OSI reference model
One of the most important networking concepts to understand is the Open Systems Interconnect (OSI) reference model. This conceptual Model, created by the International Organization for Standardization (ISO) in 1978 and revised in 1984, describes a network architecture that allows data to be passed between computer systems. OSI model and describes how it relates to real-world networking. It also examines how common network devices relate to the OSI model. Even though the OSI model is conceptual, an appreciation of its purpose and function can help you better understand how protocol suites and network architectures work in practical applications.
OSI reference model is built, bottom to top, in the following order: physical, data link, network, transport, session, presentation, and application. The physical layer is classified as Layer 1, and the top layer of the model, the application layer, is Layer 7.
7 - Application
6 - Presentation
5 - Session
4 - Transport
3 - Network
2 - Data-link
1 - Physical
The physical layer of the OSI model identifies the network’s physical characteristics, including the following specifications:
The type of media used on the network, such as type of cable, type of connector, and pinout format for cables.
The physical layer identifies the topology to be used in the network. Common topologies include ring, mesh, star, and bus.
In addition to these characteristics, the physical layer defines the voltage used on a given medium and the frequency at which the signals that carry the data operate.
The data link layer is responsible for getting data to the physical layer so that it can be transmitted over the network. The data link layer is also responsible for error detection, error correction, and hardware addressing. The term frame is used to describe the logical grouping of data at the data link layer.
The data link layer has two distinct sublayers:
Media Access Control (MAC) layer
The MAC address is defined at this layer. The MAC address is the physical or hardware address burned into each network interface card (NIC). The MAC sublayer also controls access to network media. The MAC layer specification is included in the IEEE 802.1 standard.
Logical Link Control (LLC) layer
The LLC layer is responsible for the error and flow-control mechanisms of the data link layer. The LLC layer is specified in the IEEE 802.2 standard.
The primary responsibility of the network layer is routing providing mechanisms by which data can be passed from one network system to another. The network layer does not specify how the data is passed, but rather provides the mechanisms to do so. Functionality at the network layer is provided through routing protocols, which are software components.
The basic function of the transport layer is to provide mechanisms to transport data between network devices. Primarily it does this in three ways:
Protocols at the transport layer ensure that data is sent or received correctly.
Protocols such as TCP/IP support many network services. The transport layer makes sure that data is passed to the right service at the upper layers of the OSI model.
To traverse the network, blocks of data need to be broken into packets that are of a manageable size for the lower layers to handle. This process, called segmentation , is the responsibility of the transport layer.
The session layer is responsible for managing and controlling the synchronization of data between applications on two devices. It does this by establishing, maintaining, and breaking sessions.
The presentation layer’s basic function is to convert the data intended for or received from the application layer into another format. Such conversion is necessary because of how data is formatted so that it can be transported across the network.
In simple terms, the function of the application layer is to take requests and data from the users and pass them to the lower layers of the OSI model. Incoming information is passed to the application layer, which then displays the information to the users. Some of the most basic application-layer services include file and print capabilities.