Why bother?

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Layer Overlap!

Exam Summary







"Protocols identify communication partners, determine resource availability, synchronise communication"

This layer describes the starting point of internetworking.  You first have to tell your computer to create and send data across to a second computer.  The data could be an e-mail that you want to send to a friend, or a document that you want to save on a file server, or even a request for web page from a web site.  Your computer will have an application that is able to receive your instructions and process them into data that will be sent across the network.  In simple terms, the application allows you to tell the computer what to do. You are now interacting with the computer.  This application will in turn interact with a similar application on the remote computer so that, for example, your e-mail turns up on an e-mail application on your friend’s computer.

The application layer therefore allows end-users to interact with their computers, and their computers to interact with each other.  Like will interact with like.  That means that an e-mail application will “talk” to an e-mail application, and a network management application will “talk” to a network management application, etc.

 Please understand this!  It is important that I stress two points:

·   While MS Word, Excel and PowerPoint are indeed applications, they are not really relevant to networking because the document produced using these programs stays on one computer.  Networking starts when the document has to move from one computer to another.  A document created using MS Word could for example be e-mailed as an attachment, or uploaded to a server using a file transfer program or whatever. Applications relevant to networking therefore are programs like MS Outlook or Eudora for e-mail, Cute FTP for file transfer, Netscape and MS Internet Explorer for web browsing, and such.

·   Having made the first point, I would also like to stress that layered models are not concerned with commercial applications such as the ones I have mentioned above (Outlook, Eudora, Netscape, Internet Explorer).  All these commercial applications run on top of set of rules or protocols which the computers use to communicate.  Outlook and Eudora all use a protocol called Simple Mail Transfer Protocol (SMTP) to process e-mail.  Internet browsers such as Netscape and Internet Explorer use a protocol called Hyper Text Transfer Protocol (HTTP) to send and receive web pages which are written in a protocol called Hyper Text Mark-up Language (HTML).  Therefore in exams, the term “application” will refer to protocols such as SMTP, HTML, POP3, FTP and such (do not worry about the acronyms for now, I’ll spell them out for you on the Summary page).

The term “application” therefore refers to the protocols that are involved when two or more devices are communicating with each other.

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