The OSI Model

Open System Interconnect (OSI) Model

The Open Systems Interconnection (OSI) model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. In 1984, the international Organization for Standardization (ISO) defined a standard, or set of rules, for manufacturers of networking components that would allow these networking components to communicate in dissimilar environments. This standard is known as the Open System Interconnect (OSI) model is a model made up of seven layers.

Each layer of the OSI model is responsible for a specific function or task within the stage of network communication.

    The seven layer of the OSI model, from highest to lowest are:

           LAYERS    FUNCTIONS

                

                 7 Application       Files, Print message, data base, and Application services

                

                 6 Presentation      Data encryption, decryption, compression, decompression
                                        

                 5 Session              Interhost communication, managing sessions between applications   

                 

                 4 Transport          End-to-end connections, reliability and flow control

                

                 3 Network            Path determination and logical addressing

                

                 2 Data link           Physical addressing

                

                 1 Physical            Media, signal and binary transmission

LAYER 1: Physical Layer - The physical layer does two things, it sends bits and receives bits,  The physical layer defines electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or fiber optical cable. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.
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.

 LAYER 2: Data Link -  The Data link layer provides the physical transmission of the data and handles error notification, network topology, and flow control. This means that the data link layer will ensure that messages are delivered to the proper device on LAN using hardware addresses (MAC address) and will translate messages from the network layer into bits for the physical layer to translate.

          The data link layer formats the message into pieces, each called a data frame, and a customized header containing the hardware destination and source address.

LAYER 3 : Network Layer -  The network layer is responsible for managing logical addressing (IP addressing) information in the packets and the delivery, or routing, of those packets by using information stored in a routing table.

           The network layer is responsible for working with logical addresses. The logical addresses are address types that are used to uniquely identify a system on the network. But at the same time identify the network that system resides on. The protocol that operate at network layer is TCP/IP protocol.

LAYER 4 : Transport Layer - The network layer handles transport function such as reliable and unreliable delivery of data. transport layer provide end-to-end data transport services and can establish a logical connection between the sending Host and destination Host on an internetwork. The protocol that operate in tarnsport layer is TCP and UDP.

LAYER 5 : Session Layer - The session layer controls the dialogues (connections) 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 checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls.

LAYER 6 : Presentation Layer - The presentation layer present data to the application layer and is responsible for data translation and code formatting. It encrypt and decrypt data between users.

LAYER 7 : Application Layer - The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network or the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application-layer implementations also include:

Hypertext Transfer Protocol (HTTP),

File Transfer Protocol (FTP),

Simple Mail Transfer Protocol (SMTP)

Simple Network Management Protocol (SNMP).