What is the Presentation Layer

In this lesson, we will explore the Open Systems Interconnect (OSI) model and take an in-depth look at the presentation layer. We'll also use an illustration to see how presentation layer data flow occurs within the seven functional layers of the OSI model.

What is the Presentation Layer?

The last time you paid bills online, did you give any thought to the Open Systems Interconnect (OSI) model and its presentation layer? As computer literate as you are, you may not know what the presentation layer, or data translator, has to do with paying your bills.

For example, when you accessed your bank account via the Internet, you used a secure connection provided by the presentation layer. The presentation layer also encrypted your account login information prior to transmission. Finally, at your financial institution's Internet server, the presentation layer decrypted your account login information, making it available for processing.

The OSI Model

Before we begin our discussion of the presentation layer in greater detail, let's introduce the networking environment in which the presentation layer exists: this is the Open Systems Interconnect (OSI) model, which has seven layers, and each layer performs a specific and supportive communicative task.

OSI model with data flow

These seven OSI layers are in descending order (top layer to bottom layer) and each layer provides inputs and outputs to/from the neighboring layers. Let's go over briefly what each layer is called and what it does.

Layer Seven	The Application Layer	Provides access to available resources
Layer Six	The Presentation Layer	Translates, encrypts and compresses data
Layer Five	The Session Layer	Establishes, manages and terminates communicative sessions
Layer Four	The Transport Layer	Provides reliable process-to-process message delivery and error recovery
Layer Three	The Network Layer	Moves packets from source to destination by providing inter-networking capabilities
Layer Two	The Data Link Layer	Organizes bits into frames, providing node-to-node delivery
Layer One	The Physical Layer	Transmits bits over a medium, establishing mechanical and electrical specifications

Roles, Functions and Protocols

As layer six of the OSI model, the presentation layer is primarily responsible for managing two networking characteristics: protocol and architecture. Whereas, protocol defines a standard set of guidelines under which the network operates, the network's architecture determines what protocol applies.

As the translator, the presentation layer converts the data sent by the application layer of the transmitting node into an acceptable and compatible data format based on the applicable network protocol and architecture. Upon arrival at the receiving computer, the presentation layer translates the data into an acceptable format usable by the application layer. In other words, the presentation layer takes care of any issues occurring when transmitted data must be viewed in a format different from the original format.

As a functional part of the OSI model ,the presentation layer performs a multitude of data conversion algorithms and character translation functions. The first function is:

Character-Code Translation: Where the presentation layer translates from the American standard code for information interchange (ASCII) to the extended binary code decimal interchange code (EBCDIC).

The second function is:

Data Conversion: This is where the presentation layer performs bit order reversal functions, converts CR (byte code for a carriage return) to CR/LF, (byte code for a carriage return with a line feed) and converts integer numbers to floating point numbers.

Third, the presentation layer is responsible for:

Data Compression, by reducing the number of bits requiring transmission, which improves the data throughput.

The fourth function is:

Data Encryption and Decryption: Encryption is needed for security purposes when sending data across networks. An encryption algorithm is used during transmission, while a decryption algorithm is used at the receiving node. Encryption and decryption typically involves the secure sockets layer (SSL) protocol, which has become more popular when used by the presentation layer.

The fifth function of the presentation layer is:

Data Translation: Networks provide the capability of connecting different types of computers, servers and mainframes on the same network and may employ different character sets. The presentation layer is responsible for fixing any irregularities while making translations transparent between networked systems.

Other protocols supported at the presentation layer include: