Title: The OSI Model
Course: 2016FA-MCO-110-401 > Section: Module 9: Networking

Click here for a downloadable version of this document:(EXTERNAL) The OSI Model

The OSI, or Open Systems Connection, model is the communication standard by which all networks operate regardless of their configuration and technology utilized. Ever wonder how there can be a large number of network device manufacturers utilizing proprietary network operating systems and the exponential number of  software application developers; all utilizing the same networks without issue. This is due to the fact that all networking adheres to the OSI model. The OSI model is made up of 7 layers of protocols that work together to create a normalized pathway for our data. The layers are as follows:

Layer 1 (Physical) – This layer defines the physical, or mechanical, specifications of a data network. This includes the specifications of the medium (Copper/Fiber), pin assignments, the signal timing, the voltages utilized. This layer guarantees that the hardware being utilized has the ability to transmit and receive data regardless of manufacturer.

Layer 2 (Data Link) – This Layer deals with the transmission of data packets and their protocol management.  It is utilized to handle errors found in the physical layer as well as controls flow of data and synchronization. This layer is divided into two sublayers: 1) Media Access Control (MAC) – controls how a device on the network gains access to data and assigns the permissions to transmit it.  2) Logical Link Control (LLC) – controls the synchronization of packet data and manages error checking. Examples of the layer 2 protocols would be Frame Relay and PPP (Point to Point Protocol).

Layer 3 (Network) – This is the layer at which switches and routers operate. The network layer provides these technologies with the ability to create and transmit data sequences from one device to another through addressing (IP), as well as provides error and congestion control.

Layer 4 (Transport) – This layer ensures a complete transfer of data on a network. It manages transfer of variable sized data sequences between the sending and receiving hosts. By ensuring that a data packet is received before transmitting the subsequent packet in the sequence, this layer ensures that a complete data file is received. Examples of protocols that operate within this layer would be TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

Layer 5 (Session) – This layer controls the communication sessions between hosts (computers). It manages the establishment, maintenance, and termination of a session. This is how application processes communicate with each other succinctly across a network. Functions such as security and logging are performed at this layer.  Examples of protocols at this layer would be HTTP, FTP, and Secure Shell among others.

Layer 6 (Presentation) – This layer performs the formatting of data that will be presented to the application layer. This layer is sometimes referred to as the translation layer as it translates data from network to application format and vice versa. Functions such as data compression, character code translation, and Data encryption/decryption is performed at this layer. JPEG and TIFF are examples of file protocols utilized at this layer.

Layer 7 (Application) – This layer supports the end user processes and allows sharing of networked resources commonly utilized. Functions such as directory services, remote printer access, email, and network management reside at this layer of the OSI Model.

Objectives met:
- C9
- 9.2
- 9.3