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. The Open System Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The Department of Defense (DOD), a division of the United States government, developed a model that would be used as the developing basis for their own protocol suite known as the Internet protocol suite. The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. The OSI 7 layers model has clear characteristics.

Abstract
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. The Open System Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The Department of Defense (DOD), a division of the United States government, developed a model that would be used as the developing basis for their own protocol suite known as the Internet protocol suite. The OSI model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Encapsulation is the process of inserting the information of upper layer into the data field of a lower layer. Flow control is a process of adjusting the flow of data packets to ensure the reliability of data delivery and data integrity. The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. The OSI 7 layers model has clear characteristics.

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. It was developed as part of the Open Systems Interconnection (OSI) initiative and is sometimes known as the OSI Model. From top to bottom, the OSI Model consists of the Application, Presentation, Session, Transport, Network, Data Link, and Physical layers. A layer is a collection of related functions that provides services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.

Even though newer IETF, IEEE, and indeed OSI protocol work subsequent to the publication of the original architectural standards that have largely superseded it, the OSI model is an excellent place to begin the study of network architecture. Not understanding that the pure seven-layer model is more historic than current, many beginners make the mistake of trying to fit every protocol they study into one of the seven basic layers. This is not always easy to do as many of the protocols in use on the Internet today were designed as part of the TCP/IP model, and may not fit cleanly into the OSI model.

OSI Layers
The Open System Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions. The model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered the primary architectural model for inter-computer communications. The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers.

_{The following list details the seven layers of the Open System Interconnection (OSI) reference model:}

• Layer 7 — Application
• Layer 6 — Presentation
• Layer 5 — Session
• Layer 4 — Transport
• Layer 3 — Network
• Layer 2 — Data Link
• Layer 1 — Physical

Layer 7: Application Layer
This application layer interface directly performs application services for the application processes; it also issues requests to the presentation layer. Note carefully that this layer provides services to user-defined application processes, and not to the end user. For example, it defines a file transfer protocol, but the end user must go through an application process to invoke file transfer. The OSI model does not include human interfaces. The common application services sub-layer provides functional elements including the Remote Operations Service Element (comparable to Internet Remote Procedure Call), Association Control, and Transaction Processing (according to the ACID requirements).

Layer 6: Presentation Layer
The presentation layer establishes a context between application layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol Data Units, and moved down the stack.

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 full-duplex, half-duplex, or simplex operation, and establishes check pointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session check pointing and recovery, which is not usually used in the Internet protocols suite. Session layers are commonly used in application environments that make use of remote procedure calls (RPCs).

Layer 4: Transport Layer
The transport layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/de-segmentation, 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.

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 fragmentation and reassembly, 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.

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. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multi-access media, was developed independently of the ISO work, in IEEE Project 802. IEEE work assumed sub-layering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the transport layers by protocols such as TCP, but is still used in niches where X.25 offers performance advantages.

Layer 1: Physical Layer
The physical layer defines all the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network adapters, Host Bus Adapters (HBAs used in Storage Area Networks) and more.

To understand the function of the physical layer in contrast to the functions of the data link layer, think of the physical layer as concerned primarily with the interaction of a single device with a medium, where the data link layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. The physical layer will tell one device how to transmit to the medium, and another device how to receive from it (in most cases it does not tell the device how to connect to the medium). Obsolescent physical layer standards such as RS-232 do use physical wires to control access to the medium.

_{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.

_{Benefits of OSI Model}

• Different operating systems using OSI model can communicate with each other.
• Developers can change features of one layer without changing the code simplifying the troubleshooting.
• Error-checking schemes determine whether transmitted data has become corrupt or otherwise damaged while traveling from the source to the destination. Error checking is implemented at several of the OSI layers. One common error-checking scheme is the cyclic redundancy check (CRC), which detects and discards corrupted data. Error-correction functions (such as data retransmission) are left to higher-layer protocols.

