Within the measurement infrastructure, counters are structured using a two-levelhierarchy.
在测量基础架构中，用两级层次对计数器进行结构化。
2
The Internet is a three levelhierarchy composed of backbone networks, mid-level networks, and stub networks.
因特网由三个层次组成：骨干网，中间层次网和子网络。
3
Searching variables in two levelhierarchy, it is possible to avoid the appearance of unfeasible results. So the search space is greatly reduced and efficiency is improved.