OSI Model

Simple Explanation: If I can measure the signals on its wire with a volt meter or an Oscilloscope, or hold it in my hands, or it is in any way part of the physical infrastructure of the network or computing system, it is a physical layer element. Signals in Fiber optic cable, Radio Frequencies, and Infra Red are also considered part of the physical layer.

Fancy Explanation: Responsible for error free transmission and establishing logical connections between stations. This is a achieved by packaging raw bits from the physical layer into blocks of data (frames) and sending these frames with the necessary synchronization, error control, and flow control.

Simple Explanation: So you got the bits on the wire, and the voltages and speeds are correct, and you did this in Layer 1, congratulations!. Now, you actually have to frame those little bits so the receiving end can read 'em. So, you have to figure out if you are a Token Ring, Ethernet, ATM, FDDI, Frame Relay etc. kinda network. This succinct knowledge determines how the bits are framed, and placed on the network. 98% percent of the time, your network will be ethernet.

Fancy Explanation: Responsible for addressing and control functions (e.g. routing) necessary to move data through the network. This covers establishing, maintaining, and terminating connections including packet switching, routing, data congestion, reassembly of data and translation of logical addresses to physical addresses.

Simple Explanation: If I actually want to move data between TWO or more networks, I need to ROUTE the data. If everything stays on one network, no routing is needed, thus, things like IP addresses are not strictly required, and thus no layer 3 functionality. My response assumes that you have a rock solid definition of the word network. Anytime I have to move data between two or more separate networks, routing happens. That is Layer 3. That implies a network address, and each computing device at that network requires an address on the network.

Fancy Explanation: Responsible for reliable transparent connection of data between end points. Providing end to end recovery and flow control, it deals with packet handling, repackaging of messages, dividing messages into smaller packets and error handling.

Simple Explanation: If ever something was badly named, this layer is. From dictionary.com, the definition of transport is 'To carry from one place to another; convey'. Nobody called me at the time the OSI model was being established, so we're stuck with Transport layer. Layer 1, 2 and 3 places bits on a medium (Medium is defined as ethernet cable, Fiber optic cable, Infra Red, Radio waves etc.), and moves the bits from source to destination. Layers 1,2, and 3 may discover or detect errors in the transmission, but they don't correct and retransmit. Think of the transport layer as a layer that gives you the flexibility to define the level of reliability required from transmit to receive point. For example, when transmitting real time voice or video over a network, error correction is useless. By the time a packet has been corrected and retransmitted, half a second has gone by and the receive point doesn't need the packet any more. Conversely, when transmitting sensitive documents or binaries for a program, each bit is required or the entire file is useless, therefore, a high degree of error detection and correction is required, and if it is 3 seconds late arriving at it's destination, I'll eat the three seconds as a trade off for high reliability.. Transport layer protocols allow the user the desired degree of reliability for their specific application.

Fancy Explanation: Responsible for support of connections between sessions, administrative tasks and security. Providing the control structure for communications between applications, it establishes, manages, and terminates connections (sessions) between cooperating applications.

Simple Explanation: The session layer creates a way to define which packets are port of which discreet session. The best example is that of an ATM withdrawal. Multiple transactions occur. Security and PIN verification, balance verification, instructions to disburse cash to the ATM machine, and posting of the debit to your account. If all items are completed including the account debit, but the cash is never disbursed because of a glitch, you would be annoyed bank customer. The Session layer considers the transaction complete only when all sub tasks have been completed.

Fancy Explanation: The Presentation Layer gets its name from its purpose: It presents data to the Application layer. It's basically a translator and provides coding and conversion functions. A successful data transfer technique is to adapt the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then convert the data back into its native format for reading. By providing translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another host.

Simple Explanation: The OSI has protocol standards that define how standard data should be formatted. Tasks like data compression, decompression, encryption and decryption are associated with this layer. Some Presentation layer standards are involved in multimedia operations. For instance JPEG and MPEG files, for pictures and movies respectively.

Fancy Explanation: Responsible for inter acting with the operating system of the computing device. Serving as a window for applications to access network services, it handles general network access, error recovery and file transfers.

Simple Explanation: An application is that software package that actually makes your computer a useful device. Would you like to telnet to a router? Telnet is the application. Would you like to send emails over the internet? SMTP is an internet application protocol that will send the emails. An Application will require the services of the other 6 layers.

With the predominance of IP based communications in the PC connectivity world today, we'll concentrate on how this model relates to IP centric networking. Firstly, IP maps it's protocol stack to the OSI Model like this;

OSI	TCP IP
7: Application	Application
6: Presentation
5: Session
4: Transport	TCP	UDP
3: Network	IP
2: Data Link	Network Interface
1: Physical	Network Interface

IP Assumes layer 5, 6, and 7 is so much work that will be done by other functions. IP tends not to concern itself with these layers. IP based networks pay lots of attention to layers 3 and 4, and IP is fortunate that layer 1 and 2 are as mature as they are.