Network Basics: OCI Session and Presentation Layers

The Session and Presentation layers of the standard Open Systems Interconnection (OSI) model are where network functions begin moving from physical to electronic and software matters.

Network Session Layer

The Session layer establishes conversations known as sessions between networked devices. A session is an exchange of connection-oriented transmissions between two network devices. Each of these transmissions is handled by the Transport layer protocol. The session itself is managed by the Session layer protocol.

A single session can include many exchanges of data between the two computers involved in the session. After a session between two computers has been established, it is maintained until the computers agree to terminate the session.

The Session layer allows three types of transmission modes:

  • Simplex: In this mode, data flows in only one direction.

  • Half-duplex: In this mode, data flows in both directions, but only in one direction at a time.

  • Full-duplex: In this mode, data flows in both directions at the same time.

In actual practice, the distinctions in the Session, Presentation, and Application layers are often blurred, and some commonly used protocols actually span all three layers. For example, SMB — the protocol that is the basis of file sharing in Windows networks — functions at all three layers.

Network Presentation Layer

The Presentation layer is responsible for how data is represented to applications. Most computers — including Windows, Unix, and Macintosh computers — use the American Standard Code for Information Interchange (ASCII) to represent data.

However, some computers (such as IBM mainframe computers) use a different code, known as Extended Binary Coded Decimal Interchange Code (EBCDIC). ASCII and EBCDIC aren’t compatible with each other.

To exchange information between a mainframe computer and a Windows computer, the Presentation layer must convert the data from ASCII to EBCDIC and vice versa.

Besides simply converting data from one code to another, the Presentation layer can also apply sophisticated compression techniques so that fewer bytes of data are required to represent the information when it’s sent over the network. At the other end of the transmission, the Presentation layer then uncompresses the data.

The Presentation layer can also scramble the data before it is transmitted and unscramble it at the other end by using a sophisticated encryption technique that even Sherlock Holmes would have trouble breaking.