Hard-Network

CISCO CCNA Certification knowledge to pass the exam
(Taken from the CISCO WEB site)
Knowledge of OSI Reference Model
Identify and describe the functions of each of the seven layers of the OSI reference model.
Open Systems Interconnection (OSI)
OSI consists of two environments; the OSI environment, which is made up of seven layers of OSI protocols and the local system environment, which is the end computer system. The reason for dividing the environment in this way was to avoid interfering with the innovation of the design and implementation of computer systems. OSI facilitates a vehicle to communicate between dissimilar or similar computer based systems. The local computer system environment has a closed operating system and performs its designed functions within these bounds. All application processes that do not require communicating with other systems to complete its tasks, will provide, the end result with out any problems. However when an application process needs to communicate with another application process located in a remote system, both systems must become open to the OSI environment Many operations and concepts are involved in this process. There is interaction between peer entities within a layer and interaction between layers.
Important concepts to understand OSI Layering are:
Each layer performs unique and specific task

A layer only has knowledge of its immediately adjacent layers

A layer uses services of the layer below

A layer performs functions and provides services to the layer above

A layer service is independent of the implementation
The Application layer is unique among the seven layers in that, it has no layer above. The application consists of ‘Service Elements’ that are incorporated within the application process when it needs to become a part of the OSI environment.