The OSI Model

The Open System Interconnection (OSI) model was designed by the International Organization for Standardization (ISO) as a model for helping different systems communicate. Its important to note than back in the early days of networking, you would usually be tied into a single vendor, since their systems could only communicate with like systems, and multi-vendor networks made life a huge hassle. Back then, you generally bought all IBM or all DEC equipment – once a vendor got that first sale, they knew you were pretty much locked in long term. While good for the vendor, this certainly handcuffed the customer in terms of their ability to make choices based on what best met their needs. The OSI model was developed as an architectural framework to allow different systems to communicate. It is not a set of protocols or hard and fast rules. Instead, it acts as a set of guidelines for how network communication should take place, which different sections of the model outlining the roles and responsibilities of each section.

So why take this sectional or layered approach? Well, by making each layer responsible for a different part of the communications process, we make it such that a particular portion can be changed without affecting the other independent parts. What starts off as data has to somehow be transmitted over the network, meaning that information has to be added to account for the network technologies in use, the addressing and routing of packets, the reliability of the transfer, the actual way data is formatted, how the process is initiated, and so forth. The OSI model actually maps to the packet generation process, and if you always keep that in mind, you’ll be well on your way to truly understanding how network communication takes place.

The OSI model has 7 layers, 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.

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, 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 unrecognizable 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.

The Packet Creation Process

So who cares? Good question. The OSI model is all-to-often described without any perspective of why this stuff is actually important. The answer is simple if you just take the time to put it all together in a reasonable scenario; it truly explains what happens when 2 systems actually communicate. The key to understanding the model is to look at it with respect to how a packet is built on the source system and then stripped apart at the receiving end. This process is referred to as data encapsulation.

Data encapsulation is the process by which a packet is built. It all begins which a chunk of data from the application layer. Maybe you want to view a web page or send a file over the network to another system. In order to get to that other system, information will need to be added to the data to account for reliability, how its going to get to where it needs to go, and the network technology in use.

When looking at the OSI model, the 7 Layers are often grouped into 2 main sections. The top section, comprised of the Application, Presentation and Session Layers, are often referred to as the Application layers, while the bottom four are often generically referred to as the Transport layers. For the purpose of looking at the data encapsulation process, the Applications layers are generally considered to simply pass data, while the Transport layers actually ‘add’ to the data to help prepare it to make its way across the network.

Remember that the chunk of data is passed down the layers for formatting. Once it reaches the transport layer, it makes its first big change, usually in the form of header and trailer data being added. At each subsequent layer (except the physical layer, which doesn’t change the packet) more header and trailer data is added again. This process is referred to as encapsulation, which each layer literally adding to the packet building process.

A key thing to understand is that as each layer receives the packet and adds to it, it considers everything passes down to it as simple data. That is, whatever the transport layer adds is not of concern to the network layer, and so forth. In that way, each lower layer simply considers what is being passed down to it as data. Consider the diagram below, which outlines how each of these layers changes the data (for clarity purposes, I’ve left off the trailer additions).

What happens above is easily explained, even though the diagram is highly simplified. Once data is passed to the transport layer, it adds header (and trailer, not shown) information to the data. If this were a TCP/IP packet, that would include TCP or UDP source and destination port information. The network layer would see this all as data, not concerned with what was added. Instead, it would encapsulate the data with another header, which would involve adding the source and destination IP addresses. When passed down to the Data Link layer, still another header would be added – in the case of an Ethernet network, this would include the source and destination MAC addresses. This would then be passed to the Physical layer, in the form of binary data being transmitted across the network.

At the receiving end, the data would subsequently by de-encapsulated, layer by layer in the opposite direction. For example, the receiving station would receive the data stream and pass it up to the data link layer.

1. If this station noticed that its MAC address were the destination MAC address, it would know that it has to pay attention and process the packet, striping off the data link header and passing it up to the network layer. If not, it would simply discard the packet at this point.

2. Next, it will pass the packet up to the Network Layer. At this point, it will determine whether the destination IP address is its IP address. If so, it will strip off the network header and pass it up to the transport layer. You should note that just because the MAC address belongs to the system, it doesn’t necessarily mean that the IP address does as well. For example, a router will receive many packets with its MAC address and different IP destination addresses.

3. Once the packet has been passed up to the Transport layer, it is definitely destined for this system. This layer will take a look at the destination port listed in the header, strip the header, and then pass the data to the appropriate application that is waiting for the data, assuming that application exists (it may not – imaging a request for a web page from a system not running as a web server)

Note that at each point in the process, a layer only looks at the information added by its equivalent layer at the sending machine. To that end, the only layers that a given layer ever interacts with are the equivalent layers on other systems, as well as the layers directly above and below it on the same system. The fact that layers are independent of each other is another design goal of the OSI model - changing a given layer should not effect other layers in the model

One last note with respect to the encapsulation process described above. As each layer changes the data, the reference name for that data unit is changed. ‘Packet’ is a generic name often used to describe the unit at any stage of the process. However, the following terms are commonly used to describe data at different network layers:

Data units above the Transport layer = Messages
Transport layer data units = Segments
Network layer data units = Datagrams or Packets
Data Link layer data units = Frames