To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field.Then it adds whatever headers and trailers the layer needs to perform its function.

• 要提供这种服务，下一层透过封装作业将上一层的PDU放入资料栏位中，然后加入各层所需的表头和尾标来执行功能。