Tutorial: LANs

Technologies for connecting computers within a building are called LANs.

Local Area Networks (LANs) are a way of implementing multidrop or multipoint data circuits within an office. LANs started to become popular in 1980, and have now become the standard method of implementing circuits to physically connect equipment, in the building, in the network core and on the access circuit from the building to the network.

One of the original requirements for a LAN was to connect computers in an office to a shared laser printer.

This was accomplished by running a cable down the hall and connecting all the computers and the laser printer to the cable, an architecture that looks like an electrical power distribution bus bar, and so is called a bus topology. Topology is the way the system looks viewed from the top, its layout.

All the machines were physically, electrically connected by the cable, any voltage any machine put on the cable could be detected by any other machine; in other words, everyone heard everything everyone said.

To communicate, you broadcast your message to everyone on the cable... which includes the printer.

The collection of machines connected this was is called a broadcast domain: machines that can communicate directly and there is nothing stopping them from communicating.

The concept of a broadcast domain is critical to understanding how packets actually are communicated between machines.

The bus was replaced by a passive hub, a central box all the machines plugged into with the familiar blue point-to-point LAN patch cable.  The hub was then upgraded with a processor and is called a Layer 2 switch, Ethernet switch, LAN switch or simply switch if it's clear we're not talking about a telephone switch.

This technology is now used to build telecom networks, signaling MAC frames point-to-point over fiber between two routers.  This is called Optical Ethernet.  (We, of course, explain what MAC frames are and how they work in the course.)

Ethernet Switches, a.k.a. Layer 2 or LAN switches, create broadcast domains with hardware.

A software trick that can be implemented on more expensive LAN switches called Virtual LANs (VLANs) allows the creation of broadcast domains in software.  VLANs are used to isolate users connected to a network in their own broadcast domains at the very lowest level.

Optical Ethernet technology is used on the access, between the user and the network, for 10 Mb/s to 1 Gb/s fiber to the home or office, and 40 to 100 Gigabit service to large office buildings.