So profound is the effect of technology on our lives that “Ethernet at 30” jubilee articles have even shown up in the mainstream press. And, while I don’t find myself disagreeing with the fundamental assertions in these stories, they leave out the key point that, compared with “Ethernet at birth,” today’s Ethernet really isn’t Ethernet at all. (See related story)
We need to first know where Ethernet “is,” relative to the Open Systems Interconnection model. When formally defined, Ethernet (like FDDI and token ring) consists of just the lower two layers of the protocol stack. To make it a tad more confusing, the LAN media access control actually maps into Layer 1 and the bottom half of Layer 2.
Ethernet, oversimplified, is a frame format, using an access mechanism over physical cabling. Looking at each will help you understand why I make my assertion that today’s Ethernet isn’t really Ethernet.
In the beginning, Ethernet was all about connecting stations via physical taps into a coaxial network. Even before the more fundamental move up to Fast Ethernet, that part of the specification was replaced with the more familiar 10Base-T hub model. And, as time has gone by, the so-called Ethernet PHY has been revamped and reinvented time and again as required.
The distributed, multi-user aspect of Ethernet was probably the most important attribute – especially given the mainframe/hierarchical model that was prevalent at the time. Ethernet’s approach was known as CSMA/CD for Carrier Sense, Multiple Access with Collision Detection.
It worked fine in lightly loaded environments, but as traffic grew so did collisions, and effective throughput dropped. This forced many companies to create smaller and smaller groups of Ethernet. It also gave FDDI and token ring a chance to break into the marketplace.
Both of these LANs used a deterministic, token-passing scheme and, unlike Ethernet, could be driven to nearly 100% utilization without degrading. They also offered key features such as support for priority bits and larger frame sizes – elements Ethernet’s architects did not provide.
So Ethernet “morphed” as its frame format was upgraded with four more bytes to provide virtual LAN and priority functions (IEEE 802.1p/Q). And while still not part of the standard, Jumbo Frames of 9,000 bytes commonly are supported on today’s Ethernet gear.
With the advent of switching, Ethernet left behind the most fundamental part of its heritage, CSMA/CD. A switched environment appears to each user as a dedicated LAN, thus no carrier sense, no multiple access and need for collision detection. Ethernet is no longer Ethernet. (CSMA/CD was included in the Gigabit Ethernet standard though rarely implemented. It is not included in the 10G Ethernet standard.)
At 30, Ethernet retains only the frame format from the original spec – and even that has been tinkered with. But it serves us well, and having one topology that combines the best of others is far better than supporting many.
