Open System Interconnection Model
Most of you are probably already familiar with the concept of the OSI model and its 7 layers. Each of those layers has a responsibility, important not only conceptually but also in reality. The 7 layers of the OSI model are numbered from bottom to top as follows:
Layer 7 = Application Layer
Layer 6 = Presentation Layer
Layer 5 = Session Layer
Layer 4 = Transport Layer
Layer 3 = Network Layer
Layer 2 = Data Link Layer
Layer 1 = Physical Layer
It is important to know the order and names of the layers above. To make things easy, I usually suggest creating a mnemonic to remember them. The phrase that I originally learned was All People Seem To Need Data Processing (APSTNDP), with each word representing a layer. In the beginning remembering the model can be tricky, but once you truly understand it, you'll never need to think about it again.
The function of the layers needs to be outlined, and you've probably heard this all a hundred times before. The important thing is that we can ultimately relate it all to the packet creation process, which we'll do shortly.
Application Layer - The application layer is the top level of the OSI model. Put simply, this is the layer where users actually interact with applications that create the data that will be sent over the network. Imagine you're using a web browser. You type in www.2000trainers.com, and magically a whole lot of data comes your way. The web browser doesn't participate in the actual obtaining of the data. It simple makes a request (this part is data) saying ‘hey, I need to view this certain web page' the lower layers actually take care of the rest of the request, and making sure that the data requested is actually passed back to the web browser. What the browser is more specifically doing is making a request that a web server will understand - an application on another system. Examples of application-layer programs include web browsers, ftp programs, telnet, email, and so forth. The user interacts with these directly.
Presentation Layer - Layer 6 of the OSI model is concerned with the actual format that the data is presented in. It is important to note that different systems have different ways of formatting data, and in order to ‘see' things properly, the systems have to agree on a way that data will be displayed. For example, data is often encoded using different schemes, such as ASCII, EBCIDC or UNICODE. If one system formats data using a scheme that the other doesn't understand, you're might just be staring a screen full of unrecognisable characters, and that wouldn't do you much good. This layer is home to a number of data presentation standards that you might already be familiar with, including JPG, GIF, and MPEG. However, this layer is also usually responsible for data encryption and compression.
Session Layer - Layer 5 of the OSI model is one that people always seem to have trouble with, mainly because in reality, protocols suites are often developed such that they don't map directly to the OSI model. The responsibility of this layer is to create, manage, and terminate communication sessions between systems. Think of the session layer as a coordinator of communication between systems and you would be on the right track. A variety of protocols exist at the session layer, including Remote Procedure Calls (RPCs), the Network File System (NFS), SQL, and the AppleTalk Zone Information Protocol (ZIP).
Transport Layer - Layer 4 of the OSI model brings us to a place where the actual roles and responsibilities start to get a little more concrete for people. Its responsibilities include segmenting data that comes from the upper layers, establishing a connection between end-to-end systems that defines the parameters of the connection, and ensuring that data actually gets to its destination in the correct order and error free. One way that it does this is by requiring acknowledgements from the receiver for each packet transmitted. Generally, if the packets sent don't get to the destination, the sender will never receive an acknowledgement, and will resend the data. At this layer, systems also agree on how much data should be sent at a time, a process often referred to as windowing, which will be discussed in a future article in more detail. Beyond that, this layer also handles something called flow-control - a process by which the receiving system can tell the sender to slow down, as it may be receiving data more quickly that it can process it. Examples of protocols found at this layer include TCP, UDP, and SPX.
Network Layer - Layer 3 of the OSI model is responsible for two key functions - logical addressing and routing. This layer identifies systems uniquely on our internetwork, and also handles how to get from one part of the network to another. This is the layer where most of the network configuration takes place, since protocols such as IP, IPX, and equipment such as routers work here. This layer doesn't concern itself with reliability of the data transfer, since that's the responsibility of the Transport layer. Its concern is more along the lines of ‘where am I going (the address), and how do I get there (the routing).
Data Link Layer - Layer 2 of the OSI Model is concerned with how upper level data is formatted for transmission over a given network technology. For example, if your network is running Ethernet, this layer will format the data accordingly, and follow the rules of Ethernet transmission on the network. Over the course of being transmitted over a large internetworking, the layer may change many times between the source and destination - the journey might start on Ethernet, move over an ATM network, and then end on a Token Ring LAN. The process of getting across those networks might involve a number of Data Link layer changes to the packet on even a short trip. This layer is actually made up of 2 sub-layers, one called the MAC (Media Access Control), which controls accessing the physical network media, and the other called the LLC (Logical Link Control), whose responsibility is identifying and interacting with the upper-level Layer 3 protocol being used. Examples of Data Link protocols include a variety of network communication standards - Ethernet, Token Ring, FDDI and so forth. At this layer, systems are generally identified by what is referred to as their hardware or MAC address.
Physical Layer - Layer 1 of the OSI model is generally the easiest to identify, because it defines the actual media and characteristics used to send the 1's and 0's over the network. This includes the actual wiring itself, the connectors used, voltage levels, maximum media distances and so forth. The physical layer doesn't change the actual data, although the transmission mechanism itself can vary greatly over the course of transferring data. For example, it might start on twisted-pair wiring, move onto a wireless network, and then ultimately end up on fibre - each of which using different media and methods during the transfer process. In this case, copper wiring, then air, and then fibre optic cable.
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