Presentation Layer of OSI model

The Presentation Layer acts as a data translator for network services. On the sender's computer, this layer converts the data received from the application layer into a format understandable by the layers below it. Similarly, on the destination computer, the presentation layer converts the data into a format comprehensible to the application layer.

The presentation layer performs the following tasks:

• Character code translation, such as converting from ASCII to EBCDIC format.
• Data conversion, like converting integers to floating-point numbers.
• Data compression, allowing data to be transmitted faster over the network.
• Data encryption and decryption, ensuring the security of data during transmission over the network.

Let us try to understand these functions better. First, consider character conversion. When we create data, such as an email message, in an application, it is not always in a format comprehensible to all services or applications. A universally understandable format for most computers is ASCII (American Standard Code for Information Interchange), which includes 96 characters and digits, along with 32 non-printable characters. The presentation layer typically converts data from the application layer into the ASCII format.

For IBM mainframe computers, a common format is EBCDIC (Extended Binary Coded Decimal Interchange Code), which includes 256 special characters. During data exchange with IBM mainframes, the presentation layer converts data received from the application layer into this format.

Data compression refers to reducing large data sizes. This allows data to be transmitted faster over the network. Data packets coming from the application layer often contain unnecessary elements like spaces, line breaks, etc. These can be removed to reduce the packet size. The presentation layer removes such spaces from the packets and compresses the data. This process is called data compression.

In the following diagram, Computer 1 is sending a sentence to Computer 2. At the application layer, the data includes spaces and formatting. When it reaches the presentation layer, the layer removes the spaces and formatting to reduce the packet size. The packet then passes through the other OSI layers to reach the presentation layer of Computer 2. Here, the presentation layer decompresses the data packet and sends it to the application layer. The application layer receives the formatted data.

The presentation layer performs data encryption. Encryption can be compared to a cipher code. Just as cipher codes are used to send confidential information, encryption is used to transmit sensitive data over a network. Generally, encryption is not applied to all data packets, nor is it always necessary. However, in specific cases, such as transmitting passwords or credit card numbers, encryption becomes essential. Various types of encryption are used in networks. Among these is public-private key encryption. With this method, only the person who possesses the key can read the data.

Session Layer in the OSI Model