The OSI Model when it was first created came out in two parts :
* The First part was the **Abstract Model**.
Mnemonics for memorizing these Layers in the proper order :
- All People Seem To Need Data Processing.
- Please Do Not Throw Sausage Pizza Away
* Bottom Layer
* Provides error-free transmission from one node to the next over physical media.
* Controls the operations of the subnetwork it is on.
* Ensures messages delivered error-free.
Message Segmentation Function :
* Accepts messages from session layer
* Splits message into smaller units
* Imposes message size limits on network layer protocols
* Prepares header for each smaller unit created
* Passes smaller units to network layers
* Reassembles message at destination
* Header for smaller units contain certain elements
* Header contains start and end flag
* Header contains sequence information
Other Transport Layer Functions :
* Message acknowledgment
* Provides reliable end-to-delivery of messages.
* End-to-end delivery accompanied by acknowledgments.
* Message traffic control
* Controls rate of traffic send when no buffers available
* Session multiplexing
* Breaks all the data coming in on one link into separate data streams
* Those data streams called sessions.
* Tracks which message belongs to which session.
End-to-End Layers :
* Transport layer above layers not responsible for transmission between nodes.
* Transport and above layers responsible for source to destination transmission
* Source-to-destination transmissions also called end-to-end transmissions.
* Upper layers not concerned with underlying communications facility
* Responsible for establishing sessions between processes running on different computers.
Establishment Maintenance Termination :
- Allows application processes on different machines to do several things between the machines.
- Allows processes to establish a connection.
- Allows processes to use a connection.
- Allows processes to terminate a connection.
- Each connection called a session.
Session Support :
- Performs the function of allowing processes to communicate over network.
- Performs security
- Performs name recognition.
- Performs logging on.
- Performs other functions that are less common.
* Formats data to be presented to the application layer.
Presentation Layer Functions :
* Character code translation.
* ASCII to EBCDIC
* Data conversation.
* Bit order
* Integer-floating point
* Data compression
* Reduces number of bits needed to transmit data.
* Data encryption
* Encryption of data for security purposes.
* Encryption of passwords.
* Serves as window for uses and applications to access network service.
Application Layer Common Function :
- Resource sharing.
- Device redirection
- Remote file access.
- Remote printer access.
- Network management.
- Directory services.
- Instant messaging