|  | 1.4 |
Host Layers (the Upper 4 Layers of the OSI Model)
| 1.4.1 | Application, presentation, and session layers
Application Layer In the context of the OSI reference model, the application layer (Layer 7) supports the communicating component of an application. It does not provide services to any other OSI layer. However, it does provide services to application processes lying outside the scope of the OSI model (e.g. spreadsheet programs, Telnet, WWW, etc.) A computer application can function completely by using only the information that resides on its computer. However, an application might also have a communicating component that can connect with one or more network applications. Several types are listed in the right column of the Figure . An example of such an application might include a word processor that can incorporate a file transfer component that allows a document to be transferred electronically across a network. The file transfer component qualifies the word processor as an application in the OSI context, and therefore, belongs in Layer 7 of the OSI reference model. Another example of computer application that has data transfer components is a Web browser such as Netscape Navigator and Internet Explorer. Whenever you visit a Web site, the pages are transferred to your computer. Presentation Layer The presentation layer (Layer 6) of the OSI reference model is responsible for presenting data in a form that a receiving device can understand. It serves as the translator - sometimes between different formats - for devices that need to communicate over a network, by providing code formatting and conversion. The presentation layer (Layer 6) formats and converts network application data into text, graphics, video, audio, or whatever format is necessary for the receiving device to understand it. The presentation layer is not only concerned with the format and representation of data, but also with the data structure that the programs use. Layer 6 organizes the data for Layer 7. To understand how this works, imagine that you have two systems. One system uses EBCDIC, and the other uses ASCII to represent data. When the two systems need to communicate, Layer 6 converts and translates the two different formats. Another function of Layer 6 is the encryption of data. Encryption is used when there is a need to protect transmitted information from unauthorized receivers. To accomplish this task, processes and codes located in Layer 6 must convert the data. Other routines located in the presentation layer compress text and convert graphic images into bit streams so that they can be transmitted across a network. Layer 6 standards also guide how graphic images are presented. Following are some examples: PICT -- a picture format used to transfer QuickDraw graphics between Macintosh or PowerPC programs TIFF -- tagged image file format, used for high-resolution, bit-mapped images JPEG -- from the Joint Photographic Experts Group, used for photographic quality images Other Layer 6 standards guide the presentation of sound and movies. Included in these standards are the following: MIDI -- musical instrument digital interface for digitized music MPEG -- the motion picture experts group's standard for the compression and coding of motion video for CDs, digital storage, and bit rates up to 1.5 Mbps QuickTime -- a standard that handles audio and video for Macintosh and PowerPC programs Session Layer The session layer (Layer 5) establishes, manages, and terminates sessions between applications. It coordinates the service requests and responses that occur when applications establish communications between different hosts.