Internetworking Part 4

Now is a good time to introduce the networking reference models that permit the communications within our internetworking up through the previous sessions (Part 3).

In the beginning, most computers were only able to communicate with other computers from the same manufacturer.  In the 1970s the Open Systems Interconnection (OSI) reference model was created to overcome these communications problems.  There are other models  in use such as the DoD Reference and the Cisco Hierarchical Models, which we will discuss.

First, the OSI Model.  This is a reference model, or set of guidelines, that application developers can use in the creation and implementation of applications that run on a network, which provides a  framework within which network standards can be managed.

The OSI model has 7 distinct layers, which are divided in to two groups. The upper group (top 3 layers) define how the end-to-end host applications will communicate with each other.  The bottom group (bottom 4 layers)  define how the data is to be handles and transmitted between the hosts, end-to-end.  The top group are the Application, Presentation, and Session layers; the bottom group  The following operate at all seven layers of the OSI model:  Network management stations (NMSs); web and application servers; gateways (not default gateways); and network hosts.

The upper layers:  Application layer, Presentation layer, and the Session layer furnishes a user interface, “presents” data to the application layer, and maintains data separation between different applications.

The Application Layer:  This is where you (the user) has a direct connection to the computer by inputting data, or making requests.  This layer is also responsible for resolving the availability of communication and sufficiency of resources for data input.  The protocols associated with this layer are HTTP, FTP, and SMTP.

The Presentation Layer:  As mentioned before, this layer “presents” the data to the Application layer, which is where its name originates.  It is also in control of the data translation, code formatting and conversion functions (i.e., receives generically formatted data and converst it to its original format).  The protocols associated with this layer are ASCII, EBCDIC, JPEG, GIF, and MPEG.

The Session Layer:  This layer’s operation is to create, organize, and disassemble between Presentation layer components.  In essence, this layer can open many “seesions” and will keep all of those “sessions” and their respective data separate.

Next session will be on the lower layers.

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