Layer Interactions

The following sequence outlines the basics of processing at each layer and explains how each lower layer is providing a service to the next higher layer:

1. The physical layer (Layer 1) ensures bit synchronization and places the received binary pattern into a buffer (transfer across a medium). It notifies the data link layer that a frame was received after decoding the incoming signal into a bit stream.

2. The data link layer examines the frame check sequence (FCS) in the trailer to determine whether errors occurred in transmission (error detection). If an error has occurred, the frame is discarded. Some data link protocols perform error recovery, and some do not. The data link address(es) are examined so the receiving host can decide whether to process the data further. If the address is the receiving node's MAC address, processing continues (physical addressing). The data between the Layer 2 header and trailer is given to the Layer 3 software on the receiving end. The data link layer delivers the data across the local link.

3. The network layer (Layer 3) destination address is examined. If the address is the receiving host's address, processing continues (logical addressing) and the data after the Layer 3 header is given to the transport layer (Layer 4) software, providing the service of end-to-end delivery.

4. If error recovery was an option chosen for the transport layer (Layer 4), the counters identifying this piece of data are encoded in the Layer 4 header along with acknowledgement information (error recovery). After error recovery and reordering of the incoming data, the data is given to the session layer.

5. The session layer (Layer 5) can be used to ensure that a series of messages is completed. For example, this data might be meaningless if the next four exchanges are not completed. The Layer 5 header includes fields that signify that this session flow is a middle flow, not an ending flow, in a transaction (transaction tracking). After the session layer ensures that all flows are completed, it passes the data after the Layer 5 header to the Layer 6 software.

6. The presentation layer (Layer 6) defines and manipulates data formats. For example, if the data is binary instead of character oriented, the header will state the fact. The receiver will not attempt to convert the data using the default ASCII character set of Host B. Typically, this type of header is included only for initialization flows and not with every message being transmitted (data formats). After the data formats have been converted, the data (after the Layer 6 header) is then passed to the application layer (Layer 7) software.

7. The application layer (Layer 7) processes the final header and then examines the true end-user data. This header signifies agreement to operating parameters by the applications on the sending and receiving hosts. The headers are used to signal the values for all parameters; therefore, the header is typically sent and received at application initialization time only. For example, the screen size, colors supported, special characters, buffer sizes, and other parameters for terminal emulation are included in this header (application parameters).

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