OSI Layers
Layer 7 - Application
To further our bean dip analogy, the Application Layer is the one at the top--it’s what
most users see. In the OSI model, this is the layer that is the “closest to the end user”. It
receives information directly from users and displays incoming data it to the user. Oddly
enough, applications themselves do not reside at the application layer. Instead the
layer facilitates communication through lower layers in order to establish connections
with applications at the other end. Web browsers (Google Chrome, Firefox, Safari, etc.)
TelNet, and FTP, are examples of communications that rely on Layer 7.
Layer 6 - Presentation
The Presentation Layer represents the area that is independent of data representation
at the application layer. In general, it represents the preparation or translation of
application format to network format, or from network formatting to application
format. In other words, the layer “presents” data for the application or the network. A
good example of this is encryption and decryption of data for secure transmission - this
happens at Layer 6.
Layer 5 - Session
When two devices, computers or servers need to “speak” with one another, a session
needs to be created, and this is done at the Session Layer. Functions at this layer involve
setup, coordination (how long should a system wait for a response, for example) and
termination between the applications at each end of the session.
Layer 4 – Transport
The Transport Layer deals with the coordination of the data transfer between end
systems and hosts. How much data to send, at what rate, where it goes, etc. The best
known example of the Transport Layer is the Transmission Control Protocol (TCP),
which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and
UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network
Layer.
Layer 3 - Network
Here at the Network Layer is where you’ll find most of the router functionality that most
networking professionals care about and love. In its most basic sense, this layer is
responsible for packet forwarding, including routing through different routers. You
might know that your Boston computer wants to connect to a server in California, but
there are millions of different paths to take. Routers at this layer help do this efficiently.
Layer 2 – Data Link
The Data Link Layer provides node-to-node data transfer (between two directly
connected nodes), and also handles error correction from the physical layer. Two
sublayers exist here as well - the Media Access Control (MAC) layer and the Logical Link
Control (LLC) layer. In the networking world, most switches operate at Layer 2. But it's
not th at simple. Some switches also operate at Layer 3 in order to support virtual LANs
that may span more than one switch subnet, which requires routing capabilities.
Layer 1 - Physical
At the bottom of our OSI bean dip we have the Physical Layer, which represents the
electrical and physical representation of the system. This can include everything from
the cable type, radio frequency link (as in an 802.11 wireless systems), as well as the
layout of pins, voltages and other physical requirements. When a networking problem
occurs, many networking pros go right to the physical layer to check that all of the
cables are properly connected and that the power plug hasn’t been pulled from the
router, switch or computer, for example.
From Physical to Application (bottom up):
Please Do Not Throw Sausage Pizza Away
