The OSI Model or Open System Interconnection Model is a graphical and logical representation of how network information flows. It also explains how networks transfer from one device to another, if you want to be involved in computer networking you really should understand the OSI Model.

The OSI Model consists of 7 Layers and traffic follows these layers in order, so traffic from layer 4 has to go to layer 3 then layer 2 then layer 1 it cannot skip layers. But a little more on that later, lets first go over each layer individually.

Layer 1 – Physical Layer

• This layer consists of how information is phsyically transmitted across the wire. It is on or off
• A repeater, hub, modem, NIC, Firber Media Converter are all devices that are considered layer 1 devices.

Layer 2 – Data Link Layer

• Provides the functional and precedural means to transfer data between network devices, node-to-node
• This layer is broken down into two sub layers
  • Logical Link Control (LLC) Sublayer
    • This layer provides flow control, acknowledgement, and error notification
  • Media Access Control (MAC) Sublayer
    • Determines where one frame of data starts and ends
    • Uses CSMA/CD or other logic to determine who is allowed to access the device

Layer 3 – Network Layer

• The Network Layer is responsible for end-to-end packet delivery including routing between networks through other devices (routers)
• Establishes Connection Model
  • Connection-Oriented
    • Traffic is check for errors and has garuntee delivery through acknowledgements
  • Connectionless
    • Traffic is just sent with no acknowledgements and not garuntee, but is a faster connection
    • Error recovery is usually relyed on by the application

Layer 4 – Transport Layer

• Controls the reliability of a given link through flow control, segmentation/desegmentation, and error control
• Can keep track of packets and resend those that did not make it to their destination
• TCP and UDP are protocols used under the Transport Layer

Layer 5 – Session Layer

• Establishes, manages and terminates connections between the local and remote hosts
• Full Duplex and half duplex are parts of the Session Layer

Layer 6 – Presentatiol Layer

• Transforms data into something that the Application layer can use
• Encapsulates data into Session Protocol data units

Layer 7 – Applications Layer

• Obviously this layer is what you the user are closest to and interact with
• Some application layers protocols are HTTP, FTP and SMTP

This is just a basic overview of the OSI model to give you an idea and to interest you in some basic networking concepts.