Open Systems Interconnection Model (OSI Model)

What is the Open Systems Interconnection Model (OSI Model)?

The Open Systems Interconnection (OSI) Model is a conceptual and logical layout that defines network communication used by systems open to interconnection and communication with other systems. The model is broken into seven subcomponents, or layers, each of which represents a conceptual collection of services provided to the layers above and below it. The OSI Model also defines a logical network and effectively describes computer packet transfer by using different layer protocols. The OSI Model may also be referred to as the seven-layer OSI Model or the seven-layer model.[1]

Overview of OSI Model
source: Global Knowledge

Purposes of the OSI Model[2]

OSI model was created for the following purposes:

  • To standardize data networking protocols to allow communication between all networking devices across the entire planet.
  • To create a common platform for software developers and hardware manufacturers that encourage the creation of networking products that can communicate with each other over the network.
  • To help network administrators by dividing large data exchange processes into smaller segments. Smaller segments are easier to understand, manage and troubleshoot.

The Seven Layers of OSI Model[3]

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

7. The Application Layer
This is the only layer that directly interacts with data from the user. Software applications like web browsers and email clients rely on the application layer to initiate communications. But it should be made clear that client software applications are not part of the application layer; rather the application layer is responsible for the protocols and data manipulation that the software relies on to present meaningful data to the user. Application layer protocols include HTTP as well as SMTP (Simple Mail Transfer Protocol is one of the protocols that enables email communications).

OSI Application Layer

6. The Presentation Layer
This layer is primarily responsible for preparing data so that it can be used by the application layer; in other words, layer 6 makes the data presentable for applications to consume. The presentation layer is responsible for translation, encryption, and compression of data. Two communicating devices communicating may be using different encoding methods, so layer 6 is responsible for translating incoming data into a syntax that the application layer of the receiving device can understand. If the devices are communicating over an encrypted connection, layer 6 is responsible for adding the encryption on the sender’s end as well as decoding the encryption on the receiver's end so that it can present the application layer with unencrypted, readable data. Finally, the presentation layer is also responsible for compressing data it receives from the application layer before delivering it to layer 5. This helps improve the speed and efficiency of communication by minimizing the amount of data that will be transferred.

Presentation Layer

5. The Session Layer
This is the layer responsible for opening and closing communication between the two devices. The time between when the communication is opened and closed is known as the session. The session layer ensures that the session stays open long enough to transfer all the data being exchanged, and then promptly closes the session in order to avoid wasting resources. The session layer also synchronizes data transfer with checkpoints. For example, if a 100-megabyte file is being transferred, the session layer could set a checkpoint every 5 megabytes. In the case of a disconnect or a crash, after 52 megabytes have been transferred, the session could be resumed from the last checkpoint, meaning only 50 more megabytes of data need to be transferred. Without the checkpoints, the entire transfer would have to begin again from scratch.

The Session Layer

4. The Transport Layer
Layer 4 is responsible for end-to-end communication between the two devices. This includes taking data from the session layer and breaking it up into chunks called segments before sending it to layer 3. The transport layer on the receiving device is responsible for reassembling the segments into data the session layer can consume. The transport layer is also responsible for flow control and error control. Flow control determines an optimal speed of transmission to ensure that a sender with a fast connection doesn’t overwhelm a receiver with a slow connection. The transport layer performs error control on the receiving end by ensuring that the data received is complete, and requesting retransmission if it isn’t.

The Transport Layer

3. The Network Layer
The network layer is responsible for facilitating data transfer between two different networks. If the two devices communicating are on the same network, then the network layer is unnecessary. The network layer breaks up segments from the transport layer into smaller units, called packets, on the sender’s device, and reassembles these packets on the receiving device. The network layer also finds the best physical path for the data to reach its destination; this is known as routing.

