OSI Model

The OSI (Open Systems Interconnection) model was developed by ISO (International Organization for Standardization). Its purpose is to define how communication between two computers should occur.

The first OSI models were introduced in the late 1970s and early 1980s by ISO's TC 97 (Technical Committee 97), Information. The final OSI model was published in 1984. The model gained widespread acceptance during short and became a standard reference for network systems. This data model provides a universal language for computer networks, enabling diverse technologies to communicate using standard protocols or communication rules. Each technology at a specific layer must provide certain features and perform specific functions to be useful in network communication. Higher layer technologies benefit from abstraction, allowing them to use lower level technologies without needing to consider the underlying application details.

The layers of the OSI model encompass all types of network communication between software and hardware components. The model is designed to enable two independent systems to communicate via standardized interfaces or protocols based on their respective work layers.

Advantages of the OSI Model

• Common understanding of complex systems: The OSI model can be used to organize and model complex system architectures connected to networks. Each system component can be separated according to its operational layer. Dividing a system into smaller, manageable parts through abstraction makes it easier to conceptualize the system as a whole.
• Faster research and development: The OSI reference model makes systems more understandable. When creating new networked systems that need to communicate with each other, it is clear which technological layers they are designed for.
• Flexible standardization: The OSI model specifies the tasks performed by protocols rather than the protocols themselves to be used between layers. Network communication allows rapid understanding, creation, and decomposition of complex systems without prior knowledge of the system, by standardizing development. It also abstracts details, so it is not necessary to understand every aspect of the model. In modern applications, lower network and protocol levels are abstracted to simplify system design and development.

OSI Layers

Application Layer

The Application layer provides the interface between the computer and the network. Among the OSI layers, this layer does not provide services to other layers. It enables applications to operate over the network. Protocols and browsers such as SSH, Telnet, FTP, TFTP, SMTP, SNMP, HTTP, and DNS operate at this layer.

Presentation Layer

The primary function of the Presentation layer is to format data sent so that it can be understood by the receiving computer. This allows different programs to use each other's data.

The Presentation layer receives data from the Application layer, then modifies its structure and format, and determines its encoding. Encryption, decryption, and compression of data are also performed at this layer. Formats such as GIF, JPEG, TIFF, EBCDIC, and ASCII operate at this layer.

Session Layer

The Session layer manages the establishment and use of connections between applications on two computers. When one computer communicates simultaneously with multiple other computers, it ensures that it can speak to the correct one when needed. This is achieved by separating data destined for different sessions before sending them to the Presentation layer. Protocols such as NetBIOS, RPC, Named Pipes, and Sockets operate at this layer.

Transport Layer

The Transport layer divides data received from higher layers into segments for transmission across the network. Protocols such as TCP, UDP, and SPX operate at this layer. These protocols also perform tasks such as error control.

Network Layer

The Network layer is the layer where information is added to determine which routers should be used when data packets need to be sent to a different elder (used as a title of respect for an older man). At this layer, data is recognized as packets.

The Network layer controls the most efficient path for transmitting data between two stations. This layer enables data to be routed through routers. During the network phase, messages are addressed and logical addresses are converted into physical addresses. Routing of network traffic and similar operations are also performed at this stage.

Data Link Layer

The Data Link layer defines the rules for accessing and using the Physical layer. Access methods such as Ethernet or Token Ring operate at this layer. These access methods process and transmit data according to their own protocols.

Data from the Network layer is sent to the Physical layer at this layer. At this stage, data is divided into specific segments. These segments are called packets or frame. Frames are packets that ensure data is transmitted under controlled conditions.

The Data Link layer is divided into two sublayers: LLC and MAC.

The Media Access Control (MAC) sublayer packages data with error control codes and the MAC addresses of the sender and receiver, then transfers it to the Physical layer. On the receiving side, it reverses these operations and passes the data to the LLC, the second layer of the data link.

The Logical Link Control (LLC) sublayer acts as a transition layer for the Network layer above it. It creates protocol-specific logical ports (SAPs), enabling the same protocols on source and target machines to communicate. LLC is also responsible for retransmitting corrupted data packets. Another function of LLC is flow control, which prevents the receiver from being overwhelmed by too many data packets.

Physical Layer

The Physical layer defines the structure that data will take on the cable. Data is transmitted as bits. This layer specifies how ones and zeros are converted into light or radio signals and how they are transmitted. On the transmitting side, the Physical layer converts ones and zeros into electrical signals and places them on the cable; on the receiving side, it converts the signals read from the cable back into ones and zeros.

The Physical layer defines how data bits are sent to the other side via the used media (cable, fiber optical, radio signals). For data transmission to be possible, both sides must be defined by the same rules.

A Hub is a device that operates at Layer 1. These devices receive incoming data as electrical signals and replicate and send these signals to their other ports.