Chapter Ten The Application and Presentation Layers

Objectives To look at how data packaging is selected To learn about the different syntaxes used in network transmission To examine some common file formats To get a brief introduction to file compression and encryption To examine the differences between applications and processes

OSI and the Real World Most companies these days don’t follow OSI to the letter. The Application and Presentation layers are mostly historical artifacts and not used much in real-world networking.

The Presentation Layer Deals with file formats File compression Encryption

Abstract Syntax vs. Transfer Syntax Abstract syntax –The form that data takes while it is being used Audio Graphics Video Binary executables Transfer syntax –The form that data takes as it travels over the wire

File Formats Graphics formats –Bitmap (.bmp) –Joint photographic experts group (JPEG) –Tagged image file format (TIFF) Sound files –WAV –MP3

Presentation Context Multiple file types might make up a single transmission. The overall package, including audio, video, text, and executable code makes up the presentation context.

File Compression Lossy –In reducing file size, some information is permanently lost. –This loss is considered to be acceptable loss. Lossless –No loss of information is allowed. –All data arrives intact.

Data Encryption Substitution cipher –One character is replaced by another, but the entire character set is used. –Easily hacked Transposition cipher –The characters in a block of data are put in a different order, based on a predefined sequence. DES encryption

DES Encryption A block of data of a specific size is selected. The bits themselves are scrambled based on a predefined algorithm. A “key” determines what the order will be. –A new key can be generated for each session.

The Application Layer Provides a doorway to the network for applications running on the system Unlike applications can exchange data among themselves

Applications and Processes Applications are programs running on a computer that a user launches to perform a certain task. Processes are tasks running within an application that perform a specific function to make the application do what it’s supposed to do.

Levels of Processes Common application specific elements (CASE) –Processes whose services are required by most, if not all applications running on the system Specific application specific elements (SASE) –Processes unique to a particular application Remote procedure calls (RPC) –A request made by one computer to have a service provided by another device on the network