Sometimes you get so deep into something that you don't realize how crazy it is until you take a step back. Like most technology companies, Intel has an inherent love for acronyms. The cacophony of standards bodies, advanced technologies, and intense rates of change in our industry necessitates the use of abbreviation just to be able to communicate clearly and briefly. However, while I am at least as much of a techno-phyliac as most of the folks in the technology jungle, even I sometimes run into an acronym wall. I thought to help myself and others it might be a good idea to decode one of the newer sets of network technologies that I work closely with and to decipher some of the associated names and acronyms that come along with it.
10 Gigabit Ethernet: It's here, it's real, and it's growing fast_._
Ethernet (IEEE 802.x) has evolved over the years from a new standard linking computers together at slow rates and has moved from 10 Megabit per second (Mbps), to 100Mbps, to 1 Gigabit per second (Gbps), and a few years ago to 10GbE unidirectional throughput. Over time there have been several physical connection types for Ethernet. The most common is copper (Cat 3/4/5/6/7 cabling is used as the physical medium) but Fiber has also been prevalent as well as some other more esoteric (such as BNC Coax) physical media types. The most common 10GbE adapter (until very recently) has been Optical only due the difficulty of making 10GbE function properly over copper cabling.
But this post isn't meant to discuss the past, but more to decode the present and future as it relates to 10Gig Ethernet and the variety of flavors that are available. Below I'll cover a number of acronyms for 10GbE IEEE standards that are often lumped together as '10 Gigabit' and discuss some of the differences and usages for each. After that, I'll also try to clear up some of the confusion about ‘form factor' standards for optical modules (which are separate from IEEE) and some of terms and technologies in that realm:
10GBase-T (aka: IEEE 802.3an):
This is a 10GbE standard for copper-based networking deployments. Networking silicon and adapters that follow this specification are designed to communicate over CAT6 (or 6a/7) copper cabling up to 100 meters in length. To enable this capability, a 10GbE MAC (media access controller) and a PHY (Physical Layer) designed for copper connections work in tandem.
10GBase-T is viewed as the holy grail for 10GbE because it will work within the most prevalent Cat 6/7 based infrastructure that is already in place. For this flexibility, 10GBase-T trades off higher power, and higher latency.
10Gbase-KX4 (aka: IEEE 802.3ap):
This is a pair of standards that are targeted toward the use of 10GbE silicon in backplane applications (such as a blade design). The specifically is designed for an environment where lower power is required and shorter distances (up to only 40 inches) are sufficient.
10GBase-SR (aka: IEEE 802.3ae):
This specification is for 10GbE with optical cabling over short ranges (SR = Short Range) with multi-mode fiber. Depending on the kinds of fiber, SR in this instance can mean anything between 26 - 82 meters on older fiber (50-62um fiber). On the latest fiber technology, SR can reach distances of 300m. To be able to physically support a connection of the cable, any network silicon or adapter that support 10GBase-SR would need to have a 10GbE MAC connected to an Optics module designed for multi-mode fiber. (We'll discuss optics modules in more depth further down in this post.)
10GBase-SR is often the standard of choice to use inside the datacenters where fiber is already deployed and widely used.
10GBase-LR (aka: IEEE 802.3ae, Clause 49):
LR is very similar to the SR specification except that it is for Long Range connections over single-mode fiber. Long Range in this spec is defined as 10km, but distances above that (as much as 25km) can often be obtained.
10GBase-LR is used sparsely and really only deployed where ultra long distances are absolutely required.
10GBase-LRM (aka: IEEE 802.3aq):
LRM stands for Long Range over Multimode and allows distances of up to 220 meters on older standard (50-62um) multi-mode fiber.
10GBase-LRM is targeted for those customers who have older fiber already in place but need extra reach for their network.
10GBase-CX4 +(aka: IEEE 802.3ak):+
This standard of 10GbE connection uses the CX4 connector/cabling that is used in Inifinband^TM^* networks. CX4 is a lower power standard that can be supported without a standalone PHY or optics module (the signals can be routed directly from a CX4 capable 10GbE MAC to the CX4 connector). Due to the physical specification for CX4 based 10 Gigabit, it provides a lower latency than comparable 10GBase-T copper PHY solutions. With the use of CX4 passive (copper) cables, the nominal distance you can expect between your 10GbE links is ~10-15m. There are also amplified 'active' (but still copper) cables with nominal distances up near 30m.
