This section is a reference source based on the OSI model. Each layer is discussed including the software and devices associated with each layer. This is a best place to start if you are justing learning or if you need to brush up on your skills. Just a note, the OSI model is a theoretical model and doesn't really exist. It was created to aid in understanding how real protocols operate and how they can interconnect. A solid understanding of the OSI model is very important in understanding how networks work and how network devices like hubs, bridges, switches, routers and firewalls work.

The ISO (International Standards Organization) has created a layered model called the OSI (Open Systems Interconnect) model to describe defined layers in a network operating system. The purpose of the layers is to provide clearly defined functions to improve internetwork connectivity between "computer" manufacturing companies. Each layer has a standard defined input and a standard defined output.

This is a top-down explanation of the OSI Model, starting with the user's PC and what happens to the user's file as it passes though the different OSI Model layers. The top-down approach was selected specifically (as opposed to starting at the Physical Layer and working up to the Application Layer) for ease of understanding of how the user's files are transformed through the layers into a bit stream for transmission on the network.

I've thrown together a quick video that gives "the big picture" of the OSI model and how TCP/IP fits in. It covers the layers, mapping between the two, the protocol data units, the addressing and the network devices associated with each layer:

There are 7 Layers of the OSI model and they are always presented in this manner starting with layer 7 at the top (there is an unofficial new layer 2.5 where MPLS sits).There are a few ways of remembering the OSI layers, one is the phrase "Please Do Not Take Salami Pizza Away".

7. Application Layer - Away
6. Presentation Layer - Pizza
5. Session Layer - Salami
4. Transport Layer - Take
3. Network Layer - Not
2. Data Link Layer - Do
1. Physical Layer - Please

Understanding the function of each layer is instrumental in understanding data communication within networks whether Local, Metropolitan or Wide. The Reference section is divided into the following sections:

Layer 7 - Application layer - This layer deals with network aware application. These are applications that require a network to function. Learn about:

Layer 6 - Presentation layer - The Presentation layer deals with translation of file formats, encryption of data and compression of data.

Layer 5 - Session layer - The Session layer manages the initial start-up of a session and the orderly closing of a session. The Session layer also manages Logon procedures and Password recognition and permissions.

Layer 4 - Transport layer - The Transport layers main job is to provide error free end to end delivery of data. Learn about

Layer 3 - Network layer - The Network layer is concerned with finding the shortest path to the destination. This usually means finding the fastest route through multiple networks to the destination. Learn about:

Layer 2.5 - MPLS - This layer is in between the Data Link Layer and the Network layer. It is not really part of the OSI model but aids in describing the operation of MPLS. MPLS modifies the Data Link layer frame with labels to aid in routing the data.

Layer 2 - Data Link layer - The Data Link layer is in charge of whose turn it is to talk on the wire and has the job of organizing the data in a frame. Learn about:

Layer 1 - Physical layer - The Physical layer concerns itself with the transmission of bits and the network card's hardware interface to the network. The hardware interface involves the type of cabling (coax, twisted pair, etc..), frequency of operation (1 Mbps, 10Mbps, etc..), voltage levels, cable terminations, topology (physical shape of the network: star, bus, ring, etc..), etc.. Learn about:

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Copyright July 2013 Eugene Blanchard