7 Layers of OSI Model Explained

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The 7 layers of the OSI model describe how computer systems communicate over a network. Understanding the functions and responsibilities of each layer will help you understand how network communications function.

The development of the OSI model started in the late 1970s to support the emergence of diverse computer networking methods, such as the current main method known as TCP/IP, which was competing for application in large national networking efforts in the world. In the 1980s, the model became a working product of the Open Systems Interconnection group at the International Organization for Standardization (ISO). While attempting to provide a comprehensive description of networking, the model failed to garner reliance during the design of the Internet, which is reflected in the less prescriptive Internet Protocol Suite, principally sponsored under the auspices of the Internet Engineering Task Force (IETF).

1. Physical Layer of The OSI Model

The Physical layer is layer 1. It accepts the frame from the data link layer and converts the frame into bits for transmission over the physical connection medium. The Physical layer is also responsible for receiving bits from the physical connection medium and converting them into a frame to be used by the Data Link layer.

The Physical Layer also specifies how encoding occurs over a physical signal, such as electrical voltage or a light pulse. For example, a 1 bit might be represented on a copper wire by the transition from a 0-volt to a 5-volt signal, whereas a 0 bit might be represented by the transition from a 5-volt signal to 0-volt signal. As a result, common problems occurring at the Physical Layer are often related to the incorrect media termination, EMI or noise scrambling, and NICs and hubs that are misconfigured or do not work correctly.

2. Data Link Layer of The OSI Model

The Data Link layer is layer 2. It is responsible for formatting the packet from the Network layer into the proper format for transmission The proper format is determined by the hardware and technology of the network.

IEEE 802 divides the data link layer into two sublayers:

Medium access control (MAC) layer – responsible for controlling how devices in a network gain access to a medium and permission to transmit data.
Logical link control (LLC) layer – responsible for identifying and encapsulating network layer protocols, and controls error checking and frame synchronization.
The MAC and LLC layers of IEEE 802 networks such as 802.3 Ethernet, 802.11 Wi-Fi, and 802.15.4 ZigBee operate at the data link layer.

The Point-to-Point Protocol (PPP) is a data link layer protocol that can operate over several different physical layers, such as synchronous and asynchronous serial lines.

The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete data link layer that provides both error correction and flow control by means of a selective-repeat sliding-window protocol.

Security, specifically (authenticated) encryption, at this layer can be applied with MACSec.

3. Network Layer of The OSI Model

The Network layer is layer 3. It is responsible for adding routing and addressing information to the data. The Network layer accepts the segment from the Transport layer and adds information to it to create a packet.

Message delivery at the network layer is not necessarily guaranteed to be reliable; a network layer protocol may provide reliable message delivery, but it need not do so.

A number of layer-management protocols, a function defined in the management annex, ISO 7498/4, belong to the network layer. These include routing protocols, multicast group management, network-layer information and error, and network-layer address assignment. It is the function of the payload that makes these belong to the network layer, not the protocol that carries them.

4. Transport Layer of The OSI Model

The Transport layer is layer 4. It is responsible for managing the integrity of the connection and controlling the session. It accepts Protocol Data Unit (PDU), which is a container of information or data passed between network layers.

The transport layer also controls the reliability of a given link between a source and destination host through flow control, error control, and acknowledgments of sequence and existence. Some protocols are state- and connection-oriented. This means that the transport layer can keep track of the segments and retransmit those that fail delivery through the acknowledgment hand-shake system. The transport layer will also provide the acknowledgment of the successful data transmission and sends the next data if no errors occurred.

Reliability, however, is not a strict requirement within the transport layer. Protocols like UDP, for example, are used in applications that are willing to accept some packet loss, reordering, errors or duplication. Streaming media, real-time multiplayer games, and voice over IP (VoIP) are examples of applications in which loss of packets is not usually a fatal problem.

The OSI connection-oriented transport protocol defines five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the fewest features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0-4 classes are shown in the following table:

Feature nameTP0TP1TP2TP3TP4
Connection-oriented networkYesYesYesYesYes
Connectionless networkNoNoNoNoYes
Concatenation and separationNoYesYesYesYes
Segmentation and reassemblyYesYesYesYesYes
Error recoveryNoYesYesYesYes
Reinitiate connectionaNoYesNoYesNo
Multiplexing / demultiplexing over single virtual circuitNoNoYesYesYes
Explicit flow controlNoNoYesYesYes
Retransmission on timeoutNoNoNoNoYes
Reliable transport serviceNoYesNoYesYes
a If an excessive number of PDUs are unacknowledged.

5. Session Layer of The OSI Model

The Session layer is layer 5. It is responsible for establishing, maintaining, and terminating communication sessions between two computers. It manages dialogue discipline or dialogue control.

The Session Layer also provides for full-duplex, half-duplex, or simplex operation, and establishes procedures for checkpointing, suspending, restarting, and terminating a session between two related streams of data, such as an audio and video stream in a web-conferencing application. Therefore, The session layer is commonly implemented explicitly in application environments that use remote procedure calls.

6. Presentation Layer of The OSI Model

The Presentation layer is layer 6. It is responsible for transforming data received from the Application layer into a format that any system following the OSI model can understand.

