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Although they're technically different, in networking parlance multiplexing and encapsulation are used interchangeably. Multiplexing is the act of sending multiple streams of data over a single connection. Nodes achieve multiplexing by using encapsulation, as shown in Figure 1.7.

Figure 1.7 Encapsulation of a TCP segment and IP packet inside an Ethernet frame

As an example, suppose a TCP-based client application needs to send some data to a server. The application instructs the host operating system to open a TCP connection to the server. Once the connection is open, the application uses it to send some data to the server. Here's what happens: The host's operating system creates a TCP segment, places the application data in the Data field, and populates the rest of the fields in the TCP header appropriately. The OS encapsulates the TCP packet inside an IP packet, with the Protocol field set to 6 (TCP). It then sends the IP packet to the Ethernet driver, which encapsulates the IP packet in an Ethernet frame, which has the Type field set to 0x0800 (IP). The Ethernet driver passes the frame to the network interface, which converts the data to bits and puts it out on the wire.