The Network Layer

2. The Data Link Layer
The data link layer is very similar to the network layer, except the data link layer facilitates data transfer between two devices on the SAME network. The data link layer takes packets from the network layer and breaks them into smaller pieces called frames. Like the network layer, the data link layer is also responsible for flow control and error control in intra-network communication (The transport layer only does flow control and error control for inter-network communications).

The Data Link Layer

1. The Physical Layer
This layer includes the physical equipment involved in the data transfer, such as the cables and switches. This is also the layer where the data gets converted into a bit stream, which is a string of 1s and 0s. The physical layer of both devices must also agree on a signal convention so that the 1s can be distinguished from the 0s on both devices.

The Physical Layer

The Need to Know the 7 OSI Layers[4]

Most people in the IT space will likely need to know about the different layers when they’re going for their certifications, much like a civics student needs to learn about the three branches of the US government. After that, you hear about the OSI model when vendors are making pitches about which layers their products work with. In a Quora post asking about the purpose of the OSI model, Vikram Kumar answered this way:
“The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools.”
While some people may argue that the OSI model is obsolete (due to its conceptual nature) and less important than the four layers of the TCP/IP model, Kumar says that “it is difficult to read about networking technology today without seeing references to the OSI model and its layers, because the model’s structure helps to frame discussions of protocols and contrast various technologies.” If you can understand the OSI model and its layers, you can also then understand which protocols and devices can interoperate with each other when new technologies are developed and explained.

History of the OSI Model[5]

In the early- and mid-1970s, networking was largely either government-sponsored (NPL network in the UK, ARPANET in the US, CYCLADES in France) or vendor-developed with proprietary standards, such as IBM's Systems Network Architecture and Digital Equipment Corporation's DECnet. Public data networks were only just beginning to emerge, and these began to use the X.25 standard in the late 1970s. The Experimental Packet Switched System in the UK circa 1973-5 identified the need for defining higher-level protocols. The UK National Computing Centre publication 'Why Distributed Computing' which came from considerable research into future configurations for computer systems, resulted in the UK presenting the case for an international standards committee to cover this area at the ISO meeting in Sydney in March 1977.

Beginning in 1977, the International Organization for Standardization (ISO) conducted a program to develop general standards and methods of networking. A similar process evolved at the International Telegraph and Telephone Consultative Committee (CCITT, from French: Comité Consultatif International Téléphonique et Télégraphique). Both bodies developed documents that defined similar networking models. The OSI model was first defined in raw form in Washington, DC in February 1978 by Hubert Zimmermann of France and the refined but still, the draft standard was published by the ISO in 1980. The drafters of the reference model had to contend with many competing priorities and interests. The rate of technological change made it necessary to define standards that new systems could converge to rather than standardize procedures after the fact; the reverse of the traditional approach to developing standards. Although not a standard itself, it was a framework in which future standards could be defined.

In 1983, the CCITT and ISO documents were merged to form The Basic Reference Model for Open Systems Interconnection, usually referred to as the Open Systems Interconnection Reference Model, OSI Reference Model, or simply OSI model. It was published in 1984 by both the ISO, as standard ISO 7498, and the renamed CCITT (now called the Telecommunications Standardization Sector of the International Telecommunication Union or ITU-T) as standard X.200. OSI had two major components, an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols. The OSI reference model was a major advance in the standardization of network concepts. It promoted the idea of a consistent model of protocol layers, defining interoperability between network devices and software.

The concept of a seven-layer model was provided by the work of Charles Bachman at Honeywell Information Systems.[8] Various aspects of OSI design evolved from experiences with the NPL network, ARPANET, CYCLADES, EIN, and the International Networking Working Group (IFIP WG6.1). In this model, a networking system was divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacted directly only with the layer immediately beneath it and provided facilities for use by the layer above it. The OSI standards documents are available from the ITU-T as the X.200-series of recommendations. Some of the protocol specifications were also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model were available from ISO. Not all are free of charge.

