What is the OSI model?

The OSI model emerged as a solution to communication incompatibilities between the diverse array of networking protocols in use around the turn of the century. The layers of OSI gave developers and engineers a framework for building interoperable hardware and software across networks by providing a categorical approach to networking.

At each layer of the stack—typically shown in reverse order to illustrate how data moves through a network—the OSI model provides guidelines and criteria for network components and their unique computing functions.

The layers are:

• Layer 7:The application layer initiates communication with the network, including the protocols and data manipulation processes that convert computer-readable network data into user-readable responses.

• Layer 6:The presentation layer prepares data for the application layer, including data translation, compression and encryption.

• Layer 5:The session layer initiates and terminates connections between two devices interacting on the network, making sure that resources are neither overused nor underutilized.

• Layer 4:The transport layer transmits end-to-end data between two devices interacting on the network, making sure that data isn’t lost, misconfigured or corrupted.

• Layer 3:The network layer handles data addressing, routing and forwarding processes for devices interacting across different networks. If the devices are on the same network, they don’t need the network layer to interact.

• Layer 2:Unlike the network layer, the data link layer manages data routing between two interacting devices on the same network.

• Layer 1:The physical layer comprises the physical assets, like routers and USB cables, that convert data into strings of 1s and 0s for transmission to higher layers.

The OSI model focuses on providing a list of tasks for engineers to complete in building each layer of a network architecture, instead of specifying protocols for communication between layers. Its theoretical approach enables developers to visualize and build highly complex computing networks, even without prior knowledge of the networking system itself. It also helps teams better understand how data traverses a network and tailor network functions with layer-specific coding.

Although the OSI model isn’t the direct basis for modern computer networking technologies, it’s had a profound impact on computing standards development, helping shape contemporary understandings of network architecture.

In the late 1970s and early 1980s, computer systems were becoming increasingly interconnected, but manufacturers often developed their own networking solutions, creating a patchwork of proprietary and non-interoperable systems.

Several early networking efforts attempted to address compatibility issues with the ARPANET (which laid the groundwork for the modern internet) and the TCP/IP protocol suite (commissioned by the Department of Defense). Both represented significant advancements, but they also highlighted the need for a more comprehensive and universally accepted approach.

Recognizing the growing importance of networking and the need for a universal framework, the International Organization for Standardization (ISO) and the International Telegraph and Telephone Consultative Committee (CCITT) initiated the development of a standardized networking model.

The ISO formally published the OSI model, a seminal framework for developing interoperable network solutions, in 1984. Unlike previous standardization attempts, the layered configuration of OSI enables disparate systems to communicate despite differences in their underlying architectures and protocols.

The OSI model remains integral to understanding network architecture even as technologies evolve and new models emerge. Whether a team is designing a simple local area network (LAN) or managing a complex global network, the principles of the OSI model provide a clear, structured approach to networking.

The OSI model includes seven distinct layers. The application layer (layer 7), the presentation layer (layer 6) and the session layer (layer 5) comprise the software layers of an OSI, where all transmissions to and from software apps (including operating systems and utilities, such as web browsers and email clients) occur.

The transport layer (layer 4) is the “heart of OSI,” handling all data communication between networks and systems. Finally, the network layer (layer 3), the data layer (layer 2) and the physical layer (layer 1) comprise the hardware layers of OSI, where data moves through the physical components of the network as it’s processed.

Data moves bi-directionally through the OSI model; each layer communicates with the layers below and above it in the stack. Furthermore, both the sending and receiving devices transmit data through the data layers; and senders and receivers often switch roles in the process.

For example, if a user wants to send an email to another person, the user would first write the email and send it. When the user presses “send,” their email goes to the application layer, which will choose the correct protocol (typically SMTP) and send the email to the presentation layer. The presentation layer then compresses the message data and sends it to the session layer, which initiates a communication session and sends the data to the transport layer for segmentation.

Since the email is going to another network, the email data must go to the network layer, where it’s divided into packets and then to the data link layer where it’s further broken down into frames. Those frames are subsequently transmitted through the physical layer (the recipient’s wifi), at which point the recipient’s device receives the bit stream and the email data traverses the same layers in reverse. At the end of the process, the email data lands in the application layer of the recipient’s device where it’s delivered, in human-readable form, to the recipient’s inbox.

The OSI model is foundational to protocol development, with each layer of the framework managing specific network processes.

The application layer is the OSI layer closest to the end user. It provides network services directly to user applications and facilitates communication between API endpoints and lower layers of the OSI model. In other words, software applications use the application layer to initiate communication with the network and send data to the presentation layer.

