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PDF document. This Ethernet Tutorial is provided by Fujitsu Network Communications Inc. April 11, 2006. Ethernet, a physical layer local area network ( LAN ) technology, is nearly 30 years old. In the last three decades, it has become the most widely used LAN technology because of its speed, low cost, and relative ease of installation. In the OSI ( Open Systems Interconnection ) model, we place Ethernet in the physical layer, the first layer, the most concrete, as it deals with the actual hardware of networks, and the specific methods of sending bits from one device to another. It is the domain of hardware engineers and signaling experts. Most people talk about Ethernet as being a “layer two technology”, but Ethernet specifications really deal with both layer 2 ( data link layer ) and layer 1 ( Physical Layer ). Similarly, layers three and four are often related; protocol suites are often designed so that layer three and four protocols work together; examples being TCP ( Transport Layer ) and IP ( Network Layer ) in the TCP/IP protocol suit. Ethernet is commonly described as being a packet delivery system. In reality, an Ethernet frame is made up of all the necessary parts to fit the requirements and definition of a packet. An Ethernet frame has a header ( Preamble - Length ), payload ( LLC - Pad ), and a trailer ( Frame Check Sequence ) that are bundled together ( data link layer ) in a specifically organized manner for transmission ( Physical Layer ). Each Ethernet network interface card ( NIC ) has a unique identifier called a MAC ( Media Access Control ) address that is assigned by the card manufacturer. Each MAC address is a 48-bit number, of which the first 24 bits identify the manufacturer. The number is usually programmed into the hardware so that it cannot be changed. Within a LAN, addressing is done with a MAC address. Between LANs using TCP/IP, addressing is done using IP addresses. The CSMA/CD ( Carrier Sense Multiple Access with Collision Detection, IEEE 802.3 ) standard defines how Ethernet frames get onto an Ethernet network. Because to avoid collisions only one signal at a time can be transmitted on an Ethernet network, every Ethernet device listens to hear if another device is already transmitting ( in other words, sense other carriers to determine if one of them is transmitting a signal ). If the path is clear, any Ethernet device can transmit because of multiple access to the network. But all devices, even the one transmitting, continue to listen, because they are trying to detect collisions. Frames that collide must be retransmitted.
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