用repeater有什么限制?

Andrew S. Tanenbaum 的 Computer Networks, Fourth Edition 的 4.3.1节最后一段说:

A system may contain multiple cable segments and multiple repeaters, but no two transceivers may be more than 2.5 km apart and no path between any two transceivers may traverse more than four repeaters.

这是为什么呢?

While repeaters could isolate some aspects of Ethernet segment, such as cable breakages, they still forwarded all traffic to all Ethernet devices. This created practical limits on how many machines could communicate on an Ethernet network. Also as the entire network was one collision domain and all hosts had to be able to detect collisions anywhere on the network, and the number of repeaters between the farthest nodes was limited. Finally segments joined by repeaters had to all operate at the same speed, making phased-in upgrades impossible.

 

The elimination of the collision domain(by switch and bridge) also means that all the link's bandwidth can be used and that segment length is not limited by the need for correct collision detection (this is most significant with some of the fiber variants of Ethernet).

可见"冲突检测"是一个重要原因

更详细的说明,可参见: Bridging and switching