OSI Reference Model (Open Systems Interconnection)
The International Organization introduced the OSI layer for Standardization (ISO) in 1984 in order to provide a reference model to make sure products of different vendors would interoperate in networks. OSI is short for Open System Interconnection.
The OSI layer shows WHAT needs to be done to send data from an application on one computer, trough a network, to an application on another computer, not HOW it should be done. A layer in the OSI model communicates with three other layers: the layer above it, the layer below it, and the same layer at its communication partner. Data transmitted between software programs pass all 7 OSI layers. The Application, Presentation and Session layers are also known as the Upper Layers. The Data Link and Physical layers are often implemented together to define LAN and WAN specifications.
2.2 Data Encapsulation
Data Encapsulation is the process of adding a header to wrap the data that flows down the OSI model. Each OSI layer may add its own header to the data received from above. (from the layer above or from the software program ‘above’ the Application layer.)
There are five steps of Data Encapsulation : –
- The Application, Presentation and Session layers create DATA from users’ input.
- The Transport layer converts the DATA to SEGMENTS
- The Network layer converts the SEGMENTS to PACKETS (or datagrams)
- The Data Link layer converts the PACKETS to FRAMES
- The Physical layer converts the FRAMES to BITS.
At the sending computer, the information goes from top to bottom while each layer divides the information received from upper layers into smaller pieces and adds a header. At the receiving computer, the information flows up the model discarding the corresponding header at each layer and putting the pieces back together.
The Figure shows the layered model of two directly interconnected end systems. The transmission media is not included in the seven layers and, therefore, it can be regarded as layer number zero. Functions and services of various layers
2.3 Characteristics of the OSI Layers
The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers.
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. Both users and application layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another layer in the OSI model.
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 network cabling, for example) and is responsible for actually placing information on the medium.
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 include:
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.
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.
2.4 OSI Model & Communication Between Systems
Information being transferred from a software application in one computer system to a software application in another must pass through the OSI layers. For example, if a software application in System A has information to transmit to a software application in System B. The application program in System A will pass its information to the application layer (Layer 7) of System A.
The application layer then passes the information to the presentation layer (Layer 6), which relays the data to the session layer (Layer 5), and so on down to the physical layer (Layer 1). At the physical layer, the information is placed on the physical network medium and is sent across the medium to System B. The physical layer of System B removes the information from the physical medium, and then its physical layer passes the information up to the data link layer (Layer 2), which passes it to the network layer (Layer 3), and so on, until it reaches the application layer (Layer 7) of System B. Finally, the application layer of System B passes the information to the recipient application program to complete the communication process.
2.5 Interaction between OSI model layers
A given layer in the OSI model generally communicates with three other OSI layers: the layer directly above it, the layer directly below it, and its peer layer in other networked computer systems. The data link layer in System A, for example, communicates with the network layer of System A, the physical layer of System A, and the data link layer in System B. Figure below illustrates this example.
System A System B
OSI LAYER SERVICES
One OSI layer communicates with another layer to make use of the services provided by the second layer. The services provided by adjacent layers help a given OSI layer communicate with its peer layer in other computer systems. Three basic elements are involved in layer services: the service user, the service provider, and the service access point (SAP).