Applications themselves are not part of this layer. Rather, the application layer provides the protocols (HTTP, FTP, DNS and SMTP, for instance) that enable software to send and receive data. It’s responsible for processes such as:

• File transfer.The application layer takes human-readable data files from the user’s device and transmits them to the presentation layer.

• Communication and authentication.The application layer makes sure that the receiving device can accept the data and that the communications interfaces needed for the transfer exist. It can also be used to authenticate the devices involved in the transfer.

• Remote access.The application layer enables users to access web browsers, email clients and other services from various geographical locations. It also enables users to access and manage files on a remote computer.

• Directory services.The application layer provides directory services—a shared database of information about network devices and users—to facilitate network resource management.

The presentation layer transforms data into a format that the application layer can accept for transmission across the network (from an EBCDIC-coded text file to an ASCII-coded file, for instance). Due to its role in converting data and graphics into a displayable format for the application layer, it is sometimes referred to as the syntax layer.

It supports secure sockets layer/transport layer security (SSL/TLS) protocols, JPEG protocols (for image compression) and MPEG protocols (for video The presentation layer transforms data into a format that the application layer can accept for transmission across the network (from an EBCDIC-coded text file to an ASCII-coded file, for instance). Due to its role in converting data and graphics into a displayable format for the application layer, it is sometimes referred to as the syntax layer.

It supports secure sockets layer/transport layer security (SSL/TLS) protocols, JPEG protocols (for image compression) and MPEG protocols (for video compression). The presentation layer is responsible for:

• Data translation.The presentation layer converts data into the correct format (specified by the application layer) during the encapsulation process, as outgoing messages move down the protocol stack from sender to receiver.

• Data encryption and decryption.The presentation layer encrypts data for secure transmission and decrypts it upon delivery.

• Data compression.The presentation layer reduces the size of a data stream for transmissions and decompresses it for use.

Sometimes formatting and translation are reversed during the de-encapsulation process, as incoming messages move up the protocol stack. In those instances, outgoing messages are converted into the specified format during encapsulation, while incoming messages undergo a reverse conversion during de-encapsulation.

The session layer is responsible for session management, the process of establishing, managing and terminating connections—called "sessions"—between two or more computers. It initiates the connections between local and remote applications, keeping the session open long enough to transmit the necessary data and closing them when complete to preserve network resources.

Key functions of the session layer include:

• Session interactions.The session layer manages user logon (establishment) and user logoff (termination), including any authentication protocols integrated into client software.

• Synchronization.The session layer helps ensure that data streams are properly synchronized and handles recovery points (checkpoints that allow devices to resume a session from a specific point, if interrupted).

• Session recovery.The session layer manages session failures and re-establishes connections if there are network problems.

It also establishes protocols for connecting and disconnecting sessions between related data streams, such as audio and video in web conferencing. Therefore, the session layer is often explicitly implemented in network environments that utilize remote procedure calls.

The transport layer uses protocols like transmission control protocol (TCP) and the user datagram protocol (UDP) to manage the end-to-end delivery of complete messages. It takes messages from the session layer and breaks them into smaller units (called “segments”), each with an associated header. At the destination, the transport layer reassembles the segments in the correct order to reconstruct the original message.

The transport layer also handles:

• Service point addressing.The transport layer helps ensure that messages are delivered to the correct process by attaching a transport layer header (including a service point or port address).

• Flow control.The transport layer prevents data overflow and manages the rate of data transmission between two devices interacting on the network, making sure that sending devices transmit data to receiving devices (and vice versa) at the appropriate speed.

• Multiplexing.The transport layer allows multiple network applications to use the same connection simultaneously.

At the sender's end, the transport layer receives formatted data from the upper layers, performs segmentation and implements flow and error control to ensure accurate data transmission. It adds source and destination port numbers to the header and then forwards the segmented data to the network layer.

At the receiver's end, the transport layer reads the port number from the header and forwards the received data to the appropriate application. It also handles the sequencing and reassembly of the segmented data and retransmits data if errors are detected.

The transport layer provides two types of service. 

With connection-oriented service, a three-part process including connection establishment, data transfer and termination (or disconnection), the data receiver sends an acknowledgment of receipt back to the sender when the data packet is delivered. Connectionless service, however, only involves data transfer. The receiver does not confirm receipt, which accelerates communication but can be less reliable than connection-oriented service.

The network layer of the OSI model is responsible for facilitating data transfer from one node to another across different networks. The network layer determines the best path (routing) for data to travel between nodes. If segments are too large, the network layer breaks them up into smaller “packets” for transport and reassembles them on the receiving end.

