以太网(Ethernet)

以太网(Ethernet)是一种计算机局域网组网技术。IEEE制定的IEEE 802.3标准给出了以太网的技术标准。它规定了包括物理层的连线、电信号和介质访问层协议的内容。以太网是当前应用最普遍的局域网技术。它很大程度上取代了其他局域网标准，如令牌环、FDDI和ARCNET。 以太网的标准拓扑结构为总线型拓扑，但目前的快速以太网（100BASE-T、1000BASE-T标准）为了最大程度的减少冲突，最大程度的提高网络速度和使用效率，使用交换机（Switch）来进行网络连接和组织，这样，以太网的拓扑结构就成了星型，但在逻辑上，以太网仍然使用总线型拓扑的CSMA/CD介质访问控制方法。

历史：以太网是由日本施乐公司与DEC和Intel公司于1980年合作开发的一个局域网协议。

以太网中继器和集线器
As Ethernet grew, the 以太网集线器 was developed to make the network more reliable and the cables easier to connect.

For signal degradation and timing reasons, Ethernet segments have a restricted size which depends on the medium used. For example, 10BASE5 coax cables have a maximum length of 500 米s (1,640 英尺). A greater length can be obtained by using an Ethernet 中继器, which takes the signal from one Ethernet cable and repeats it onto another cable. Repeaters can be used to connect up to five Ethernet segments, three of which can have attached devices. This also alleviates the problem of cable breakages: when an Ethernet coax segment breaks, all devices on that segment are unable to communicate; repeaters allowed the other segments to continue working.

Like most other high-speed busses, Ethernet segments must be terminated with a resistor at both ends. For coaxial cable, each end of the cable must have a 50-欧姆 resistor and heatsink attached, called a terminator and affixed to a male N or BNC connector. If this is not done, the result is the same as if there is a break in the cable: the AC signal on the bus will be reflected, rather than dissipated, when it reaches the end. This reflected signal is indistinguishable from a collision, and so no communication can take place. A repeater electrically isolates the segments connected to it, regenerating and retiming the signal. Most repeaters have an "auto-partition" function, which partitions (removes from service) a segment when it has too many collisions or collisions that last too long, so that the other segments are not affected by the broken one. The repeater reconnects the segment when it detects activity without collisions.

People recognized the usefulness of cabling in a star topology, and network vendors started creating repeaters having multiple ports. Multi-port repeaters are now known as hubs. Hubs can be connected to other hubs and/or a coax backbone.

The first hubs were known as "multiport transceivers" or "fanouts". The best-known example is DEC's DELNI. These devices allow multiple hosts with AUI connections to share a single tranceiver. They also allow creation of a small standalone Ethernet segment without using a coax cable.

Network vendors such as DEC and SynOptics sold hubs which connected many 10BASE-2 thin coaxial segments.

The development of Ethernet on unshielded twisted-pair cables (UTP), beginning with StarLAN and continuing with 10BASE-T eventually made Ethernet over coax obsolete. These variations allowed unshielded twisted-pair Cat-3/Cat-5 cable and RJ45 telephone connectors to connect endpoints to hubs, replacing coaxial and AUI cables. Hubs made Ethernet networks more reliable by preventing problems with one cable or device from affecting other devices on the network. Twisted-pair Ethernet resolves the termination problem by making every segment point-to-point, so termination can be built into the hardware rather than requiring a special external resistor.

Despite the physical star topology, hubbed Ethernet networks are half-duplex and still use CSMA/CD, with only minimal cooperation from the hub in dealing with packet collisions. Every packet is sent to every port on the hub, so bandwidth and security problems aren't addressed. The total throughput of the hub is limited to the speed of a single link, either 10 or 100 Mbit/s, minus the overhead for preambles, inter-frame gaps, headers, trailers, and padding. Collisions also reduce the total throughput, especially when the network is heavily loaded. In the worst case when there are lots of hosts with long cables that transmit many short frames, excessive collisions that seriously reduce throughput can happen with loads as low as 50%. A more typical configuration can tolerate higher loads before collisions seriously reduce throughput.

桥接和交换
While repeaters could isolate some aspects of Ethernet segments, such as cable breakages, they still forward all traffic to all Ethernet devices. This creates significant limits on how many machines can communicate on an Ethernet network. To alleviate this, bridging was created to communicate at the data link layer while isolating the physical layer. With bridging, only well-formed packets are forwarded from one Ethernet segment to another; collisions and packet errors are isolated. Bridges learn where devices are, by watching MAC addresses, and do not forward packets across segments when they know the destination address is not located in that direction. Control mechanisms like 生成树协议 enable a collection of bridges to work together in coordination.

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