Ethernet

Like the check being placed into the envelope, the Ethernet protocol encapsulates data passed to it from higher layers. It also does the reverse: it decapsulates data that is presented to it from the physical layer. Thus, it stuffs envelopes when data is moving down through the layers, and it opens envelopes and passes the contents upward at the receiving end. The Ethernet envelope is called a frame.

Ethernet technology is everywhere. It is believed that more than 90% of network installations use Ethernet. The remaining network connections are a combination of Token Ring, Fiber Distributed Data Interface (FDDI), Asynchronous Transfer Mode (ATM), and other protocols. Ethernet gained acceptance because of its simplicity of installation and management.

The Ethernet standard was defined in 1985 by the Institute of Electrical and Electronic Engineers (IEEE) in a specification known as IEEE 802.3. The standard specifies the physical medium, carrier sense multiple access with collision detection (CSMA/CD) access method, and frame format.

In the CSMA/CD access method, each station contends for access to the shared medium. If two stations try sending the packets at the same time, a collision will result. The CSMA/CD access method is designed to restore the network to normal activity after a collision occurs, and collisions are normal in an Ethernet shared network.

The original 10 Mbps shared Ethernet network was based on coaxial cable physical medium, and later the standard was extended to shielded and unshielded twisted pair, and fiber optic cable media. The most common physical media is unshielded twisted pair (UTP), because it is inexpensive, easy to install, and allows star topology.

Note: Star topology is so named because it allows all hosts in a network to be logically (and, in effect, physically) connected at a central point. The central point of connectivity means that the loss of any individual host on the network will not affect the remaining connected hosts.

Compare this to chain topology, where the loss of a host in the chain would cause a disruption in connectivity.

The 10 Mbps twisted pair standard is referred to as 10Base-T.

Fast Ethernet is an extension of the popular 10Base-T Ethernet standard, supporting both 10 Mbps and 100 Mbps media speed. Fast Ethernet retains the data format and protocols of 10 Mbps Ethernet, so no changes are required in higher level protocols and applications.

Fast Ethernet standards provide for auto-negotiation of media speed, allowing vendors to provide dual-speed Ethernet interfaces that can be installed and run at either 10 or 100 Mbps. With dual speed products, users who are planning future 100 Mbps implementations can purchase a 10/100 Mbps product today and use the 10 Mbps speed in their existing networks, and then later upgrade to 100 Mbps when and where it is needed.

Gigabit Ethernet

Gigabit Ethernet is an extension to 10 Mbps and Fast Ethernet. It provides seamless interoperability with the existing 10 Mb and Fast Ethernet (10/100 Mbps) and is compatible with existing networking protocols, networking operating systems, network applications, and networking management tools. It uses a combination of proven protocol technologies adopted by the original IEEE 802.3 Ethernet specification and Fiber channel specification.

Gigabit Ethernet retains the standard 10/100Base-T frame size and format and the same CSMA/CD scheme. However, it can use fiber channel's physical layer as the underlying transport mechanism. The full duplex implementation of Gigabit Ethernet as in Fast Ethernet does not require the CSMA/CD scheme, but retains support for the Ethernet frame format.

The initial Gigabit Ethernet offering supported one fiber physical interface. Two common fiber types in use today are single mode fiber, for longer distances up to 60 kilometers, and multimode fiber for shorter distances in the range of 300 to 500 meters. They are covered by the 1000Base-LX and the 1000Base-SX specification, respectively.

A standard has been defined by the IEEE 802.3ab task force for Gigabit Ethernet over copper physical medium.

10 Gigabit Ethernet

The evolution of Ethernet speeds continues. An OSA-Express2 card is also capable of supporting the 802.3 suite of standards (802.3ae) in the form of 10 Gbps. As with 1-Gigabit Ethernet, there is a copper medium option (802.3ak).