The standard used today is the TCP/IP 4 layer model; however, for academic purposes the OSI 7 layer is taught because of how "detailed" it is. In reality you will almost NEVER use all 7 layers to communicate electronically over the Internet. Here we discuss the TCP/IP 4 layer model.
- OSI 7 layer model
- TCP/IP 4 layer model
TCP has 4 layers Application Layer - HTTP/POP3/SMTP (Layer 7 on OSI) Transport Layer - TCP, UDP (Layer 4 on OSI) Internet Layer - IP (Layer 3 on OSI) Network layer - Ethernet, Frame Relay (Layer 2 on OSI) The application layer provides services to the application software running on a computer. Essentially this provides an interface between the software and the network.
Let's take a break for a moment to discuss something called "Same layer communication" between two different computers. Let's say that computer one has a web browser open and you want to access a website that is housed on server two. Each computer will utilize the HTTP protocols to communicate to each other and the information to be exchanged is carried in "Headers". Of course, these headers are called HTTP headers and can contain various codes to communicate.
Computer one sends a HTTP header with a GET command and if that file is available on server two is responds with a HTTP OK and then the requested file in an HTTP header back to computer one. This process is called same layer communication among the two remote computers. AS you can see same layer communication happens between two different computers.
Now, in order for this to happen that HTTP header is sent down the protocol stack of each computer in a process called "Adjacent layer communication". This process happens on just the one computer that is requesting these services. So, in the above example computer one needs to send a packet to server two for a file (web page) and in order to do this the Application layer (HTTP) needs to be sent down the various layers below it in order to be sent out to the network in a process called "Encapsulation".
HTTP is an application layer protocol and in the TCP/IP stack the layer below it is the transport layer and is in charge of breaking up the data into segments and numbering it to ensure guaranteed delivery and error recovery.
Below this is Internet layer that is responsible for end to end routing and logical addressing. This layer adds the necessary information to route the packet to it's final destination and give the proper logical addressing for the proper return of the packet. This layer includes comptuer one's IP address as the originating IP and server two's IP address as the intended recepient of the packet (There is more to this but that is for later).
Finally, you have the network layer which is responsible for taking that packet and encapsulating it into a frame for delivery onto the physical network. This layer is responsible for puting the data onto the network with the proper encoding schemes and also handles access on the line (CSMA/CD for example).
There you have it, the process at which data is encapsulated and information is transmitted. Hopefully this explains adjacent and same layer communications. There are no current application layer protocols that provide error recovery and thus require such protocols as TCP to ensure end-to-end delivery
Robert M Parten II
