Fortunately, there will be no difficult learning curve; it's still pretty much plug and play. How's that for scalability? The biggest change is that CSMA/CD (Carrier Sense Multiple Access/Collision Detection) has been eliminated because 10 Gig will be implemented in full-duplex mode only, meaning that collision detection is turned off. Although some Ethernet purists might consider this a drastic change, it will make your life easier by eliminating the duplexity mismatches that have plagued some Fast Ethernet and Gigabit Ethernet installations. Obviously there will be no shared-media hubs for 10 Gig, but this shouldn't be a big surprise. Practically speaking, shared-media hubs are rarely used today, especially for high-speed versions of Ethernet.

Other major changes involve the interface. The seven types of physical interfaces, or PHYs, are all fiber--there's no IEEE working group focusing on a copper standard. If 10 Gig ever does run on twisted pair, distances would be limited. Each PHY comprises a PCS (Physical Coding Sublayer), which is responsible for controlling the transmitted bit patterns, and a PMD (Physical Media Dependent) layer, which is responsible for converting bits into light signals. The PMD is sometimes referred to as the "optics." These layers were designed to be independent of one another.

With Gigabit Ethernet, you have just two types of standardized fiber interfaces to keep straight: those that support multimode fiber and those that support single-mode fiber. The major difference between single mode and multimode is the light frequencies supported and the corresponding difference in range. Longer wavelengths running on single mode provide more distance.

In contrast, 802.3ae supports three unique light frequencies, represented by corresponding PMDs: 850 nm on multimode, and 1,310 nm and 1,550 nm on single mode (see chart below).

The other major change is that there are now LAN and WAN PHYs for each PMD. Multiplying three optics by two PHYs gives six unique interfaces. The seventh interface, sometimes referred to as the LX4 interface, is a LAN PHY and uses light frequencies in the 1,310-nm range. The main difference is that, though the other PMDs convert bits to light in a serial manner, the WWDM (Wide Wavelength Division Multiplexing) interface uses WDM technology to multiplex the bits across four light waves. This interface is the most versatile because it supports both 62.5 micron multimode fiber for short distances (300 meter) and single-mode 9-micron fiber for long-range (10 kilometers) connections.

If you're wondering why there are so many different versions, you're not alone, especially when you consider that there is overlap between LX4 and some of the other standards. A lot of the variation is based on cost, range and the desire to take advantage of existing technologies and installed fiber. For example, the 850-nm optics that drive multimode fiber short distances are less expensive to build than the optics for single-mode fiber going longer distances. The thinking is, Why pay for what you don't need? This makes sense, but distance for the 850-nm PMD is limited to only 26 meters for existing (62.5 micron) fiber. Going 65 meters will require 50-micron fiber, which is much less common.

If you're connecting switches and servers with patch cords in a data center, this isn't a big deal--aside from the burden of keeping track of different fiber types. But for cable installed in structured-wiring plans, it's a different ball of wax. One thing is clear: If you're running fiber today, you should pull some single mode (9 micron) fiber, especially if the runs are more than 300 meters. It will cost you a bit more initially, but the labor cost to add it down the road could be even higher. The newer, higher grades of fiber will increase distances even further.