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CISCO CCNA Certification knowledge to pass the exam
(Taken from the CISCO WEB site)

Knowledge of OSI Reference Model

1) 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 systemenvironment, 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 thesebounds. 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 isinteraction 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.

CONCEPT OF A LAYER

Each layer contains a logical groupings of functions that providespecific services for facilitating a communication. A function, or a group of functions, making up a functional unit is a logical entity that accepts one or more inputs (arguments) and produces a single output (value) determined by the nature of the function. Functions can be grouped in a collective unit, which is then defined as (N) layer having (N+1) layer an upper layer boundary and (N-1) layer asa lower boundary. The N layer receives services from N-1 layer and provides services to N+1 layer.

SEVEN LAYERS OF THE OSI MODEL AND THEIR FUNCTIONS

• Layer 7 is the APPLICATION layer: provides services directly to applications. Responsible for identifying and establishing the availability of the intended partner, and required resources. It is also responsible for determining if there existsufficient communication resources to reach the remote partner.

• Layer 6 is the PRESENTATION layer: Data encryption, decryption, compression and decompression are functions of this layer. It does this by using Abstract Syntax Notation 1 (ASN.1) ASN.1 standardization allows differing computer architectures to exchange data that are from differing computer architectures.

• Layer 5 is theSESSION layer: facilitates a dialog between communicating systems and controls the dialog. Offers three different dialogs, simplex, half-duplex and full duplex. Session is set up by connection establishment, data transfer and connection release.

• Layer 4 is the TRANSPORT layer: Segments data and also reassembles data from upper layers. Delivers data in a connection and connection less modes.Includes simplex (one way) half duplex (both ways one at a time) full duplex (both ways simultaneously). Also flow control and error recovery.

• Layer 3 is the NETWORK layer: Establishes a connection between two nodes by physical and logical addressing. Includes routing and relaying data through internetworks. This layer’s primary function is to deliver packets from the source network to the...
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