What is the topology used by Ethernet?

The topology used by Ethernet is a bus topology. Ethernet is a computer local area network technology. In order to reduce conflicts and maximize network speed and efficiency, the current Fast Ethernet uses switches to connect and organize the network.

The operating environment of this tutorial: Windows 7 system, Dell G3 computer.

The topology used by Ethernet is basically bus type.

Ethernet is a computer local area network technology. The IEEE 802.3 standard of the IEEE organization formulates the technical standard of Ethernet, which specifies the content including physical layer wiring, electronic signals and media access layer protocols. Ethernet is currently the most commonly used LAN technology, replacing other LAN technologies such as Token Ring, FDDI and ARCNET.

The standard topology of Ethernet is a bus topology, but the current Fast Ethernet (100BASE-T, 1000BASE-T standards) uses Switches for network connectivity and organization. As a result, the topology of Ethernet becomes a star; but logically, Ethernet still uses bus topology and CSMA/CD (Carrier Sense Multiple Access/Collision Detection). bus technology.

Ethernet realizes the idea of ​​multiple nodes of a radio system on the network sending information. Each node must obtain a cable or channel to transmit information, sometimes also called Ether. (The name comes from the electromagnetic radiation medium hypothesized by 19th-century physicists - optical ether. Later research proved that optical ether does not exist.) Each node has a globally unique 48-bit address, which is the MAC address assigned to the network card by the manufacturer. , to ensure that all nodes on the Ethernet can identify each other. Because Ethernet is so common, many manufacturers integrate Ethernet cards directly into computer motherboards.

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The story of Ethernet begins during the ALOHA period. The exact time was when a student named Bob Metcalfe obtained a bachelor's degree from MIT degree, and then moved across the river to Harvard University to pursue a Ph.D. During his studies, he came across the work of Abramson, which interested him. After graduating from Harvard, he decided to stay in Hawaii for a vacation before taking a formal job at Xerox Palo Alto Research Center in order to help Abramson work. When he arrived at the Palo Alto Research Center, he saw that researchers there had designed and built machines that would later be called personal computers, but these machines were all in isolation; he used the knowledge gained from helping Abramson work with his colleague David Boggs designed and implemented the first local area network. The LAN uses a long thick coaxial cable and runs at 3Mbps.

They named this system Ethernet, and it was once thought that electromagnetic radiation could be transmitted through it.

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Shared Medium

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) technology provides for sharing among multiple computers A channel method. This technology first appeared in the ALOHAnet, developed by the University of Hawaii in the 1960s, which uses radio waves as a carrier. This method is simpler than Token Ring or Master Control Network. When a computer wants to send a message, it transitions between the following actions and states:

• Start - If the line is free, start the transmission, otherwise jump to step 4.

• Send - If a conflict is detected, continue sending data until the min echo receive interval is reached to ensure that all other repeaters and terminals detect the conflict, and then jump to the 4 steps.

• Successful transmission - Report successful transmission to the higher layer network protocol and exit transmission mode.

• Line Busy - Keep waiting until the line is free.

• Line Idle - Before reaching the maximum number of attempts, go to step 1 and try again at random intervals.

• Exceeded the maximum number of transmission attempts - report the transmission failure to the higher layer network protocol and exit transmission mode.

Because all communication signals are transmitted on shared lines, even if the information is only intended to be sent to one of the terminals (destination), it will be sent to the terminal on the line in the form of broadcast. All computers. Under normal circumstances, the network interface card will filter out information that is not sent to itself, and will issue an interrupt request to the CPU only when it receives information whose target address is its own, unless the network card is in promiscuous mode. This "one speaks, everyone listens" quality is a security weakness of shared-medium Ethernet, because a node on the Ethernet network can choose whether or not to listen to all information transmitted on the line. Sharing the cable also means sharing the bandwidth, so Ethernet can be very slow in some situations, such as after a power failure when all network terminals are rebooted.

Repeater

Due to signal attenuation and delay, there are distance restrictions for Ethernet segments based on different media. For example, 10BASE5 coaxial cable has a maximum distance of 500 meters (1,640 feet). The maximum distance can be achieved through Ethernet repeaters, which amplify the signal in the cable and transmit it to the next segment. Repeaters can connect up to 5 network segments, but can only have 4 devices (that is, a network segment can connect up to 4 repeaters). This can alleviate problems caused by cable breaks: when a section of coaxial cable is disconnected, all devices on this section cannot communicate, and the repeater can ensure that other network segments work normally.

Similar to other high-speed buses, Ethernet segments must be terminated with resistors at both ends. For coaxial cables, the terminals at both ends of the cable must be connected to a 50-ohm resistor called a "terminator" and a heat sink. If this is not done, a situation similar to a cable break will occur: when the AC signal on the bus arrives It will be reflected when it terminates and cannot dissipate. The reflected signal will be considered a collision, making communication unable to continue. Repeaters can electrically isolate, strengthen and synchronize signals between two network segments connected to it. Most repeaters have a feature called "auto-isolation" that isolates segments that have too many conflicts or that conflict for too long, so that other segments are not affected by the damaged segment. The repeater can restore the network segment's connection after detecting that the conflict has disappeared.

hub

Although the Ethernet using a hub network is physically a star structure, it is still logically a bus type. The half-duplex communication method uses CSMA/CD. Collision Detection Method,The hub does little to reduce packet collisions. Every packet is sent to every port on the hub, so bandwidth and security issues remain unresolved. The total throughput of a hub is limited by the speed of a single connection (10 or 100 Mbit/s), taking into account the minimum overhead in preamble, transmission interval, header, trailer, and encapsulation. Collisions also often reduce throughput when the network is overloaded. The worst-case scenario is that when many hosts with long cables transmit many very short frames, the network may be fully loaded at only about 50% due to too many collisions. In order to maximize the load on the network before conflicts seriously reduce the transmission volume, some settings are usually made to avoid similar situations.

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