11标准定义了3种物理层通信技术,这3种技术不包括(19)。Watch a baby between six and nine months old, and you will observe the basic concept of geometry being learned. Once the baby has mastered the idea that space is three dimensional, itreaches out and begins grasping various kinds of objects. It is then, from perhaps nine to fifteen months, that the concepts of sets and numbers are formed. So far, so good. But now an ominous development takes place. The nerve fibers in the brain insulate themselves in such a way that the baby begins to hear sounds very precisely. Soon it picks up language, and it is then brought into direct communication with adults. From this point on, it is usually downhill all the way for mathematics, because the child now becomes exposed to all the nonsense words and beliefs of the community into which it has been so unfortunate as to have been born. Nature, having done very well by the child to this point, having permitted it the luxury of thinking for itself for eighteen months, now abandons it to the arbitrary conventions and beliefs of society. But at least the child knows something of geometry and numbers; and it will always retain some memory of the early halcyon days, no matter what vicissitudes it may suffer later on. The main reservoir of mathematical talent in any society is thus possessed by children who are about two years old, children who have just learned to speak fluently.描述了项目的可交付物和产生这些可交付物所必须做的项目工作,就此在所有项目干系人之间建立了共识。。