Ethernet Technologies

Ethernet Technologies

As discussed earlier, weve referred to Ethernet as if it were a single protocol standard. But in fact, Ethernet comes in many different flavors, with somewhat bewildering acronyms such as 10BASE-T, 10BASE-2, 100BASE-T, 1000BASE-LX, and 10GBASE-T. These and many other Ethernet technologies have been standardized over the years by the IEEE 802.3 CSMA/CD (Ethernet) working group [IEEE 802.3 2009]. While these acronyms may appear bewildering, there is actually considerable order here. The first part of the acronym refers to the speed of the standard: 10, 100, 1000, or 10G, for 10 Megabit (per second), 100 Megabit, Gigabit, and 10 Gigabit Ethernet, respectively. "BASE" refers to baseband Ethernet, meaning that the physical media only carries Ethernet traffic; almost all of the 802.3 standards are for baseband Ethernet. The final part of the acronym refers to the physical media itself; Ethernet is both a link-layer and a physical-layer specification and is carried over a variety of physical media including coaxial cable, copper wire, and fiber. Usually, a "T" refers to twisted-pair copper wires.

In the past, an Ethernet was initially conceived of as a segment of coaxial cable, as shown in "Ethernet" Figure 1. The early 10BASE-2 and 10BASE5 standards specify 10 Mbps Ethernet over two types of coaxial cable, each limited in length to 500 meters. Longer runs could be obtained by using a repeater - a physical-layer device that receives a single on the input side, and regenerates the signal on the output side. A coaxial cable, as in "Ethernet" Figure 1, corresponds nicely to our view of Ethernet as a broadcast medium - all frames transmitted by one interface are received at other interfaces, and Ethernets CDMA/CD protocol nicely solves the multiple access problem. Nodes simply attach to the cable, and voila, we have a local area network.

Ethernet has passed through a series of evolutionary steps over the years, and todays Ethernet is very different from the original bus-topology designs using coaxial cable. In most installations today, nodes are connected to a switch via point-to-point segments made of twisted-pair copper wires or fiber-optic cables, as illustrated in Figure 1.

In the mid-1990s, Ethernet was standardized at 100 Mbps, 10 times faster than 10 Mbps Ethernet. The original Ethernet MAC protocol and frame format were preserved, but higher-speed physical layers were defined for copper wire (100BASE-T) and fiber (100BASE-FX, 100BASE-SX, 100BASE-BX). Figure 2 shows these different standards and the common Ethernet MAC protocol and frame format. 100 Mbps Ethernet is limited to a 100 meter distance over twisted pair, and to several kilometers over fiber, allowing Ethernet switches in different buildings to be connected.

Gigabit Ethernet is an extension to the highly successful 10 Mbps and 100 Mbps Ethernet standards. Offering a raw data rate of 1,000 Mbps, Gigabit Ethernet maintains full compatibility with the huge installed base of Ethernet equipment. The standard for Gigabit Ethernet, referred to as IEEE 802.3z, does the following:

● Uses the standard Ethernet frame format ("Ethernet Frame Structure" Figure 1) and is backward compatible with 10BASE-T and 100BASE-T technologies. This allows for easy integration of Gigabit Ethernet with the existing installed base Ethernet equipment.

A link-layer switch inter-connecting six nodes

● Allows for point-to-point links as well as shared broadcast channels. Point-to-point links use switches while broadcast channels use hubs, as described earlier. In Gigabit Ethernet jargon, hubs are called buffered distributors

● Uses CSMA/CD for shared broadcast channels. In order to have acceptable efficiency, the maximum distance between nodes must be severely restricted

● Allows for full-duplex operation at 1,000 Mbps in both directions for point-to-point channels.

100 Mbps Ethernet standards - a common link layer, different physical layers

Initially operating over optical fiber, Gigabit Ethernet is now able to run over category 5 UTP cabling. 10 Gbps Ethernet (10GBASE-T) was standardized in 2007, providing yet higher Ethernet LAN capacities.

Lets conclude our discussion of Ethernet technology by posing a question that may have begun troubling you. In the days of bus topologies and hub-based star topologies, Ethernet was clearly a broadcast link (as defined in "Multiple Access Protocols") in which frame collisions occurred when nodes transmitted at the same time. To deal with these collisions, the Ethernet standard included the CSMA/CD protocol, which is particularly effective for a wired broadcast LAN spanning a small geographical radius. But if the prevalent use of Ethernet today is a switch-based star topology, using store-and-forward packet switching, is there really a need for an Ethernet MAC protocol? As well see in "Link-layer Switches" a switch coordinates its transmissions and never forwards more than one frame onto the same interface at any time. Moreover, modern switches are full-duplex, so that a switch and a node can each send frames to each other at the same time without interference. In other words, in a switch-based Ethernet LAN there are no collisions and, therefore, there is no need for a MAC protocol.

As weve seen, todays Ethernets are very different from the original Ethernet conceived by Metcalfe and Boggs more than 30 years ago -  speeds have increased by three orders of magnitude, Ethernet frames are carried over a variety of media, switched-Ethernets have become dominant, and now even the MAC protocol is often unnecessary. Is all of this really still Ethernet? The answer, of course, is "yes, by definition." It is interesting to note, however, that through all of these changes, there has indeed been one enduring constant that has remained unchanged over 30 years - Ethernets frame format. Perhaps this then is the one true centerpiece of the Ethernet standard.


Tags

baseband ethernet, ethernet traffic, physical media, repeater, coaxial cable, star topology

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