以太网(Ethernet)

www.net130.com     日期:2005-7-5    浏览次数:
出处:维基百科、网络大典

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.

Early bridges examined each packet one by one, and were significantly slower than hubs (repeaters) at forwarding traffic, especially when handling many ports at the same time. In 1989 the networking company Kalpana introduced their EtherSwitch, the first Ethernet switch. An Ethernet switch does bridging in hardware, allowing it to forward packets at full wire speed.

Most modern Ethernet installations use 以外网交换机es instead of hubs. Although the wiring is identical to hubbed Ethernet, switched Ethernet has several advantages over shared medium Ethernet including greater bandwidth and better isolation from misbehaving devices. Switched networks typically have a 星型拓扑, even though they may still implement a single Ethernet shared medium from the viewpoint of attached machines, if they use the half-duplex option. Full-duplex Ethernet in the 10BASE-T and later standards is not a shared-medium system.

Initially, Ethernet switches work like Ethernet hubs, with all traffic being echoed to all ports. However, as the switch "learns" the end-points associated with each port, it ceases to send non-broadcast traffic to ports other than the intended destination. In this way, Ethernet switching can allow the full wire speed of Ethernet to be used by any given pair of ports on a single switch.

Since packets are typically only delivered to the port they are intended for, traffic on a switched Ethernet is slightly less public than on shared-medium Ethernet. Despite this, switched Ethernet should still be regarded as an insecure network technology, because it is easy to subvert switched Ethernet systems by means such as ARP spoofing and MAC flooding, as well as for network administrators to use monitoring functions to copy traffic from the network.

When only a single device (anything but a hub) is connected to a switch port, full-duplex Ethernet becomes possible. With only two devices on the Ethernet segment, collision detection is not required and both devices can transmit at the same time. This doubles the aggregate bandwidth of the link (although the bandwidth for each direction remains the same), but more importantly the lack of collisions allows nearly the entire bandwidth to be used.

It is essential that both the switch port and the device connected to it use the same duplex setting. Most 100BASE-TX and 1000BASE-T devices support auto-negotiation, where they signal the speed and duplex to use. However, if auto-negotiation is disabled or not supported, the duplex must be set by auto-detection or manually on both the switch port and the device to prevent duplex mismatch, a common cause of problems with Ethernet (the device set to half-duplex will report late collisions and the device set to full-duplex will report runts). Many low-end switches lack the ability for manual speed and duplex setting, so ports always try to auto-negotiate. When auto-negotiation is enabled but does not succeed (e.g., because the other device does not support it), auto-detection sets the port to half-duplex. The speed can be automatically sensed, so connecting a 10BASE-T device to a 10/100 switch port with auto-negotiation enabled will correctly result in a half-duplex 10BASE-T connection. But connecting a device configured for full duplex 100 Mbit operation to a switch port configured to auto-negotiate (or vice versa) will result in a duplex mismatch.

Even when both ends of a cable are capable of autosensing speed and duplex settings, it is very common for them to guess wrongly and fall back to 10 Mbit mode. Therefore, if performance is worse than expected, one should check whether a computer has put itself into 10 Mbit mode, and if one knows the other end is 100 Mbit capable, manually force it into the correct mode.

Problems also occur when two nodes try to operate at speeds faster than the cable can support, such as attempting 100BASE-T on Category 3 cable or 1000BASE-T on Category 3 or Category 5 cable. Unlike ADSL and conventional dialup modems, which perform an elaborate "training" sequence to determine the maximum data rate supported by the link, Ethernet nodes merely exchange speed capability messages and choose the highest speed supported by both ends. No attempt is made to see if the link can actually run at that speed, so if it's beyond the cable's capability, then the link will fail. The solution is to force either or both ends down to a speed supported by the cable.


以太网类型Varieties
Other than the framing types mentioned above, most of the other differences between Ethernet varieties have all been variations on speed and wiring. Therefore, in general, network protocol stack software will work identically on most of the following types.

The following sections provide a brief summary of all the official ethernet media types. In addition to these official standards, many vendors have implemented proprietary media types for various reasons—often to support longer distances over fiber optic cabling.

很多以太网卡和交换设备都支持多速率,设备之间通过自动协商设置最佳的连接速度和双工方式。如果协商失败,多速率设备就会探测另一方使用的速率但是默认为半双工方式。10/100以太网端口支持10BASE-T和100BASE-TX。10/100/1000支持10BASE-T,100BASE-TX,和1000BASE-T。

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