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CCNA Training Course: OSI Model Part 2

Posted: October 21st, 2016

How Layers are used

In a layered system, a layer is considered to be a service provider to the layer above it. The upper layer is considered to be a service user by the lower layer. The service user avails itself of the services of the layer below by sending a transaction to the provider. This transaction informs the provider as to the nature of the service that is to be provided.


A layer cannot be by passed, even if the end user does not require the services of that layer, the user must still “pass though” the layer on the way to the next adjacent layer, this pass though will only invoke a small set of code and therefore translates to some processing overhead.

The OSI Model refers to the layers with the terms N, N+ and N-. The layer with is the focus of attention is designated layer N. The layer above is N+1 and the layer below is N-1.

If the Network layer is the focus of attention then the Network layer is N, the Transport layer is designated N+1, and the data link layer is designated N-1.



Each layer contains entities that exchange data and provides functions (horizontal communications)

With peer entities at other computers.

For example Layer N in machine A communicates logically with Layer N in machine B, layers N+ communicate logically with one another likewise with layers N-.

At each layer at a transmitting station adds header information to data. The headers are used to establish peer-to-peer sessions across nodes (horizontal communications). At the receiving side, the layer entities use the headers to implement actions created by the peer entity at the transmitting side.

The figure below shows how a machine (a) sends data to another machine (b). Data is passed from the upper layers or the user application to layer N+1. Layer N+1 adds a header to the data (labelled N+1 in the figure). Layer N+1 also performs actions based on the information in the transaction that accompanied the data from the upper layer.


Layer N+1 passes the data unit and its header to the layer N. This layer performs some actions based on the information in the transaction, and adds its header N to the incoming traffic. This traffic is passed across the communications line (or through a network) to the receiving machine B.

At ‘B’, the process is reversed. The headers that were created at machine ‘A’ are used by machine ‘B’ to determine what actions are to be taken at the peer layer. As the data and the existing headers are sent up the layers, the respective layer removes its header, performs the defined actions, and passes the traffic on up to the next layer.

The user application at machine ‘B’ is presented only with user data, which was created by the sending user application (machine A). These user applications are unaware of the many operations in each layer that were invoked to support the end-user data transfer.

The headers created and used at the peer layers are not to be altered by any non-peer layer. As a general rule, the headers from one layer are treated as “Transparent data” by any other layer.

There are exceptions to the rules, as examples, data may be altered by a non-peer layer for the purposes of compression, encryption or other forms of syntax alteration. This type of alteration is allowed if the data are restored to the original syntax when presented to the receiving peer layer.

An exception to the exception, the presentation layer may alter the syntax of the data permanently, because the receiving application layer has requested the data in a different syntax (ASCII instead of a bit string)