Overview of Presentation Layers
The Presentation Layer is Layer 6 of the seven layers in the Open Source Interconnection (OSI) model of computer networking. An understanding of the presentation layer and the OSI model is general is of increasing importance for system administrators and IT professionals who are implementing virtual desktop management environments to facilitate application delivery, remote access, virtual network reliability and realize any of the other various benefits associated with VDI environments. In order to understand this layer, it’s important to grasp how OSI works.
How OSI Works
OSI divides network architecture into seven interdependent layers. A layer is simply a collection of conceptually similar functions that make services available to the layer immediately above it, while receiving service from the layer below it.
The seven layers and their functions, in ascending order, are the: Physical Layer; Data Link Layer; Network Layer; Transport Layer; Session Layer; Presentation Layer; and the Application Layer.
What the Presentation Layer Does
The Presentation Layer’s name clues one in as to its role in the OSI model. As in, it handles the presentation of data. This layer has two sublayers; the Common Application Service Element (CASE) and the Specific Application Service Element (SASE).
About the Two Sublayers of the Presentation Layer
CASE: This sublayer has a dual function. It supplies services for the layer above it, the Application Layer. It also gives support to the Presentation Layer with common application services like the Reliable Transfer Service Element (RTSE) and the Remote Operation Service Element (ROSE).
SASE: This sublayer is very helpful to the Presentation Layer. It offers it application-specific help, eg, Remote Database Access (RDA) and Manufacturing Messaging Service (MMS), to name a couple. There are many more.
As can probably be deduced, the Presentation Layer deals with any issues that may come up when data is transmitted from one system to another. For example, if the data sent from one system needs to be viewed in another form by the receiving system, the Presentation Layer would deal with this.
The Presentation Layer also oversees the special processing of data when it is transported, ie, from the time an application sends it until the time it is transmitted over a network.
When the Presentation Layer Is Not Needed
There are a couple of instances where the Presentation Layer is not needed.
- Service Not Used: For example, if compression of a file is not needed, then this layer would not be used in the network communications process; and
- Handled by Another Layer: In certain situations, the function that would normally be handled by the Presentation Layer is performed by the Application Layer, rendering the need for the Presentation Layer unnecessary.