A tutorial/intro by cwk9
The OSI Reference Model
Proudly being rammed down the throats of unsuspecting networking students since 1984
Definition:
OSI reference model: Open System Interconnection reference model.
Short History:
Back in 1984 many of the networks being built were incompatible with each other. So the ISO (International Organization for Standardization) got together and decided to lay down the law. What they came up with was the OSI reference model. I couldn’t find anything on the actual process of creating it but I assume it involved locking 20 nerds in a room with a keg and telling them to have something by the next morning.
Why It’s Important:
Basically OSI is the theoretical stuff behind networking. It servers as a foundation for building networks and trouble shooting. But more importantly it helps put those big complicated networks into perspective. If you’ve taken any Cisco orientated courses you’ll know that they make sure that you know OSI like the back of your hand before you even see a router. It’s dry and boring but it really does change the way you look at computer networks.
It’s all in the layers:
Transferring data form one computer to another is divided into 7 layers. Each one performing its own function.
Layer7: The application layer
Layer6: The presentation layer
Layer5: The session layer
Layer4: The transport layer
Layer3: The network layer
Layer2: The data link layer
Layer1: The physical
It’s important to note that no layer can work with out the help of the layers below it. If you were to send a picture to another computer the data would start at the application layer and move down every respective layer. The computer receiving would start at layer 1 and build the data backup to the app layer. When sending data flows down the layers and when receiving data flows up the layers. The top three layers deal with the data formatting and use interface while 4,3,2,1 deal with the delivery of the data.
The Application Layer: The layer that cares.
This is the only layer that you can see. Think of the application layer as the user interface. Its there so you don’t have to worry about all those other layers.
The Presentation Layer: The application layers little helper
Data Formatting. That’s the only thing you have to remember about this layer. Things like compression and encoding happen here.
The Session Layer:
Handles the over all connection between two host. It decides what needs to be sent and when to send it. Also keeps data for different programs separated.
Some examples of stuff you would find at these layers
Application Layer: Internet explorer, Quake, Kazaa,
Presentation Layer: ASCII, Audio compression, encryption
Session Layer: The operating system, Application Access Scheduling.
The above layers are more programming orientated were as the bottom four are were the networking gurus play. Layers 4,3,2,1 are were things like routers and tcp/ip go.
The Transport layer: The buffer between the upper layers and the lower ones
Five basic services happen at this layer
-Segmenting upper-layer application data
-Providing flow control
-sending segments from one end host to another end host
-ensuring data reliability
-Establishing end-to-end operations
If your wondering were the error checking was this is were it happens. The Transport layer determines if data needs to be resent. It’s also were two computers decide who sends what and when.
Stuff that you would find at the Transport layer: UDP, SPX, TCP(not TCP/IP just tcp), Windowing, Error checking.
The Network Layer: Were stuff happens
This is the layer were logical addressing happens. The network layer finds out were to send stuff that isn’t directly linked by mac address. Think routing.
Some things you will find and the network layer. Routers, IPX, IP(not tcp/ip just ip)
The Data Link Layer: (insert your own clever subtitle here)
This is the first point of logical organization. It deals with preparing the data to make the transit across the network media. The data link layer is concerned with physical addressing not logical. The data link layer can be summed up as Media access control.
Stuff you find at the data link layer. HDLC 802.3/802.2, NIC cards and switches, EIA/TIA-232
The Physical Layer: The Layer you can kick
The actual sending of the electric signal. Getting those 1’s and 0’s from point A to point B. The general rule is if you can kick it its working in the Physical layer. Just keep in mind just because a device is working in layer 1 doesn’t mean it only works there.
Stuff you can kick that works only in the physical layer: Wire, Hubs, Repeaters and most likely any other networking device without a chip in it.
Encapsulation:
Encapsulation is an important term when referring to the OSI model.
A text book definition: Encapsulation is the wrapping of data in a particular protocol header. For example, upper-layer data is wrapped in a specific Ethernet header before network transit. Also, when bridging dissimilar networks, the entire frame from one network can simply be placed in the header used by the data link layer protocol of the other network.
That definition never really helped me much. Basically as data moves down the OSI model each protocol throws in its own extra bit of data, like IP and mac address.
When de-encapsulation occurs the extra data is striped as the packet/frame/segment …ect moves up the layers.
A quick run down of Encapsulation
1. Build the data
2. Prepare the data for end-to-end transport
3. Add the network address to the header: contains source and destination IP
4. Add the MAC address to the data link header:
5. Convert the frames into 1’s and 0’s and fire them off over the network media
Data Encapsulation at different layers of the OSI model
Application =>Data
Presentation =>Data
Session =>Data
Transport =Segments
Network =>Packets
Data Link =>Frames
Physical =>bits
Now there is another model that important to learn and that’s TCP/IP. The TCP/IP model is arguably more important than OSI because it has protocols were as OSI is very generic and theoretical. A common misconception is that TCP/IP is simpler because it has fewer layers.
Some Differences between the two models
-TCP/IP combines a few layers
TCP/IP Layers:
Application
Transport
Internet
Network Access
OSI Layers:
Application
Presentation
Session
Transport
Network
Data Link
Phycical
-TCP/IP has well known protocols
Some similarities between the two
-You should know both
-They both have layers
