There is the concept of QoS because there is some relation of service between the neighboring layers or peer-to-peer layers in network.

网络各个层次之间、对等层次之间都存在服务与被服务的关系，因此就存在服务质量的概念。

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To do so they're pushing the frontiers of imaging technology to peer beneath miles of water, rock and salt layers thousands of feet thick--all in the hopes of finding billions of barrels of oil trapped below.

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