Ethernet IEEE 802.3 tutorial

- an overview or tutorial of Ethernet, IEEE802.3 used widely for local area network, LAN applications.

Ethernet, defined under IEEE 802.3, is one of today's most widely used data communications standards, and it finds its major use in Local Area Network (LAN) applications. With versions including 10Base-T, 100Base-T and now Gigabit Ethernet, it offers a wide variety of choices of speeds and capability. Ethernet is also cheap and easy to install. Additionally Ethernet, IEEE 802.3 offers a considerable degree of flexibility in terms of the network topologies that are allowed. Furthermore as it is in widespread use in LANs, it has been developed into a robust system that meets the needs to wide number of networking requirements.


Ethernet, IEEE 802.3 history

The Ethernet standard was first developed by the Xerox Corporation as an experimental coaxial cable based system in the 1970s. Using a Carrier Sense Multiple Access / Collision Detect (CSMA/CD) protocol to allow multiple users it was intended for use with LANs that were likely to experience sporadic use with occasional heavy use.

The success of the original Ethernet project lead to a joint development of a 10 Mbps standard in 1980. This time three companies were involved: Digital Equipment Corporation, Intel and Xerox. The Ethernet Version 1 specification that arose from this development formed the basis for the first IEEE 802.3 standard that was approved in 1983, and finally published as an official standard in 1985. Since these first standards were written and approved, a number of revisions have been undertaken to update the Ethernet standard and keep it in line with the latest technologies that are becoming available.


Ethernet network elements

The Ethernet IEEE 802.3 LAN can be considered to consist of two main elements:

  • Interconnecting media:   The media through which the signals propagate is of great importance within the Ethernet network system. It governs the majority of the properties that determine the speed at which the data may be transmitted. There are a number of options that may be used:

    • Coaxial cable:   This was one of the first types of interconnecting media to be used for Ethernet. Typically the characteristic impedance was around 110 ohms and therefore the cables normally used for radio frequency applications were not applicable.
    • Twisted Pair Cables   Type types of twisted pair may be used: Unshielded Twisted Pair (UTP) or a Shielded Twisted Pair (STP). Generally the shielded types are better as they limit stray pickup more and therefore data errors are reduced.
    • Fibre optic cable:   Fibre optic cable is being used increasingly as it provides very high immunity to pickup and radiation as well as allowing very high data rates to be communicated.
  • Network nodes   The network nodes are the points to and from which the communication takes place. The network nodes also fall into categories:

    • Data Terminal Equipment - DTE:   These devices are either the source or destination of the data being sent. Devices such as PCs, file servers, print servers and the like fall into this category.
    • Data Communications Equipment - DCE:   Devices that fall into this category receive and forward the data frames across the network, and they may often be referred to as 'Intermediate Network Devices' or Intermediate Nodes. They include items such as repeaters, routers, switches or even modems and other communications interface units.

Ethernet network topologies

There are several network topologies that can be used for Ethernet communications. The actual form used will depend upon the requirements.

  • Point to point:   This is the simplest configuration as only two network units are used. It may be a DTE to DTE, DTE to DCE, or even a DCE to DCE. In this simple structure the cable is known as the network link. Links of this nature are used to transport data from one place to another and where it is convenient to use Ethernet as the transport mechanism.
  • Coaxial bus:   This type of Ethernet network is rarely used these days. The systems used a coaxial cable where the network units were located along the length of the cable. The segment lengths were limited to a maximum of 500 metres, and it was possible to place up to 1024 DTEs along its length. Although this form of network topology is not installed these days, a very few legacy systems might just still be in use.
  • Star network:   This type of Ethernet network has been the dominant topology since the early 1990s. It consists of a central network unit, which may be what is termed a multi-port repeater or hub, or a network switch. All the connections to other nodes radiate out from this and are point to point links.

Despite the fact that Ethernet has been in use for many years, it is still a growing standard and it is likely to be used for many years to come. During its life, the speed of Ethernet systems has been increased, and now new optical fibre based Ethernet systems are being introduced. As the Ethernet standard is being kept up to date, the standard is likely to remain in use for many years to come.

By Ian Poole


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