2. intro to ethernet

Developed in the early 1970’s, Ethernet has proven to be one of the most simple, reliable, and long-lived networking protocols ever designed. The high speed and simplicity of the protocol has resulted in its widespread use.

Although Ethernet works across a variety of layer one media, the three most popular forms are 10BaseT, 10Base2, and 10BaseF, which use unshielded twisted pair (UTP), coaxial, and fiber optic cables respectively. UTP is used in a “star” configuration, in which all nodes connect to a central hub. 10Base2 uses a single coaxial cable to connect all workstations together in a “bus” configuration, and does not require a hub. 10BaseF uses fiber optics, which, though expensive, can travel long distances (2km) and through electrically noisy areas.

An interesting difference between coaxial Ethernet and other types is that coax Ethernet is truly a one-to-many (or, ‘point-to-multipoint’) connection; fiber and UTP connections

are, from a layer one perspective, one-to-one (or, ‘point-to-point’) connections, and require an additional networking device (typically, a repeater, or Ethernet hub) to connect to multiple other workstations. This is why coax Ethernet does not require a hub, and Ethernet over other media typically does.
Ethernet Topologies
Pro Con Typical Use
10BaseT *Very reliable – one fault usually doesn’t affect entire network. *Relatively short distance from hub to workstation (100m).
*Requires a lot of wiring (a separate link for each workstation.) *Offices and home networks.
10Base2 *Cheap – no hub required, no wiring except from station to station.
*Well shielded against electrical interference.
*Can transmit longer distances (200m). *Any break in connectivity disrupts entire network segment.
*Problems can be very difficult to troubleshoot. *Small or home networks, hub to hub links.
10BaseF *Long distance networking (2000m).
*Immune to electrical interference. *Very expensive to install. *Long distance hub-to-hub or switch-to-hub links.

Ethernet is like a bunch of loud people in an unmoderated meeting room. Only one person can talk at a time, because communication consists of standing up and yelling at the top of your lungs. People are allowed to start communicating whenever there is silence in the room. If two people stand up and start yelling at the same time, they wind up garbling each others’ attempt at communication, an event known as a “collision.” In the event of a collision, the two offending parties sit back down for a semi-random period of time, then one of them stands up and starts yelling again. Because it’s unmoderated, the likelihood of collisions occurring increases geometrically as the number of talkers and the amount of stuff they talk about increases. In fact, networks with many workstations are generally considered to be overloaded if the segment utilization exceeds 30-40%. If the collision light on your hubs is lit more often than not, you probably need to segment your network. Consider the purchase of a switch, described below.

Ethernet hubs are used in 10BaseT networks. A standard hub is just a dumb repeater – anything it hears on one port, it repeats to all of its other ports. Although 10BaseT is usually wired with eight wire jacks (known as RJ45 connectors), only four wires are used – one pair to transmit data, and another pair to receive data. While transmitting, an Ethernet card will listen to its receive pair to see if it hears anyone else talking at the same time.



2. intro to ethernet