The Presentation Layer handles protocol conversion, data encryption, data decryption, data compression, data decompression, incompatibility of data representation between OSs, and graphic commands. The presentation layer transforms data into the form that the application layer accepts, to be sent across a network. Since the presentation layer converts data and graphics into a display format for the Application Layer, the Presentation Layer is sometimes called the syntax layer. For this reason, the Presentation Layer negotiates the transfer of syntax structure through the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or an serialization of objects and other data structures from and to XML.

7. Application Layer of The OSI Model

The Application layer is layer 7. It is responsible for interfacing user applications, network services, or the operating system with the protocol stack.

Application-layer functions typically include file sharing, message handling, and database access, through the most common protocols at the application layer, known as HTTP, FTP, SMB/CIFS, TFTP, and SMTP. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. The most important distinction in the application layer is the distinction between the application-entity and the application. For example, a reservation website might have two application-entities: one using HTTP to communicate with its users, and one for a remote database protocol to record reservations. Neither of these protocols has anything to do with reservations. That logic is in the application itself. The application layer has no means to determine the availability of resources in the network.

How do I remember the 7 OSI layers?

To make the most of the OSI model, you must first be able to remember the names of the seven layers in their proper order. One of the best common methods to memorize them is to create a mnemonic from the initial of the last names so they are easier to remember.

A common mnemonic used is Please Do Not Teach Surly People Acryonms, which allows you to memorize the layers from the Physical layer up to to the Application layer. If you want to remember it the opposite way, another mnemonic used is All Presidents Since Truman Never Did Pot.

What are the OSI layers and their functions?

The Physical layer is layer 1. It accepts the frame from the data link layer and converts the frame into bits for transmission over the physical connection medium. The Physical layer is also responsible for receiving bits from the physical connection medium and converting them into a frame to be used by the Data Link layer.

The Data Link layer is layer 2. It is responsible for formatting the packet from the Network layer into the proper format for transmission The proper format is determined by the hardware and technology of the network.

The Network layer is layer 3. It is responsible for adding routing and addressing information to the data. The Network layer accepts the segment from the Transport layer and adds information to it to create a packet.

The Transport layer is layer 4. It is responsible for managing the integrity of the connection and controlling the session.

The Session layer is layer 5. It is responsible for establishing, maintaining, and terminating communication sessions between two computers. It manages dialogue discipline or dialogue control.

The Presentation layer is layer 6. It is responsible for transforming data received from the Application layer into a format that any system following the OSI model can understand.

The Application layer is layer 7. It is responsible for interfacing user applications, network services, or the operating system with the protocol stack.

What is the most important layer in the OSI model?

The Network layer also known as layer 4 is the most important layer of the OSI model. It is responsible for adding routing and addressing information to the data. The Network layer accepts the segment from the Transport layer and adds information to it to create a packet.

The network layer provides the functional and procedural means of transferring packets from one node to another connected in “different networks”. A network is a medium to which many nodes can be connected, on which every node has an address, and which permits nodes connected to it to transfer messages to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting the network find the way to deliver the message to the destination node, possibly routing it through intermediate nodes. If the message is too large to be transmitted from one node to another on the data link layer between those nodes, the network may implement message delivery by splitting the message into several fragments at one node, sending the fragments independently, and reassembling the fragments at another node. It may, but does not need to, report delivery errors.

Message delivery at the network layer is not necessarily guaranteed to be reliable; a network layer protocol may provide reliable message delivery, but it need not do so.

A number of layer-management protocols, a function defined in the management annex, ISO 7498/4, belong to the network layer. These include routing protocols, multicast group management, network-layer information and error, and network-layer address assignment. It is the function of the payload that makes these belong to the network layer, not the protocol that carries them.

OSI Model versus TCP/IP Model

The design of protocols in the TCP/IP model of the Internet does not concern itself with strict hierarchical encapsulation and layering. RFC 3439 contains a section entitled “Layering considered harmful”.[39] TCP/IP does recognize four broad layers of functionality that are derived from the operating scope of their contained protocols: the scope of the software application; the host-to-host transport path; the internetworking range; and the scope of the direct links to other nodes on the local network.[40]

Despite using a different concept for layering than the OSI model, these layers are often compared with the OSI layering scheme in the following manner:

The Internet application layer maps to the OSI application layer, presentation layer, and most of the session layer.

The TCP/IP transport layer maps to the graceful close function of the OSI session layer as well as the OSI transport layer.

The internet layer performs functions as those in a subset of the OSI network layer.

The link-layer corresponds to the OSI data link layer and may include similar functions as the physical layer, as well as some protocols of the OSI’s network layer.

These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in the internal organization of the network layer.

The OSI protocol suite that was specified as part of the OSI project was considered by many as too complicated and inefficient, and to a large extent unimplementable. Taking the “forklift upgrade” approach to networking, specified eliminating all existing networking protocols and replacing them at all layers of the stack. This made implementation difficult and was resisted by many vendors and users with significant investments in other network technologies. In addition, the protocols included so many optional features that many vendors’ implementations were not interoperable.

Although the OSI model is often still referenced, the Internet protocol suite has become the standard for networking. TCP/IP’s pragmatic approach to computer networking and to independent implementations of simplified protocols made it a practical methodology. Some protocols and specifications in the OSI stack remain in use, one example being IS-IS, which was specified for OSI as ISO/IEC 10589:2002 and adapted for Internet use with TCP/IP as RFC 1142.

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