Ethernet Basics

Links
UNH Interoperability Lab

The most popular Local Area Networking technology in use today is Ethernet running at 10 Mbps. This network type is flexible, inexpensive, and reasonably fast for most applications. Actually, the only time it is not a good choice is for especially demanding applications such as video conferencing or very large numbers of users.


Varieties of Ethernet

Ethernet has many variations, each with its own rules. Therefore, each subject has been given its own page. Click on the links below to get detailed information on each subject:

Thick Wire (10 Base-5) Technology
Thin Wire (10 Base-2) Technology
10 Base-T (Ethernet over UTP) Technology
10 Base-FL (Ethernet over Fiber Optics) Technology
Repeater Count Rules
Switched Ethernet Networks

History & Standards

Ethernet is a 10 Mbps Local Area Network technology originally developed by Digital Equipment Corporation (DEC), Xerox, and Intel. In 1983, the specification was formalized by the IEEE as the 802.3 specification. At first, the technology only used coaxial cable in a bus topology, however it has changed with the times to take into advantage of new technologies such as twisted pair cabling (10 Base-T), fiber optics (10 Base-FL), and 100 Mbps operation (100 Base-X or Fast Ethernet). The current standard is known as IEEE 802.3u.

This page will discuss only the 10 Mbps variations of Ethernet. For information on Fast Ethernet (the 100 Mbps version), please see the  Fast Ethernet Page available which discusses it in detail.


What Is CSMA/CD?

All versions of Ethernet use a Media Access Control (MAC) protocol called CSMA/CD. This protocol controls which devices can transmit data to the network, and when they can do so.

CSMA/CD is an acronym for Carrier Sensed Multiple Access with Collision Detection. This fancy term basically means "Don't speak while someone else is speaking, but if it happens, everyone stop and try again later." Let's look at how it works in a bit more detail:

Carrier Sense Multiple Access (CSMA)
This is a way for many devices to access the same network (Multiple Access). The way it works is that every device needing to send data to the network first "listens" to the wire to determine if anyone else is sending anything (Carrier Sense). If there is any activity at the time, then the device needing to transmit will wait until the transmission ends, otherwise it sends its data.
Collision Detection (CD)
Once a device determines that the network is clear, it begins sending data. While sending, it monitors what data is actually on the wire, and compares it to what is being sent. It is possible for two devices to see that the network is open at the same time and for both to start sending. When this happens, the data on the wire will be garbled and neither device will see that what is actually on the wire matches what it sent. This is called a collision. The devices detect this collision, and both stop their current transmission, wait a random amount of time, and try again.

Every Ethernet network, regardless of type, behaves in this manner. Naturally, this access method is most efficient under periods of light load, as there is a greater chance the network will be available when the device wants to transmit, and there is also less chance another device will try to transmit at the same time. As the load on the network increases, collisions increase with it. Eventually, it is possible to get to the point where there are too many collisions and devices spend more time retransmitting and waiting than they do actually sending data.

The CSMA/CD protocol is usually robust enough to keep the network running very well, however there are situations which can cause it to fail. Proper design of the network is important to prevent a breakdown of the basic CSMA/CD protocol. Design mistakes which cause problems are most often either exceeding the maximum repeater count or exceeding the distance limit of a network segment.

One thing which should be kept in mind is that CSMA/CD happens automatically in each machine's network interface card, and there is nothing the user needs to program or set up to implement it, nor is the user usually notified of collisions.


Types Of Ethernet

Ethernet has several different variations, each of which uses different cable types, topologies, and distance limitations. However, all variations use the same CSMA/CD protocol. The different types are:

10 Base-5 (Thick Ethernet)
This is the original version of Ethernet. It consists of a coaxial cable about 10mm (0.4") thick with transcievers connecting the individual devices on the network to the cable. It allows up to 255 physical connections to a cable segment, and an overall cable length of 500 meters (1650 feet).
10 Base-2 (Thin Ethernet)
This is the second variation of Ethernet. Here, we dispense with the external transcievers and simply daisy-chain all of the devices on the network together with a thinner version of the cable used in 10 Base-5.
10 Base-T
This is the most popular version of Ethernet. It uses Unshielded Twisted Pair (UTP) cable. Each device has its own cable run of up to 100 meters (330 feet) back to a common hub.
10 Base-FL
This is basically a version of Ethernet which runs over fiber optic cable. It is set up in much the same manner as 10 Base-T, except that it uses fiber optic cable. This allows it to support very long distances of up to 2000 meters (6600 feet) per run.

Each of the above types is a subject in itself, so simply click on the name of each technology above to get more detailed information on it.