Ethernet Basics

Ethernet :

In the OSI model, Ethernet technology operates at the physical and data link layers - Layers One and Two respectively. Ethernet supports all popular network and higher-level protocols, principally IP.

Traditional Ethernet supports data transfers at the rate of 10 Megabits per second (Mbps). Over time, as the performance needs of LANs have increased, the industry created additional Ethernet specifications for Fast Ethernet and Gigabit Ethernet.

Fast Ethernet extends traditional Ethernet performance up to 100 Mbps and Gigabit Ethernet up to 1000 Mbps speeds. Although products aren't yet abvailable to the average consumer, 10 Gigabit Ethernet (10000 Mbps) also remains an active area of research.

To connect Ethernet cables to a computer, a person normally uses a network adapter, also known as a network interface card (NIC). Ethernet adapters interfaces directly with a computer's system bus. The cables, in turn, utilize connectors that in many cases look like the RJ-45 connector used with modern telephones.

Ethernet Topologies and Protocols:

Traditional Ethernet employs a bus topology, meaning that all devices or hosts on the network use the same shared communication line. Each device possesses an Ethernet address, also known as MAC address. Sending devices use Ethernet addresses to specify the intended recipient of messages.

Data sent over the Ethernet exists in the forms of frames. An Ethernet frame contains a header, a data section, and a footer having a combined length of no more than 1518 bytes. The Ethernet header contains the addresses of both the intended recipient and the sender.

Data sent over the Ethernet is automatically broadcast to all devices on the network. By comparing their Ethernet address against the address in the frame header, each Ethernet device tests each frame to determine if it was intended for them and reads or discards the frame as appropriate. Network adapters incorporate this function into their hardware.

Devices wanting to transmit on the Ethernet first perform a preliminary check to determine whether the medium is available or whether a transmission is currently in progress. If the Ethernet is available, the sending device transmits onto the wire. It's possible, however, that two devices will perform this test at approximately the same time and both transmit simultaneously.

In traditional Ethernet, this protocol for broadcasting, listening, and detecting collisions is known as CSMA/CD (Carrier Sense Multiple Access / Collision Detection). Some newer forms of Ethernet do not use CSMA/CD. Instead, they use the so-called full duplex Ethernet protocol, which supports point-to-point simulatenous sends and receives with no listening required.

Ethernet Devices:

As mentioned earlier, Ethernet cables are limited in their reach, and these distances (as short as 100 meters) are insufficient to cover medium-sized and large network installations.

A repeater in Ethernet networking is a device that allows multiple cables to be joined and greater distances to be spanned.

A bridge device can join an Ethernet to another network of a different type, such as a wireless network.

One popular type of repeater device is an Ethernet hub. Other devices sometimes confused with hubs are switches and routers.