# CCNA Certification/The OSI Model

The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnection initiative. It is also called the OSI seven layer model.

## History

In 1977, the International Organization for Standardization (ISO), began to develop its Open Systems Interconnection (OSI) networking suite. OSI has two major components: an abstract model of networking (the Basic Reference Model, or seven-layer model), and a set of concrete protocols. The standard documents that describe OSI are for sale and not currently available online.
Parts of OSI have influenced Internet protocol development, but none more than the abstract model itself, documented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it. Protocols enable an entity in one host to interact with a corresponding entity at the same layer in a remote host.

## Description of OSI layers

OSI Model :
| | Data unit | Layer | Function |
| Host layers | Data | Application | Network process to application |
| Host layers | Data | Presentation | Data representation and encryption |
| Host layers | Data | Session | Interhost communication |
| Host layers | Segments | Transport | End-to-end connections and reliability (TCP) |
| Media layers | Packets | Network | Path determination and logical addressing (IP) |
| Media layers | Frames | Data link | Physical addressing (MAC & LLC) |
| Media layers | Bits | Physical | Media, signal and binary transmission |

### Layer 7: Application layer

The Application layer is the seventh level of the seven-layer OSI model. It interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer.
The common application layer services provide semantic conversion between associated application processes. Note: Examples of common application services of general interest include the virtual file, virtual terminal, and job transfer and manipulation protocols.
The application layer of the TCP/IP models corresponds to a mixing of the application, presentation, and session layers in the OSI models.
The user always interacts with the application layer. This layer provides protocol and services for the user.

### Layer 6: Presentation layer

The Presentation layer transforms data to provide a standard interface for the Application layer. [ MIME] encoding, data encryption and similar manipulation of the presentation is done at this layer to present the data as a service or protocol developer sees fit. Examples of this layer are converting an EBCDIC- text file to an ASCII-coded file, or serializing object and other data structures into and out of XML. http protocol works in the presentation layer.

### Layer 5: Session layer

The Session layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for either full-duplex or half-duplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, and also for session checkpointing and recovery.
In the TCP/IP model, "graceful close", and other connection related parts, are part of the Transport layer.

### Layer 4: Transport layer

The Transport layer provides transparent transfer of data between end users, thus relieving the upper layers from any concern while providing reliable data transfer. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the transport layer can keep track of the segments and retransmit those that fail.
The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP). The transport layer is the layer that converts messages into TCP segments or User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc.

### Layer 3: Network layer

The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing functions, and might also perform segmentation/desegmentation, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical.
The best known example of a layer 3 protocol is the Internet Protocol (IP). Perhaps it's easier to visualize this layer as the actual Air Mail or Consolidated Carrier that transfers the mail from Point A to Point B.
The network layer is responsible for getting the packets to the remote host.

### Layer 2: Data Link layer

The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. The best known example of this is Ethernet. Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and Aloha for local area networks. On IEEE 802 local area networks, and some non-IEEE 802 networks such as FDDI, this layer may be split into a Media Access Control (MAC) layer and the IEEE 802.2 Logical Link Control (LLC) layer. It arranges bits from physical layer into logical chunks of data, known as frames. The Data Link Layer's job is to move the data to the next hop.
This is the layer at which the bridges and switches operate. Connectivity is provided only among locally attached network nodes forming layer 2 domains for unicast or broadcast forwarding.

### Layer 1: Physical layer

The Physical layer defines all the electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. Ethernet hubs, repeaters, network adapters and Host Bus Adapters. The major functions and services performed by the physical layer are:
* Establishment and termination of a connection to a communications medium.
* Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
* Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.
Parallel SCSI buses operate in this layer. Various physical-layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the data-link layer. The same applies to other local-area networks, such as Token ring, Fiber distributed data interface (FDDI), and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

## Interfaces

In addition to standards for individual protocols in transmission, there are also interface standards for different layers to talk to the ones above or below (usually operating-system–specific). For example, Microsoft Windows' Winsock, and Unix's Berkeley sockets and System V Transport Layer Interface, are interfaces between applications (layers 5 and above) and the transport (layer 4). Network Driver Interface Specification (NDIS) and Open Data-Link Interface (ODI) are interfaces between the media (layer 2) and the network protocol (layer 3).

## External links

* ISO standard 7498-1:1994 (ZIP file format)
* Cybertelecom — Layered Model of Regulation
* OSI Reference Model — The ISO Model of Architecture for Open Systems Interconnection, Hubert Zimmermann, IEEE Transactions on Communications, vol. 28, no. 4, April 1980, pp. 425 - 432.
* Internetworking Basics