2ETHERNETWe now turn to a deeper analysis of the ubiquitous Ethernet LAN protocol. Current user-level Ethernet today(2013) is usually 100 Mbps, with Gigabit and 10 Gigabit Ethernet standard in server rooms and backbones,but because the potential for collisions makes Ethernet speeds scale in odd ways, we will start with the 10Mbps formulation. While the 10 Mbps speed is obsolete, and while even the Ethernet collision mechanismis largely obsolete, collision management itself continues to play a significant role in wireless networks.The original Ethernet specification was the 1976 paper of Metcalfe and Boggs,[MB76]. The data rate was10 megabits per second, and all connections were made with coaxial cable instead of today’s twisted pair.The authors described their architecture as follows:We cannot afford the redundant connections and dynamic routing of store-and-forward packetswitching to assure reliable communication, so we choose to achieve reliability through sim-plicity. We choose to make the shared communication facility passive so that the failure of anactive element will tend to affect the communications of only a single station.Classic Ethernet was indeed simple, and – mostly – passive. In its most basic form, the Ethernet medium wasone long piece of coaxial cable, onto which stations could be connected viataps. If two stations happenedto transmit at the same time – most likely because they were both waiting for a third station to finish – theirsignals were lost to the resultantcollision. The only active components besides the stations wererepeaters,originally intended simply to make end-to-end joins between cable segments.Repeaters soon evolved into multiport devices, allowing the creation of arbitrary tree (that is, loop-free)topologies. At this point the standard wiring model shifted from one long cable, snaking from host to host,to a “star” network, where each host connected directly to a central multipoint repeater. This shift allowedfor the replacement of expensive coaxial cable by the much-cheaper twisted pair; links could not be as long,but they did not need to be.Repeaters, which forwarded collisions, soon gave way toswitches, which did not (2.4Ethernet Switches).Switches thus partitioned an Ethernet into disjointcollision domains, or physical Ethernets, through whichcollisions could propagate; an aggregation of physical Ethernets connected by switches was then sometimesknown as avirtualEthernet. Collision domains became smaller and smaller, eventually down to individuallinks and then vanishing entirely.