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10 GIG Can't Wait to Interoperate

In many ways, 10 Gigabit Ethernet is the same as the original 10-Mbps version that the father of Ethernet, Bob Metcalfe, scribbled on a napkin around 25 years ago--only faster. It still has the same header format, the same 8-byte preamble, and the same minimum (64 byte) and maximum (1,518 byte) frame sizes.

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).

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