DOD Model
The Department of Defense (DOD), a division of the United States government, developed a model that would be used as the developing basis for their own protocol suite known as the Internet protocol suite. A protocol suite indicates a group of protocols that were designed and meant to be used together. This model has four layers compared to the seven layers of the OSI model. The Host-to-Host layer maps out to the Transport layer and the Internet layer maps out to the Network layer. The graphic above shows how the layers of the DOD model map out to the layers of the OSI model. Since there is a relationship between the layers of each of the models, some of the developed protocols in the Internet suite, at a particular layer, functions much like the equivalent layers of the OSI model. An example would be a protocol like Routing Information Protocol (RIP), which functions at the Internet layer of the DOD. Since the Internet layer of the DOD maps out to the Network layer of the OSI model, RIP would have the same responsibility of route discovery, which is an OSI Network layer responsibility.

The Network Access layer is different. The DOD did not develop any protocols for this layer. The DOD developed protocols for the Process/Application, Host-to-Host, and Internet layers only. The DOD did not develop any protocols for the Network Access layer, because they wanted to create a generic suite of protocols that would function on any vendor’s system. It was the responsibility of the individual vendors to create a set of protocols that would allow the Internet suite to work with their hardware. This generic solution by the Department of Defense allows the capability of making the Internet as popular as it is today on such a vast number of systems.

Protocols
The OSI model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. In the context of data networking, a protocol is a formal set of rules and conventions that governs how computers exchange information over a network medium. A protocol implements the functions of one or more of the OSI layers.

A wide variety of communication protocols exist. Some of these protocols include LAN protocols, WAN protocols, network protocols, and routing protocols. LAN protocols operate at the physical and data link layers of the OSI model and define communication over the various LAN media. WAN protocols operate at the lowest three layers of the OSI model and define communication over the various wide-area media. Routing protocols are network layer protocols that are responsible for exchanging information between routers so that the routers can select the proper path for network traffic. Finally, network protocols are the various upper-layer protocols that exist in a given protocol suite. Many protocols rely on others for operation. For example, many routing protocols use network protocols to exchange information between routers. This concept of building upon the layers already in existence is the foundation of the OSI model.

Encapsulation and De-capsulation
Encapsulation is the process of inserting the information of upper layer into the data field of a lower layer. Let's say you want to send a message from your PC to another PC on the Internet. First you type a message that you want to send. This message is converted into 1s and 0s by the application layer. Then, the presentation layer takes this message, and adds its own header and footer bytes to it. Your message itself has not been changed; it is contained in the data field of the presentation layer. When the packet arrives to its destination, the receiving computer performs the same process in reverse order (de-capsulation).

Flow Control
Flow control is a process of adjusting the flow of data packets to ensure the reliability of data delivery and data integrity. Flow control is performed at the Transport layer of the OSI model. When packets are received by the destination computer, they are put into buffer while being processed. If buffer becomes full, any additional data packets will be discarded.

Summary
The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. The OSI 7 layers model has clear characteristics. The layers 7-4 deal with end to end communications between data source and destinations. Layers 3 to 1 deal with communications between network devices. On the other hand, the seven layers of the OSI model can be divided into two groups: upper layers (layers 7, 6 & 5) and lower layers (layers 4, 3, 2, 1). The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the wires, for example) and is responsible for placing data on the medium.

References
Data Encapsulation & Decapsulation in the OSI Model. (n.d.). Retrieved from Firewall.cx:
     http://www.firewall.cx/osi-encap-decap.php

Encapsulation - Decapsulation. (n.d.). Retrieved from Walters State Community College:
     http://ws.edu.isoc.org/workshops/2004/SANOG-IV/ip-services/presentations/ip-intro/ipbasics/sld026.htm

Internetworking Basics. (n.d.). Retrieved from Cisco:
     http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Intro-to-Internet.html

OSI Layers. (n.d.). Retrieved from Webopedia:
     http://www.webopedia.com/quick_ref/OSI_Layers.asp

OSI Model. (n.d.). Retrieved from Javvin:
     http://www.javvin.com/osimodel.html

OSI Model. (n.d.). Retrieved from Wikipedia:
     http://en.wikipedia.org/wiki/OSI_model