What Is a Presentation Layer?

The presentation layer services the application layer upwards and accepts the services from the session layer downwards. The presentation layer provides a representation service for the information transmitted between application processes. It only cares about the syntax and semantics of the information sent.

The presentation layer services the application layer upwards and accepts the services from the session layer downwards. The presentation layer provides a representation service for the information transmitted between application processes. It only cares about the syntax and semantics of the information sent.

Chinese name

Presentation layer

Foreign name

Presentation Layer

Layers

Layer 6 of the OSI reference model

Presentation layer specific introduction

The presentation layer is located in the sixth layer of the OSI layered structure. One of its main functions is to provide a common language for heterogeneous machine communication in order to be interoperable. This type of service is needed because different computer architectures use different data representations. Unlike the fifth layer, which provides transparent data transport, the presentation layer handles all issues related to data representation and transport, including conversion, encryption, and compression. Each computer may have its own internal method of representing data, such as ASCII and EBCDIC codes, so a representation layer agreement is required to ensure that different computers can understand each other.

For example , the IBM mainframe uses EBCDIC encoding, while most PCs use ASCII. In this case, a presentation layer is needed to complete this conversion. If you want to remember this layer 6 with as few words as possible, it is "a common data format."

The presentation layer provides three services for the application layer

Grammar conversion: Grammar conversion involves code conversion and character set conversion, data format modification, data structure operation adaptation, data compression, data encryption, and so on.

Grammar selection: Grammar selection is a means of providing an initial selection of a grammar and subsequently modifying this selection.

Connection management: Use the services provided by the session layer to establish a presentation connection, manage the data transport and synchronization control on this connection, and terminate the connection normally or abnormally. ^[1]

Actual function of the presentation layer

Functions of the presentation layer

Network security and confidentiality management; text compression and packaging; Virtual Terminal Protocol (VTP).

Grammar conversion: convert abstract grammar to transmission grammar, and achieve the opposite conversion on the other side (ie, transfer grammar to abstract grammar). The contents involved are code conversion, character conversion, modification of data format, and adaptation to data structure operations, data compression, encryption, and so on.

Syntax negotiation: Negotiate and select the appropriate context according to the requirements of the application layer, that is, determine the transmission syntax and transmit it.

Connection management: This includes establishing a presentation connection using a session layer service, managing data transport and synchronization control over the connection (using the corresponding service at the session layer), and terminating the connection normally or abnormally. ^[1]

Through the previous introduction, we can see that the five layers below the session layer have completed end-to-end data transmission and are reliable and error-free. However, data transmission is only a means rather than a purpose, and ultimately the use of data is to be achieved. Because the definition of data is not the same in various systems, the most understandable example is the keyboard, and the meaning of certain keys on many systems is different. This naturally creates obstacles to the use of data from other systems. The layer and application layer are responsible for removing this obstacle.

For user data, it can be analyzed from two aspects, one is that the meaning of the data is called semantics, and the other is the representation of the data, called grammar. Like text, graphics, sound, text, compression, encryption, etc. It belongs to the grammatical category. The presentation layer is designed with 15 types of 3 functional units, of which the context management functional unit is to communicate data coding rules between users so that both parties have a consistent data form and can recognize each other.

OSI OSI presentation layer

It has developed a series of standards such as DP8822, DP8823, DIS6937 / 2 for services, protocols, and text communication symbols. The presentation layer is like a translator between the application and the network. It mainly solves the problem of syntactic representation of user information, that is, it provides formatted representation and data conversion services. Data compression, decompression, encryption, and decryption are all done at this layer.

How the presentation layer works

At the presentation layer, data is formatted according to a scheme that the network can understand; this formatting also varies depending on the type of network used. Decryption and encryption of presentation layer management data, such as the processing of system passwords. If you query your bank account on the Internet, a secure connection is used. Your account data is encrypted before it is sent. At the other end of the network, the presentation layer will decrypt the received data. In addition, the presentation layer protocol also decodes and encodes picture and file format information.

Encryption is divided into link encryption and end-to-end encryption. For the presentation layer, the participating encryption is end-to-end encryption, which means that the information is automatically encrypted by the sender, enters the TCP / IP data packet encapsulation, and then enters the Internet as unreadable and unrecognizable data. After reaching the destination, it will be automatically decrypted enough to become readable data. End-to-end encryption is intended for high-level subjects of the network. It does not encrypt the underlying protocols. The protocol information is transmitted in clear text. User data does not need to be decrypted at the central node.