Chapter 5: General OSI Reference Model Issues and Concepts

• The layers of the OSI model represent how close the layer is to the physical hardware.\
• The higher the level, the more abstract it is
• Layers 1-4(physical, data link, network, transport) represents the lower layers which are in charge of formatting, encoding, and transmission of data. They generally don't care what the data is, only how it gets to its destination. These layers also provide services to the upper layers.

• layers 1 and 2 generally work together as do layers 3 and 4

• Layers 5-7(session, presentation, application) are in charge of how it interacts with the user and implementing the application.
• N Notation: can be used to describe layers relative to each other i.e. this layer provides a service to layer N+1
• Interfaces are used as communication between layers. i.e. layer 6/7 interface works between layer 6 and 7
• Horizontal Communications are crucial so that each layer can pass information and then decode using the same layer

• Protocols are the rules that control horizontal communications. Generally, they are referred to as protocol data units or PDU's. N-1 is in charge of passing its service data unit or SDU into layer Ns PDU format