OSI Model

lastmod 2025-07-17created 2015-03-0427 edits

The OSI model (Open Systems Interconnection Reference Model) is a model developed by the International Organization for Standardization (ISO) that describes computer network protocol design and communication by dividing them into layers. It is commonly called the OSI 7-layer model.

Layer 1: Physical layer

The physical layer defines the electrical and physical details needed to connect actual devices. For example, pin layouts, voltages, and cable specifications belong to this layer. Hubs and repeaters are physical layer devices. It specifies mechanical structure and electrical characteristics. The important tasks performed in the physical layer are as follows.

Establishing and terminating connections to a physical information transmission medium. Taking part in efficiently distributing communication resources among multiple users, for example, contention resolution and flow control. Converting and modulating the digital data of user devices transmitted over a communication channel into corresponding signals. These signals are carried over copper wire or optical fiber; for example, SCSI belongs here It is the layer that transmits data bits over the network, provides the means to establish, maintain, and release physical connections for bit transmission between data link entities, and the ways of exchanging data in the physical layer are circuit switching, message switching, and packet switching. The transmission medium defines how signals are sent.

Layer 2: Data link layer

The data link layer is the layer that guarantees reliable point-to-point (Point to Point) transmission, and requires CRC-based error control and flow control. It provides the functional and procedural means to transfer data between entities on a network and to detect and correct errors that may occur in the physical layer. Addresses are assigned physically, meaning the MAC address (MAC address) is fixed from the time the network card is made. The addressing scheme is a flat structure with no hierarchy. The best-known example of the data link layer is Ethernet. Others include point-to-point (point-to-point) protocols such as HDLC and ADCCP, packet switching networks, and local area network protocols such as LLC and ALOHA. Network bridges and switches operate at this layer and can only connect to directly attached nodes.

Layer 3: Network layer

The network layer is the layer that finds the route each time data passes through multiple nodes; it delivers variable-length data across networks and provides the functional and procedural means to deliver the quality of service (QoS) required by the transport layer along the way. The network layer performs routing, flow control, segmentation (segmentation/desegmentation), error control, internetworking (Internetworking), and so on. Routers operate at this layer, and some switches also operate at this layer. It is the layer that makes the Internet possible by delivering data through other connected networks. It has a logical address structure (IP), that is, a structure in which network administrators assign addresses directly, and it is hierarchical (hierarchical).

As the top layer of the subnet, it sets up routes and manages billing information. It provides the functions of establishing, maintaining, and releasing network connections between open systems, and of exchanging Network Service Data Units (NSDU : Network Service Data Unit) between transport layers.

Layer 4: Transport layer

The transport layer lets users at both ends (End to end) exchange data reliably, so that upper layers don't have to think about the validity or efficiency of data delivery. It uses sequence-number-based error control. The transport layer controls the validity of a given connection, and some protocols are stateful (stateful) and connection-oriented (connection oriented). This means the transport layer checks whether packet transmission is valid and retransmits packets that failed to be delivered. The best-known example of the transport layer is TCP.

It is the lowest layer that deals with end-to-end (end-to-end) communication, transmitting data reliably and efficiently end to end, and its functions include error detection and recovery and flow control.

Layer 5: Session layer

The session layer provides a way for application processes at both ends to manage communication. Along with simultaneous send/receive (duplex), half-duplex (half-duplex), and full-duplex (Full Duplex) communication, it performs checkpointing, idling, termination, and restart procedures. This layer is responsible for creating and tearing down TCP/IP sessions.

It synchronizes communicating users and handles error recovery commands in bulk.

Layer 6: Presentation layer

The presentation layer handles translation between codes, relieving the application layer of the burden of dealing with differences in data format on user systems. Operations such as MIME encoding and encryption take place at this layer. For example, converting a document file encoded in EBCDIC into a file encoded in ASCII is the job of the presentation layer.

Layer 7: Application layer

The application layer works directly with application processes and performs common application services. Common application services provide transitions between related application processes. Examples of application services include virtual terminals (for example, Telnet) and the "Job transfer and Manipulation protocol" (JTM, standard ISO/IEC 8832).

Packet-based explanations

• [Recommended] (Understanding) the OSI 7 layers and the TCP/IP 4 layers | STEVEN J. LEE
• [Recommended] Understanding packets, Ethernet frames, IP .. : Naver Blog
• Linux ETC - Understanding the OSI 7 Layer

OSI/IP Model

OSI Model vs Internet protocol suite

The two models are related but do not map onto each other exactly. The biggest difference is the number of layers. The Internet protocol suite uses four layers, or five (if the link layer is split into the physical layer and the data link layer), while the OSI model uses seven layers.

The OSI model fits better when describing SSL or TLS. SSL and TLS are usually used as session layer protocols that sit above TCP and below applications running over HTTP, SFTP, and others such as stunnel or VPN.

Example with TCP/IP HTTPS

• OSI model - Wikipedia
• What is the OSI model? | OSI 7 layers | Cloudflare

See also

• ComputerNetwork:Basic - computer network basics
• L3 Switch/L4 Switch
• Internet protocol suite
• Medium access control (aka media access control; MAC)
  • MAC address
• Network switch - includes the differences between hubs, switches, and routers.
• Classful network

Favorite site

• Wikipedia (en) description of the OSI model
• OSI reference model (Reference model, OSI 7 Layer)
• OSI model – how Internet works
• [Recommended] Introduction to Ethernet, IP, TCP/UDP headers ¹
• [Recommended] Explanation of how MAC addresses, IP addresses, and PORT numbers are used ²
• Win32 socket programming tutorial - History of TCP/IP ³ (see Network socket#Win32 socket programming tutorial)

References