OSI Model
The reference model most IT professionals use to describe networks and network applications is the OSI Model.
The original idea behind this model was to describe a complete set of production network protocols, but in the end the project was unviable due to the cost and complexity of the government processes involved in defining the OSI network.
While OSI designers discussed who would do what and who would be responsible, TCP/IP took over and became the world’s favorite protocol.
The OSI Model has Seven Layers:
Layer Name
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
There is a useful mnemonic to remember the layers of the OSI model: “All People Seem To Need Data Processing”
7 Application = All
6 Presentation = People
5 Session = Seem
4 Transport = To
3 Network = Need
2 Data Link = Data
1 Physical = Processing
The functions of this seven layers are the following:
Layer Seven
The user interacts with this layer, that’s why its called the Application Layer. It is responsible for file transfers, electronic messaging, e-mail, virtual terminal access, and network management.
Layer Six
The Presentation Layer is the one responsible for defining the syntax network hosts use to communicate. Encryption and compression are also some of its functions.
Layer Five
It is called the Session because it is responsible the establishment of process-to-process communications between networked hosts.
Layer Four
The layer tasked with delivering messages between networked hosts is the Transport Layer. It is responsible for the fragmentation and reassembly of the information.
Layer Three
This is the Network Layer, it is the one that establishes paths for data transfer through the network. Routers operate at this Layer.
Layer Two
The layer responsible for communications between adjacent network nodes, hubs and switches, is appropriately called the Data Link layer.
Layer One
The that defines items such as: connector types, cable types, voltages, and pin-outs is called the Physical layer. It is responsible for bit-level transmission between network nodes.
The OSI Model in the Real World.
The hardest thing with the OSI model is that it doesn’t get along very well with the real world. The reason is that it was created after many of the protocols in use today were already in production.
These protocols, for example TCP/IP, were designed and built around the needs of real users while the OSI model was created by academicians.
However OSI model remains the only well-recognized standard we have which describes networked applications.
The logical thing would be to map the real-world protocols to the model .The problem arises when you realize that there is no consensus as to which protocol fits in a specific OSI layer.
As an example, the map could be something like this:
Layer Name Common Protocols
7 Application SSH, telnet, FTP
6 Presentation HTTP, SMTP, SNMP
5 Session RPC, Named Pipes, NETBIOS
4 Transport TCP, UDP
3 Network IP
2 Data Link Ethernet
1 Physical Cat-5
But remember that this is a subject that could be argued endlessly.
On the other hand, TCP/IP is a much more accurate model of real-world networking:
TCP/IP Model
• Application Layer
• Transport Layer
• Internet Layer
• Network Interface Layer
If you want to know more about why the OSI model should follow the path of Betamax, read Kill the Beast: Why the Seven-Layer Model Must Die.
Additional Reading on the OSI Model