OSI was an industry effort, attempting to get industry participants to agree on common network standards to provide multi-vendor interoperability. It was common for large networks to support multiple network protocol suites, with many devices unable to interoperate with other devices because of a lack of common protocols. For a period in the late 1980s and early 1990s, engineers, organizations and nations became polarized over the issue of which standard, the OSI model or the Internet protocol suite, would result in the best and most robust computer networks. However, while OSI developed its networking standards in the late 1980s, TCP/IP came into widespread use on multi-vendor networks for internetworking.

The OSI model is still used as a reference for teaching and documentation; however, the OSI protocols originally conceived for the model did not gain popularity. Some engineers argue the OSI reference model is still relevant to cloud computing. Others say the original OSI model doesn't fit today's networking protocols and have suggested instead a simplified approach.

OSI Vs. TCP/IP Model[6]

The Transfer Control Protocol/Internet Protocol (TCP/IP) is older than the OSI model and was created by the US Department of Defense (DoD). A key difference between the models is that TCP/IP is simpler, collapsing several OSI layers into one:

  • OSI layers 5, 6, and 7 are combined into one Application Layer in TCP/IP
  • OSI layers 1, and 2 are combined into one Network Access Layer in TCP/IP – however TCP/IP does not take responsibility for sequencing and acknowledgment functions, leaving these to the underlying transport layer.

OSI Model Vs TCP/IP Model
source: Imperva

Other important differences:

  • TCP/IP is a functional model designed to solve specific communication problems, and which is based on specific, standard protocols. OSI is a generic, protocol-independent model intended to describe all forms of network communication.
  • In TCP/IP, most applications use all the layers, while in OSI simple applications do not use all seven layers. Only layers 1, 2 and 3 are mandatory to enable any data communication.

Advantages and Disadvantages of OSI Model[7]

Advantages of OSI Model

  • Network Support: OSI model is generic on default. Which means that it is supported by a wide range of device manufacturers. Most computer networks use OSI as their standard model.
  • Layer Changes: Each layer in the OSI model is separated from the other. Therefore, any changes in the layer will not cause any effects on the other. However, this cannot be guaranteed if there are changes in the layer interface.
  • Layer Identification: Each layer in the OSI model is assigned the task of services, protocols, and interfaces. But the OSI model is able to clearly distinguish the task in each layer. Hence, all the devices that work with the OSI model will be able to support each other.
  • Flexibility: OSI model is also flexible in nature since it is can work with both connection-oriented and connectionless services. If there are situations where reliability needs to be maintained then it is possible to use connection-oriented services. In the contrast, if the speed of data transmission is the concern, then connectionless services will be the best option to use.
  • Troubleshooting: Since each layer in the OSI model is separated from the other, troubleshooting is made easier. In case of any failure, the network administrator could identify the issue more effectively by looking at each layer. No time is wasted here by analyzing the entire network.

Disadvantages of OSI Model

  • Implementation: OSI is entirely a theoretical model. This means that its practical implementation is almost impossible. Even in the absence of appropriate technology. And also, the cost involved in the implementation is usually higher here.
  • Adaptation: Many companies were initially reluctant to use this OSI model due to the popularity of the TCP/IP model. The TCP/IP model was in use much before the ISO model. Therefore, companies were not ready to accept this adaptation.
  • Effectiveness: Unlike TCP/IP, an OSI model failed to meet practical expectations. As a result, it is not effective as a TCP/IP model. Due to this, most people began to consider the OSI model as not up to the standard.
  • Complexity: Compared to a TCP/IP model, an OSI model is complex in its structure. This is because of the presence of different layers which is not optimized. For example, data link and network layer functions are not done by the same layer. Moreover, there are also duplications of services. Meaning two or more layers process the same task.
  • Collaboration: OSI model also poses some complications while working. Each layer in the OSI model will not be able to work in parallel. Unless the data is passed from the previous layer, the layers in the OSI model cannot work.

See Also