LANs and Ethernet

A local-area network, or LAN, is a system consisting of

•         physical links that are, ultimately, serial lines

•         common interfacing hardware connecting the hosts to the links

•         protocols to make everything work together

We will explicitly assume that every LAN node is able to communicate with every other LAN node. Sometimes this will require the cooperation of intermediate nodes acting as switches.

Far and away the most common type of (wired) LAN is Ethernet, originally described in a 1976 paper by Metcalfe and Boggs. Ethernet’s popularity is due to low cost more than anything else, though the primary reason Ethernet cost is low is that high demand has led to manufacturing economies of scale. The original Ethernet had a bandwidth of 10 Mbps (megabits per second; we will use lower-case “b” for bits and upper-case “B” for bytes), though nowadays most Ethernet operates at 100 Mbps and gigabit (1000 Mbps) Ethernet (and faster) is widely used in server rooms. (By comparison, as of this writing (2015) the data transfer rate to a typical faster hard disk is about 1000 Mbps.) Wireless (“Wi-Fi”) LANs are gaining popularity, and in many settings have supplanted wired Ethernet to end-users.

Many early Ethernet installations were unswitched; each host simply tapped in to one long primary cable that wound through the building (or floor). In principle, two stations could then transmit at the same time, rendering the data unintelligible; this was called a collision. Ethernet has several design features intended to minimize the bandwidth wasted on collisions: stations, before transmitting, check to be sure the line is idle, they monitor the line while transmitting to detect collisions during the transmission, and, if a collision is detected, they execute a random backoff strategy to avoid an immediate recollision. While Ethernet collisions definitely reduce throughput, in the larger view they should perhaps be thought of as a part of a remarkably inexpensive shared-access mediation protocol.

In unswitched Ethernets every packet is received by every host and it is up to the network card in each host to determine if the arriving packet is addressed to that host. It is almost always possible to configure the card to forward all arriving packets to the attached host; this poses a security threat and “password sniffers” that surreptitiously collected passwords via such eavesdropping used to be common.