OSI+Model

=__ OSI (Open Systems Interconnection Reference) model: __= Note: Rough content, information contained in the page are notes taken in class without any real refining.

The OSI model provides air free communication across a network and provides the path needed by applications above it, all while it calls the next lower layer to send and receive packets that make up the contents of that path.


 * ISO ( International Organization for Standardization)** are responsible for standardizing networks, avoiding having different complex ones which are not compatible.

** The OSI Model consists of 7 layers: **
__ The 7th Layer: __ The __application layer__ of the **“Application set”**: This is the layer that actually interacts with the operating system or the application, whenever the user decides to transfer a file, receive a file or transfer messages etc… Displays data. Protocol data unit (PDU): **Data**.

__ The 6th Layer: __
The __presentation layer__ of the **“Application set”**: Takes the data provided by the application layer and converts it into a standard format that the other layers can understand. Encrypts, decrypts, and compresses data. Protocol data unit (PDU): **Data**.

__ The 5th Layer: __
The __session layer__ of the “Application set”: The session layer establishes, maintains and ends communication with the receiving device and also tracks connection of the receiving device. Protocol data unit (PDU): **Data**.

__ The 4th Layer: __
The __transport layer__ of the “Transport set”: This layer controls flow of data and provides air checking and recovery of data between devices. Flow control means that the transport layer sees that the data is coming from more than one application and integrates each applications data into a single stream for the network layer. Protocol data unit (PDU): **Segments**.

__ The 3rdLayer: __
The __network layer__ of the “Transport set”: This layer determines the way that the data will be sent to the recipient device. Such as Protocols, routing, addressing. MAC address. Protocol data unit (PDU): **Packets.** __ The 2nd Layer: __ The __data link layer__ of the “Transport set”: In this layer the appropriate physical protocol is assigned to the data and also the type of network and the packet sequencing is defined. Protocol data unit (PDU): **Frames.** __ The1st Layer: __ The __physical layer__ of the “Transport set”: This is the actual hardware layer, it defines the physical characteristics of the network, for example; connections, voltage levels, and timing. Protocol data unit (PDU): **Bits.**

**Disadvantages of the OSI model:** **Advantages of the OSI model:**
 * 1) The OSI model is big and complex to comprehend thoroughly.
 * 2) Some layers such as session and presentation stay empty when implemented.
 * 3) The OSI model was forced upon existing models such as the TCP/IP model.
 * 1) The OSI is a universal and standardized model, making it so that it is applicable everywhere and for any device.
 * 2) All application implements the OSI model somehow.
 * 3) The OSI model uses object oriented programming.
 * 4) It is not necessary to develop a separate layer for each and every function outlined in the OSI model, but developers are ensured that certain level of compatibility is maintained by following the general guidelines provided by the OSI model.

**Protocol stacks:** They work together to allow software or hardware to perform a function, for example; the TCP/IP uses 4 layers of the OSI model:
__ The1stLayer: __ **Network Interface:** combines physical and data layers and roots the data on the between devices on the same network, and also manages the exchange in data between the network and other devices. __ The 2nd Layer: __ **Internet:** This layer works with the network layer. It uses the IP (Internet Protocol) address consisting of a network identifier and host identifier to determine the address of the device it is communicating with. __ The3rdLayer: __ **Transport:** This layer is the part of the protocol stack where the TCP (Transport control protocol) can be found. TCP works by asking other devices in the network if it is willing to accept information from the local device. __ The 4th Layer: __ **Combines the session, presentation and application layer:** Protocols for specific functions such as emails (simple mail transfer protocol/SMTP) and file transfer (FTP file transfer protocol) reside in layer 4.

Created By: D3LTA Last update:08/11/12

Sources:
 * Class notes. 6/11/12. Dimitry Mtchedlidze.