advantage：
1、provide checkpoints by stages for the project
2、waterfall model can be applied in iteration model
disadvantage：
1、The division of each stage is completely fixed, and a large number of documents are generated between stages, which greatly increases the workload
2、It can not adapt to the changes of users'needs
fixed order 固定顺序
瀑布模型，就是将软件生存周期的各项活动规定为按固定顺序而连接的若干阶段工作，关键在于顺序
the activities of software life cycle as several stages of work connected in a fixed order