Encapsulation/Decapsulation - the process of adding/removing control information as a message passes through the layered model Source Destination Message Application Application Message 4 Message Transport Transport 4 Message Router 3 4 Message Network 3 4 Message Network 3 4 Message Source Encapsulation Application Layer Develops the payload (message) to be transmitted Transport Layer Adds the transport layer header to the payload Contains Identifiers of source and destination application programs Flow, error, and congestion control Resulting packet called segment (TCP) or user datagram (UDP) Network Layer Takes segment or user datagram and adds Addresses of source and destination devices, etc. Resulting packet called a datagram Data link Layer Takes datagram and adds Adds a link layer address of the next device to be reached Resulting packet called a frame Router Encapsulation/Decapsulation Router conducts both decapsulation and encapsulation because it is connected to two or more links When bits are received at the data link layer the frame is decapsulated and the datagram is sent to the network layer The network layer inspects the source and destination addresses in the data gram header and consults its forwarding table to determine the next hop (i.e. the next device in the procession of the message) The datagram should remain unchanged unless there is a need to fragment the datagram because it is too big to pass through the next data link The datagram is passed to the next link’s data link layer The data link layer encapsulates the datagram into a frame and passes the frame to the physical layer for transmission Destination Decapsulation Each layer decapsulates the packet received, removes the payload, and delivers the payload to the next-higher layer until the message is received by the application Decapsulation at the destination involves error checking 2 3 4 Message Data Link 2 3 4 Message Data Link 2 3 4 Message 1 2 3 4 Message Physical 1 2 3 4 Message Physical 1 2 3 4 Message