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Explain Operating System Interface OSI Layer 6 – Presentation Layer

At number 6 in the OSI model we have the presentation layer.

About Presentation Layer – OSI Model

– The context between the entities of the application layer are established by this layer only.
– Here, different syntax and semantics are used by the high level entities only if a mapping service is provided between them by the services of the presentation layer.
– If the mapping service is provided by the presentation layer, it encapsulates the data units of the presentation service in to data units of the session protocol and passes them down in to the stack.
– The presentation layer translates the application and network formats thus providing independence from the encryption i.e., the data representation.
– The data is transformed in to a form that is acceptable by the application by this layer.
– Furthermore, it is this layer that formats as well as encrypts the data that is to be sent across the network.
– This is the reason why this layer is sometimes referred to as the syntax layer.

– For the original presentation structure, the following were used:
+ Basic encoding rules of ASN. 1 (i.e., the abstract syntax notation one)
+ Capabilities such as:
a) Conversion of the EBCDIC – coded text file in to an ASCII coded file
b) Serialization of objects
c) Serialization of data structures from XML and to XML.

– This 6th layer of OSI model is the data translator of the whole network.
– The responsibility of delivering and formatting the information so that it can be displayed or processed further by the application layer is the responsibility of this layer.
– Because of this, the application layer is relieved of the issues concerning the syntactical differences that arise in data representation within the user systems.
– It is at this layer that the data structure and presentation is considered by the application programmers.
– Therefore, this is the lowest layer that concerns them in the network structure.
– Data structures and presentation is considered in place of the data grams or data packets that are exchanged between the hosts.
– Issues concerning the string representation are also dealt by this layer.
– An example of such issues is whether the strings use the C/ C++ method i.e., the null terminated strings or the pascal method i.e., a specified amount of bytes follow the integer length field.
– The basic behind this whole thing is that the data to be move must be pointed to by the application layer and the rest things will be taken care of by the presentation layer.
– The second capability i.e., the serialization of the objects and data structures that we mentioned above is one of the key functionality of the presentation layer.
– The data structures and objects are serialized in to flat – byte strings via mechanisms like XML, TLV and so on.
– Typically, encryption is done at this layer also apart from other layers such as the application layer, session layer, network layer and the transport layer.
– But encrypting at each of these levels has its own pros and cons.
– Presentation layer also deals with the decryption rather than just dealing with the encryption part.
– Example: When you log off from a banking site, the data as it was received will be de-crypted by the presentation layer.
– Another example is of the structure representation that is standardized at this layer, done via XML.
– Both simple and complicated pieces of data are standardized at this level. For example:
a) Exact way of transmitting a streaming video and
b) Objects in object oriented programming.

– A number of applications and protocols do not make any differences between the application layer and the presentation layer.
– HTTP or the hypertext transfer protocol is the best example.
– It is considered to be an application layer protocol layer even though it has many aspects of the presentation layer such as ability of identifying the proper character encoding for conversion.
– This is then carried out in the application layer itself.

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