Ethernet Fundamentals

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. These two behaviors (listen for silence before talking, and detect other people talking at the same time) are described by the as CSMA/CD, or "Carrier Sense Multiple Access, Collision Detection."

One hundred megabit Ethernet (100BaseTX) works just like ten megabit Ethernet, only ten times faster. On high-quality copper (known as Category 5, or CAT 5 UTP), 100BaseTX uses the same two pair of copper to communicate. If you have standard network-quality copper, an alternative is to use 100BaseT4, which uses all four pairs, but can communicate at 100Mbps on CAT 3 UTP.

Gigabit Ethernet works just like hundred megabit Ethernet, only ten times faster (1000Mbps, or 1Gbps.) There are some Gigabit Ethernet devices floating around out there, but it's unlikely that you'll find such devices on the small LANs that you'd find on the "Near Side of the 'Net."

If your conference room gets too busy, you may consider splitting them into two groups by putting a partition wall with a door between the halves, and putting a person in the doorway. This person would listen to the conversations in both rooms, memorize the names (Ethernet card addresses) of everyone in each room, and forward messages from room to room when necessary. A device to do this is called a "transparent bridge." It's called "transparent" because it's smart enough to learn the Ethernet addresses on its own without the workstations suspecting anything is going on. ["Source-route bridges" are uncommonly used so I'm not going to discuss them.]

Ethernet switches are little more than high-speed, multi-port bridges. They learn the Ethernet addresses of everyone attached to each port, and make intelligent forwarding decisions based on Ethernet card address (aka MAC address.) Because communication between 100Mbps and 10Mbps networks requires buffering, Ethernet switches are often used for this purpose. Many inexpensive switches have many 10Mbps ports and one or two 100Mbps ports. Typically, you would connect your server(s) to the 100Mbps port(s), and workstations or entire hubs to the 10Mbps ports. The buffering and intelligent forwarding allows another interesting feature to exist-- "full-duplex" Ethernet. "Half-duplex" means you can either talk or listen, but not both, at a given time, such as when using a radio. "Full-duplex" communication means you can talk and listen at the same time, such as when on the phone. Since 10BaseT uses separate pairs of copper for sending and receiving, it's physically possible to do both if there are no other workstations on your network segment-- which is the case if you are directly attached to a switch. Note that both the switch port and your network card must be configured for full duplex operation for this to work, but the result is worth it: a full 20Mbps for "regular" Ethernet and a whopping 200Mbps of bandwidth available for full-duplex fast Ethernet. Since collisions are eliminated, the 30% rule does not apply. When considering the purchase of a switch, there are a few important considerations, not all of which may apply to your requirements:

• Does the switch support 100Mbps on any ports? How many, and will it autodetect 10/100BaseT?
• Does the switch support full duplex? Even on the 100Mbps ports?
• How many MAC (Ethernet card) addresses does it store? 500? 5000? "Unlimited" is not a rational answer.
• Some "workgroup" switches only allow one MAC address per port, so these would not be suitable if you plan to connect hubs to switch ports.
• You tend to get what you pay for. If a switch seems unreasonably inexpensive compared to other switches that appear to have similar specs, look closer, or check the detailed specs on the manufacturer's web site. Often, you'll find that a cheap switch either isn't a switch at all (see last item) or only allows one workstation per port (see item above last item.)