The Open Systems Interconnection (OSI) Reference model is an important concept in network communication. Its aim is to divide the internet communication system into layers. The OSI model is broken into seven layers that describe the types of networks communication. Let us have a look at the seven separate layers and their functionality:
Layer 7 – Application
The application layer is the layer that the user mostly interacts with. An example is the graphical user interface of an application, such as a web browser. E.g., Mozilla FireFox, Google Chrome, Microsoft Internet Explorer.
Layer 6 – Presentation Layer
The presentation layer provides a mapping between the application layer 7, and the layers beneath. The presentation layer also forms the protocols for communication, formatting functions or compression.
Layer 5 – Session Layer
The session layer controls the connection between devices. The session layer initiates, maintains and terminates the communication session.
Layer 4 – Transport Layer
The transport layer is responsible for the transmission of data for the above layers. The transport layer focuses on flow control, segmentation and de-segmentation, and error control. It essentially ensures that the information between two devices is successfully delivered.
Layer 3 – Network Layer
The network layer provides the end-to-end routing of packets from a device on one network, to a device on another network, still maintaining the quality of service set by the transport layer. Its primary focus is on network addressing and routing. In this layer, Routers are used to route the data between networks, thus routers work at layer 3 and are routed based on the IP address.
Layer 2 – Data Link
The data link layer is responsible for the transfer of data between networks. This layer has two sub layers the Logical Link Control (LLC) that provides the protocol to protocols for flow control, acknowledgement, and error notification; and the Media Access Control (MAC) to ensure successfully communication of packets through frame synchronisation. A switch operates at layer 2 the switch and forward frames within the network based on the MAC address.
Layer 1 – Physical Layer
The physical layer is the actual sending bits (1’s and 0’s) over a communication channel.
The TCP/IP Protocol architecture
Similar to the OSI model, the TCP/IP Protocol Architecture follows a similar structure; however, differentiates by having a 5 layer structure by combining:
· Application and Presentation combined with an overlap of the session layer in the OSI Model to form the Application layer in the TCP/IP model
· Transport and an overlap of the session layer the OSI Model to form Transport layer in the TCP/IP model
· Overlap of the Network layer in the OSI model to form the Internet layer in the TCP/IP layer
· Overlap of the Network layer and the Data Link layer in the OSI model to form the Network Access layer in the TCP/IP layer
· And the physical layer are the same in both models
Layered protocols outline the rules on how systems to communicate to each other. It contains:
· HTTP – Sending of the identical message end-to-end
· TCP – Sending of the identical segments end-to-end
· IP – sending of the identical datagram
· Ethernet – sending of the proper frame
· Network Interface and Wiring – sending of the actual bits
Layer 2 (Data Link Layer) Addresses
At layer two, switches forward frames based on the MAC address contained within. The MAC address is determined by the manufacture and the type of equipment it is. The MAC address is a unique identify. The first 3 bytes are the manufacture (MFG) assigned by the manufacture by the IEEE, the last 3 bytes are the serial number.
Layer 3 (Network Layer) Addresses
The network administrator mostly defines the addresses in Layer 3, by creating local subnets. The subnet of a device would usually change based on its location, but for layer 2, the address will remain the same. Thus why routers are required to be able to forward packets to the correct location.