Ethernet Topology, Protocol, and Devices
An Article by your Guide Bradley Mitchell
Ethernet Topologies and ProtocolsTraditional 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 simulatenously.
By design, as a performance tradeoff, the Ethernet standard does not prevent multiple simulatenous transmission. These so-called collisions, when they occur, cause both transmissions to fail and require both sending devices to re-transmit. Ethernet uses a algorithm based on random delay times to determine the proper waiting period between re-transmissions. The network adapter also implements this algorithm.
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.
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.
Ethernet network adapters also exist in multiple forms. Newer personal computers often include a built-in Ethernet adapter. Otherwise, one can purchase and install an add-in card. PCI cards are most popular for desktop computers and PCMCIA ("credit card") adapters most popular for notebooks. USB Ethernet adapters also exist for both desktops and laptops. Wireless Ethernet adapters can also be configured to work with newer computers.
Ethernet is one of the Internet's key technologies. Despite its advanced age, Ethernet continues to power many of the world's local area networks and continually is improving to meet future needs for high-performance networking.
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