Below is an image of a standard CX4 based socket that would be on a 10GBase-CX4 NIC:
There are also what referred to as ‘active optical' cables are for CX4, which actually have an optics module in the termination of the cable, and the cable body is fiber. This kind of active design increases cable reach and improves flexibility (fiber is smaller than copper pairs) but also increases cost. These active cables can increase reach up to 100m.
Intel has recently released our own series of active optical CX4 cables.
For short distances (such as inside the rack in a datacenter), CX4 offers one of the lowest cost ways to deploy 10GbE from switch to server. Because of its design, CX4 also achieves very low latencies as well.
</end of IEEE standards ramble>
Ok, so we've summarized the majority of the IEEE 10GbE standards. But the immediate question arises: "Why are there so many?" Is the IEEE standards body for 10GbE just throwing out all these standards for every possible niche application? The answer is no. For any new standard IEEE phy interface standard to be approved, it must pass on several criteria including "distinct identity" and "broad market potential". While all of these standards certainly won't apply to any given institution's network, they all do all meet real market needs.
X2, XFP, SFP+... say what?
A final mystery that I've alluded to above has to do with the various optical module form factors that are available for 10GbE. XENPAK, X2, XPAK, XFP and SFP+ are standard optics module form factors that are used by both switch and NIC vendors in the industry. These modules that go along with the 10GbE networking products are an interesting beast. They are not specified by IEEE, but are standardized by a group of industry participants through what is known as a Multi-Source Agreement (MSA).
XENPAK, XPAK and X2 are the older module standards originally used for 1GbE, followed by XFP which shrunk the form factor of the actual module as well as the fiber cable pairs. SFP+ is a newer form factor that is now gaining momentum with switch and NIC vendors. An SFP+ optics module can use the same fiber pairs used with XFP (no new fiber cable needed), but the form factor of the cage in the switch or NIC as well as the optics module itself are smaller. The key advantage of using SFP+ is the new form factor can drive lower costs, lower thermals, and higher densities at the switch.
Here is an image of an older X2 optics module:
And here is a comparison of the size of XFP (right) relative to SFP+ (left):
The optics modules are driven by a low power interface from the 10GbE MAC. The interfaces are XAUI (for X2 modules), XFI (for XFP modules), and SFI (for SFP+ modules). These interfaces generally are supplied directly from the 10GbE based MAC to the module cage. One of the things the module MSA standards bodies agree on is not only a form factor for the module itself but also the electrical specifications of the driver interface that can be accepted from the MAC.
The key thing I want to hammer home here is that IEEE specification (such as 10GBase-SR) is separate from the module form factor used.
For example, you can have a Short Range optical NIC that uses X2, XFP, or SFP. So asking for an "SFP NIC" isn't actually specific enough, because that could mean a lot of different things. You'd have to specify a 10GBase-SR NIC, with SFP+ optics.
Now that I've thoroughly confused everyone, I'll take it one step further. Not only can each module form factor be used with different IEEE MAC specifications, but each module doesn't even need to be used for a fiber connection at all. An interesting example of using an ‘optics' module form factor for a non-optical design is SFP+Direct Attach.
SFPDA is similar in concept to CX4 but provides a bit more flexibility. Normally, you may have a switch or NIC that is designed to be able to support the addition of SFP based optics modules for a 10GbE fiber connection. Direct Attach allows for passive Twin-Axial (2 pair copper) cables to be plugged directly into the SFP+ cage (in place of an optical module) to carry the serial signal from the MAC directly over the cable to another SFP+ form factor enabled NIC or switch.
Again, the downside is that without either a standalone PHY, or optics module to send the signal over a long distance, a passive cable with SFPDA has a reach in the ~10-15m range. The real advantage for SPFDA over CX4 is that on the switch side the SFP+ module design allows higher density switches than CX4 can provide.
For a top of the rack switch, SFP+DA will likely provide excellent cost, power and latency characteristic and still have enough reach to be very feasible inside the rack.
10GbE - The Infrastructure is Ready!
I hope that I've lifted a little bit of the fog that surrounds the 10GbE market and the related technologies. The last thing I want to leave you with is the fact that 10GbE infrastructure is now starting to roll into the mainstream. CX4 switches are available broadly in the market today and SFP+ type designs for both optical modules as well as Direct Attach connections have been demonstrated and will be getting rolled out very soon by various vendors.
Intel is already selling a wide variety of NICs and silicon to meet the various form factors and standards based market needs I listed above along with other vendors in the market place.
After years of anticipation, 10GbE is finally hitting its stride. Next stop... 10_0_GbE...