Layer 6: Presentation Layer

Layer 6 of the OSI model is unique to all the other layers.  While I can say that with any other layer, the presentation layer is unique in that it is optional.  That’s right.  There are cases when data is sent from the source computer to the destination computer while skipping layer 6 completely.  However, layer 6 has its own uses that can benefit any platform over any type of network.

The three big services that the presentation layer has to offer are:

• Data format
• Compression
• Encryption

The presentation layer got its name because of its data format services.  Using unified standards, a Macintosh, Windows, Unix and Linux user can view and share the same information, regardless of their platform.  So, even though I typed out this post using my Microsoft Windows XP computer, if you are on a Mac, you can obviously still read this.  But how?  That is because my Windows machine and your Macintosh can use the same standard for encoding text.  One of these big standards is ASCII (pronounced as-key-2,  American Standard Code for Information Interchange).

Another service that the presentation layer can do is convert data from one format to another.  If my computer is using ASCII while your computer is using another text format (such as EBCDIC), the presentation layer will convert between the two so that both machines can communicate.

The presentation layer can also compress data.  This is useful for network communication because the smaller the data, the faster the transmission.  It might be a big surprise, but most people has at least heard some of these layer 6 standards:

• JPEG (Joint Photographic Experts Group) – used to compress pictures
• MIDI (Musical Instruments Digitally Interface) -  For music, tells what note to be played using what instrument
• QuickTime – Used for video and audio

There are more standards, I just listed these as an example.

Finally, layer 6 can do encryption.  Encryption is another optional service.  It can be handled by lower layers using protocols such as IPSec or WPA, it can be handled by layer 6, or there can be no encryption at all.  Transmitting information, particularly text, without encryption is called open text.  The most popular protocol on layer 6 using encryption is called SSL (Secure Socket Layer).

Please note: I have seen conflicting data as to if SSL is indeed on layer 6 or if it is considered on layer 7.  However, based on what SSL does and what layer 6 is, I am posting that it is a layer 6 protocol here on my blog :p.

We are almost done.  In the next posting of the OSI model series, I will discuss about layer 7, the application layer.

Resorces