A network serves as a medium where multiple nodes (each with a unique address) can connect. The network layer allows nodes to send messages to nodes on other networks by providing the message content and the destination address, leaving the network to determine the optimal delivery path (which may involve routing through intermediate nodes).

The network layer primarily uses the Internet Protocol v4 (IPv4) and IPv6 and is responsible for:

• Packet fragmentation and reassembly.The network layer splits large packets (those that exceed the size limits of the data link layer) into smaller ones for transmission and reassembles them at the destination.

• Traffic control.The network layer manages network traffic to prevent congestion and safeguard efficient data flow. 

Reliability isn’t guaranteed in the network layer; while many network layer protocols offer reliable message delivery, some do not. Furthermore, error reporting isn’t mandatory at this layer of OSI, so data senders may or may not receive confirmation of delivery.

The data link layer’s primary function is to manage error-free data transfer between multiple devices interacting on the same network.

The DLL is divided into two sublayers.

The logical link control (LLC) layer—which serves as an interface between the media access control (MAC) layer and the network layer—handles flow control, synchronization and multiplexing (where two or more data streams share a single connection to the host). The MAC layer controls how devices access network mediums and transmit data.

When the DLL receives a packet from the network layer, it divides the packet into data “frames”—according to the frame size of the network interface card (NIC)— and transmits it to the host using its MAC address.

DLL functions include:

• Framing.The DLL allows the sender to transmit a set of bits (data) that are meaningful to the receiver by attaching special bit patterns to the beginning and end of the frame.

• Physical addressing.The DLL uses the Address Resolution Protocol (ARP) to convert IP addresses to MAC addresses and then adds the MAC addresses of the sender and receiver to the header of each frame after framing is complete.

• Error control.The DLL detects damaged or lost frames and manages retransmission (if necessary) to ensure data integrity.

• Flow control.To prevent corruption, the DLL dictates how much data a sender can send before receiving an acknowledgment of delivery, keeping the data rate consistent on both sides.

• Access control.When multiple devices share a single communication channel, the MAC sublayer determines which device has control over the channel at a given moment.

The physical layer comprises the physical network components responsible for transmitting raw data—in the form of “bits,” or strings of 1s and 0s—between devices (connectors, routers, repeaters and fiber optic cables, for instance) and a physical medium (like wi-fi).

The physical layer is responsible for:

• Bit rate control.The physical layer defines the data transmission rates, often in bits per second.

• Bit synchronization.The physical layer imposes a clock on bit streams, ensuring that the sender and receiver are synchronized at the bit level. 

• Transmission mode.The physical layer defines how data will flow between connected devices (as simplex, half duplex or full duplex transmission).

• Physical topologies.The physical level specifies how network devices and nodes are situated (in bus, star or mesh topologies, for example). Standards like USB, Bluetooth and Ethernet include physical layer specifications. 

The physical layer also defines how encoding occurs over a physical signal (using electrical voltage, radio or light pulses, for example). 

The OSI reference model provides a theoretical underpinning that helps engineers and developers understand the intricacies of network communication. However, it’s sometimes compared with another networking model: the transmission control protocol/internet protocol (TCP/IP) model.

Unlike the OSI model, the TCP/IP model is based on standardized protocols that are widely and directly implemented and in real-world networks. It consists of four layers—instead of seven—but each layer corresponds to one or more layers of the OSI model.

• Network access layer.Also called the data link layer or the physical layer, the network access layer of a TCP/IP network includes both the hardware and software components necessary for interfacing with the network medium, combining the OSI model’s physical and data link layers. It handles physical data transmission—by using Ethernet (for LANs) and ARP protocols—between devices on the same network.

• Internet layer.Similar to the OSI model's network layer, the internet layer is responsible for logical addressing, routing and packet forwarding. It primarily relies on the IP protocol and the Internet Control Message Protocol, which manages addressing and routing of packets across different networks.

• Transport layer.The TCP/IP transport layer serves the same function as the OSI model's transport layer; it enables reliable data transfer between upper and lower layers. Using TCP and UDP protocols, it also provides mechanisms for error checking and flow control.

• Application layer.TCP/IP’s application layer encompasses the OSI model’s session, presentation and application layers. It uses HTTP, FTP, Post Office Protocol 3 (POP3), SMTP, DNS and SSH protocols to provide network services directly to applications and manages all the protocols that support user applications. 

The OSI model's primary value lies in its educational utility and its role as a conceptual framework for designing new protocols, making sure that they can interoperate with existing systems and technologies.

However, the TCP/IP model's practical focus and real-world applicability have made it the backbone of modern networking. Its robust, scalable design and horizontal layering approach has driven the explosive growth of the internet, accommodating billions of devices and massive amounts of data traffic.