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Ethernet Network

In 1973 Xerox invented Ethernet to solve the problem of transferring data between computers. Digital Equipment and Intel collaborated with Xerox in 1973 to publish the DIX networking standard. In the early 1980s Xerox turned over control of the Standard to the Institute of Electrical and Electronics Engineers (IEEE). The IEEE sets up committees to define industry standards. The IEEE 802 committee sets the standards for networking. The IEEE subcommittee 802.3 sets the standard for Ethernet.

An Ethernet packet contains four main parts: The data, the MAC address of the packets source, the MAC address of the packets destination and an error checking code. When creating a packet, the sending computer performs a mathematical operation on the data and attaches the result, called the cyclic redundancy check (CRC), to the packet. The receiving computer performs the same operation on the data and compares its result with the CRC. If the two results do not match, an error has occurred in transmission of the data. The receiving computer will send a request to the sending computer to retransmit the packet.

CSMA/CD

Ethernet networks use the access method known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Multiple access means that several computers share the same cable. But only one computer can place a data packet on the cable at a given instant. Carrier sense means that before a computer places a data packet on the cable, it checks or senses the cable to make sure no other computer is using the cable. If it senses traffic on the cable, it will wait until the cable is free. When the computer senses that there is no traffic on the cable, it will send out its data packet.

If two computers sense that there is not traffic on the cable and start transmitting at exactly the same instant, the data packets will collide. Collision detection means that both computers can detect if a collision has occurred. If they detect a collision, both computers stop transmitting. Each computer then generates a random number and waits for a period of time equal to that number before attempting to transmit again.

On very long cables, beyond 2,500 meters, computers on opposite ends of the cable cannot detect the start of a transmission on the opposite end of the cable quick enough to avoid an undetected collision. An undetected collision will result in corruption of the data packet.

You might think a system designed with the possibility of having data packet collisions is somewhat crude. But data collisions occur rarely during normal operation of an Ethernet network and they have little affect on the efficiency of the network. As network traffic increases, collisions occur more frequently and have the effect of limiting the bandwidth (maximum operating speed) of the network.

A modification of the CSMA/CD access method features collision avoidance. In the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) access method, a computer broadcasts a message on the network to signal its intent to transmit before it places a data packet on the cable. The other computers on the network then refrain from transmitting data, thus avoiding collisions. However, the broadcasting of messages increases the amount of traffic on the cable, resulting in little or no performance gain over CSMA/CD.

More Networking Topologies Articles:
• What is an Ethernet Bridge?
• Bluetooth Basics
• Network Interface Cards (NIC)
• Cisco Switching Fundamentals
• What Are Pseudo-Wires?
• A Guide to Broadband Internet Connections
• ISP Multihoming Explained
• The Secret of Maintaining Your Fiber Optic Network
• Understanding the Basics of All-Optical Switching
• Distance Vector vs. Link State vs. Hybrid Routing

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