1. 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