Gigabit Ethernet is dual duplex!

James Hsieh recently blogged about Ethernet autonegotiation. Which reminds me of one very cool feature of Gigabit Ethernet over copper. When 10BASE-T (10 Megabit Ethernet over unshielded twisted pair (UTP)) was standardized, it was a breakthrough technology which allowed people like myself to inexpensively and easily network large numbers of systems together. At the time, I was the Manager of Network Support for the College of Engineering at Auburn University. A few years previously, the entire campus got a telecom overhaul and every building was wired for the future networking technologies. We had UTP wiring with spare pairs everywhere. And we had fiber in a home-run configuration to the central switch room. It was quite an awesome blank canvas and we proceded to try to network everything. Anyway, back to the technology... when 100BASE-T (FastEthernet, 100 Megabit, over UTP) came out, it was basically the same technology as 10BASE-T. Both used 2 pairs, one for transmit and one for receive. The big irritation with this is that if you wanted to connect two devices together without using a hub (or later, switch) you could use a "null-Ethernet" cable where you cross-wire the transmit and receive pairs. While this wasn't nearly as irritating as the RS-232 cabling fiasco, it still causes lots of myths and problems.

With Gigabit Ethernet over UTP (1000BASE-T) something had to give. They couldn't just bump up the speed by an order of magnitude. What they did was very, very clever and only possible with the advances in semiconductor technology at the time. First, they use all 4 pairs in a cat-5 cable, rather than 2. Next, they use a signalling method which allows a signal level to represent more than one bit (no, I won't explain that here, see the official IEEE site for details). Finally, they put little DSP engines on each pair so they could operate in full-duplex mode which simultaneously receiving and transmitting on the same wires. This is known as dual-duplex. The smarts in a Gigabit Ethernet UTP interface are clever enough to be able to negotiate all of this including negotiating back to prior, slower, standards. All-in-all it is vastly simpler to implement. Just plug it in and it works! Goodbye null-Ethernet cables, I never liked you anyway!

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