Seven Layers of the OSI Model

Your Guide to the Seven Layers of the OSI Model

The open systems interconnection (OSI) model is a conceptual model created by the ISO or International Organization for Standardization and is now considered as an architectural model for inter-computer communications. It describes how a network functions and standardizes the way that systems send information to one another. The OSI model doesn’t perform any functions in the networking process – it’s a conceptual framework so we can better understand the complex interactions that are happening.

The OSI model breaks down network communication into seven layers and provides a visual design of how each communications layer is built on top of the other, starting with the physical cabling, all the way to the application trying to communicate with other devices on a network. The layers help network professionals visualize what’s happening within their networks and to troubleshoot network problems by breaking them down and isolating the source.  Most telecommunications vendors also describe their products and services in relation to the OSI model. This helps them differentiate among the various communications packaging methods, transport protocols, and addressing schemes.

The Seven Layers of the OSI Model

The main concept of OSI is that the process of communication between two endpoints in a network can be divided into seven distinct groups of related layers or functions. In this architecture, each layer takes care of a very specific job and then passes the data onto the next layer – each layer serves the layer above it and, in turn, is served by the layer below it. Layers 1-4 are considered the lower layers and mostly concern themselves with moving data around. Layers 5-7, called the upper layers, contain application-level data.

The seven layers of the OSI model can be defined as follows, from top to bottom:

Layer 7- Application 

The application layer is the only layer that directly interacts with the data from the end-user. Software applications such as email clients and web browsers rely on the application layer to initiate communications. It provides protocols that allow the software to send and receive information as well as present meaningful data to users. The protocol used depends on the information the user wants to send.

The application layer synchronizes communication, identifies communication partners, and handles resource allocation. Other functions of the OSI model application layer are the mail and directory services and Network Virtual Terminal and FTAM-File transfer access. Some common protocols include SMTP, POP3, FTP, Telnet, DNS, and HTTP.

Layer 6 – Presentation

The Presentation layer formats or translates data for the application layer based on the syntax or semantics that the application accepts. Because of this, it’s at times also called the syntax layer. The presentation layer is responsible for translation, encryption, and compression of data – it defines how two devices should encode, encrypt, and compress data, so it’s received correctly on the other end.

For example, if two devices are communicating over an encrypted connection, the presentation layer is responsible for adding the encryption on the sender’s end and decoding the encryption on the receiver’s end so that it can present the application layer with unencrypted, readable data. And if two computers use different encoding methods, the presentation layer converts the data from sender-dependent format into a common format and changes the common format into receiver-dependent format at the receiving end.

Layer 5 – Session

When two computers or other networked devices need to speak with one another, a session needs to be created, which is done at the session layer. The session layer is responsible for establishing, coordinating, and termination of conversations, exchanges, and dialogues between the applications at each end of the session. It’s also responsible for authentication and reconnections, and it can set checkpoints during a data transfer — if the session is interrupted, devices can resume data transfer from the last checkpoint.

Layer 4 – Transport 

The transport layer deals with the coordination of the data transfer between end systems and hosts. The basic function of the transport layer is to accept data from the layer above, split it up into smaller units, pass these data units to the network layer, and ensure that all the pieces arrive correctly at the other end.

Functions of the transport layer include:

• Service-point addressing: The responsibility of the transport layer is to transmit the message to the correct process.
• Error control: The transport layer performs error control end to end to ensure that the complete message arrives at the receiving transport layer without any error.
• Connection control: The transport layer provides two services; connectionless service and connection-oriented service. A connectionless service treats each segment as an independent packet, and they all travel in different routes to reach the destination. A connection-oriented service connects with the transport layer at the destination machine before delivering the packets – all the packets travel in a single route.
• Flow control: The transport layer is also responsible for flow control, but it is performed end-to-end rather than across a single link.
• Reassembling and segmentation: When the transport layer receives the message from the upper layer, it divides the message into multiple segments, and each segment contains a sequence number, which enables this layer to reassemble the message. The message is reassembled correctly upon arrival at the destination and replaces packets that were lost in transmission.
• Multiplexing: The transmission of multiple packet streams from unrelated applications or other sources (multiplexing) across a network requires some very dedicated control mechanisms found in the transport layer. This multiplexing allows simultaneous applications over a network, such as when different internet browsers are opened on the same computer.
• Byte orientation: Some applications prefer to receive byte streams instead of packets; the transport layer allows for the transmission of byte-oriented data streams if required.

Layer 3 – Network

The network layer is responsible for connections between different networks. Anything that has to do with inter-network connections takes place at the network layer. This includes setting up the routes for data packets, checking to see if a server in another network is up and running, and addressing and receiving IP packets from other networks. The network layer provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host via one or more networks while maintaining the quality-of-service functions.

Layer 2 – Data Link

The data link layer is the protocol layer in a program that handles data moving into and out of a physical link in a network. The data link layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the physical layer.

Data bits are encoded, decoded, and organized in the data link layer before being transported as frames between two adjacent nodes on the same LAN or WAN. The data link layer also determines how devices recover from collisions that may occur when nodes attempt to send frames simultaneously.

Layer 1 – Physical

The physical layer is the lowest layer of the OSI Model and is responsible for the wireless connection or physical cable between network nodes. This can include the cable type, radio frequency link (as in a Wi-Fi network), and the layout of pins, voltages, and other physical requirements.

The physical layer defines the means of transmitting raw bits instead of logical data packets over a physical link connecting network nodes. The bitstream may be grouped into symbols or code words and converted to a physical signal transmitted over a hardware transmission medium. This layer is not concerned with the meaning of the bits and deals with the setup of physical connection to the network and transmission and reception of signals.

Wrapping Up

The OSI model is an essential theory for understanding modern computer network technology in a connection-oriented way. When you understand the OSI model and its layers, you can also understand which protocols and devices can interoperate when new technologies are developed and explained.

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