以太网(Ethernet)

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。

早期以太网varieties

• 施乐以太网 -- the original, 3-Mbit/s Ethernet implementation, which in turn had two versions, Version 1 and Version 2, during its development. The version 2 framing format is still in common use.
• 10BROAD36 -- Obsolete. An early standard supporting ethernet over longer distances. It utilized broadband modulation techniques similar to those employed in cable modem systems, and operated over coaxial cable.
• 1BASE5 -- Also known as StarLAN, was the first implementation of Ethernet on 双绞线 wiring. It operated at 1 Mbit/s and was a commercial failure.

10 Mbps以太网

• 10BASE5 (also called Thickwire or Yellow Cable) -- this is the original 10 Mbit/s implementation of Ethernet. The early IEEE standard uses a single 50-ohm 同轴电缆 of a type designated RG-8, of maximum length 500 米. Transceivers could be connected by a so-called "vampire tap", which was attached by drilling into the cable to connect to the core and screen, or using N connectors at the end of a cable segment. An AUI cable then connected the transceiver to the Ethernet device. Largely obsolete, though due to its widespread deployment in the early days, some systems may still be in use. It requires precise termination at each end of the cable.
• 10BASE2 (also called Thinwire or Cheapernet) -- 50欧姆RG-58同轴电缆,200米, 连接所有的计算机,每台计算机使用T适配器连接到带有BNC连接器的网卡。线路两头需要终结器。For many years this was the dominant 10 Mbit/s Ethernet standard.
• StarLAN 10 -- First implementation of Ethernet on twisted pair wiring at 10 Mbit/s. Later evolved into 10BASE-T.
• 10BASE-T -- 使用3类双绞线或者5类双绞线的4根线 (2对绞线) 100米. A 以太网集线器 or 以太网交换机 sits in the middle and has a port for each node.
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