers, and it’s easy to see why!

Let’s wrap it up:

1)      Dual vs. Quad isn’t just about port density

2)      Intel makes both dual and quad adapters

3)      Thanks for your interest in Intel® Ethernet products