204 CHAPTER 8 Fundamentals of communication networks Internet, wireless local area network (LAN), wireless mesh networks, and wireless sensor networks are based on the packet switching technology. The asynchronous transfer mode (ATM) network is a virtual circuit switching network. 8.2.3 Encapsulation and Multiplexing At a source host, application data are sent down through the layers in the protocol stack, while each layer adds a header (and may be a trailer) to the data received from its higher layer (called protocol data unit, PDU). The encapsulation process is shown in Figure 8.2. When the packet arrives at the destination, it is sent up through the protocol stack. At each layer, the corresponding header and trailer are stripped and processed for control functions in that layer. The recovered higher-layer data are delivered to the upper layer. In TCP/IP, different higher-layer protocols can use the service provided by the same lower-layer protocol, and the same higher-layer protocol can use the service provided by different lower-layer protocols. In the first case, each packet sent down to the lower layer should have an identifier indicating to which higher-layer module it belongs. Multiplexing and demultiplexing are performed at different layers using