連結
Layer Services and Data Encapsulation
As we’ve already discussed in our look at protocols, the communication between layers higher than layer one is logical; the only hardware connection is at the physical layer. Thus, in order for a protocol to communicate, it must pass down its PDU to the next lower layer for transmission. We’ve also already seen that using OSI terminology, lower layers are said to provide services to the layers immediately above them. One of the services each layer provides is this function: to handle and manage data received from the layer above.
At any particular layer N, a PDU is a complete message that implements the protocol at that layer. However, when this “layer N PDU” is passed down to layer N-1, it becomes the data that the layer N-1 protocol is supposed to service. Thus, the layer N protocol data unit (PDU) is called the layer N-1 service data unit (SDU). The job of layer N-1 is to transport this SDU, which it does in turn by placing the layer N SDU into its own PDU format, preceding the SDU with its own headers and appending footers as necessary. This process is called data encapsulation, because the entire contents of the higher-layer message are encapsulated as the data payload of the message at the lower layer.