The Open Systems Interconnection is generally known as the OSI. A model is a reference tool for understanding data communications between any two networked systems. It differentiates the communications processes into seven layers. Each layer both performs specific functions to support the layers above it and provides services to the layers below it. The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process.
Generally OSI describes how information from a software application in one computer moves through a physical medium to the software application in another computer. It is said that the OSI model was developed by the International Organization for Standardization (ISO) in 1984. Presently, it is considered as an architectural model for the inter-computer communications. OSI model breaks the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task. Each layer is self-contained, so that task assigned to each layer can be performed independently and efficiently.
Applications of OSI
A OSI model is a reference tool for understanding data communications between any two networked systems. It differentiates the communications processes into seven layers. Each layer both performs specific functions to support the layers above it and provides services to the layers below it. The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process. Generally OSI describes how information from a software application in one computer moves through a physical medium to the software application in another computer. It is said that the OSI model was developed by the International Organization for Standardization (ISO) in 1984. Presently, it is considered as an architectural model for the inter-computer communications. The main benefits of the OSI model include the following
- Helps the operator understand the big picture of networking
- Assists the operator understand how hardware and software elements function together
- Makes troubleshooting easier by differentiating the networks into manageable pieces
- Defines terms that networking professionals can use to compare basic functional relationships on different networks
- Helps the operator understand new technologies as they are developed
- Aids in interpreting vendor explanations of product functionality
Layers of OSI
OSI model breaks the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task. Each layer is self-contained, so that task assigned to each layer can be performed independently and efficiently. It differentiates the communications processes into seven layers. Each layer both performs specific functions to support the layers above it and provides services to the layers below it. The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process. Generally OSI describes how information from a software application in one computer moves through a physical medium to the software application in another computer. It is said that the OSI model was developed by the International Organization for Standardization (ISO) in 1984.
Layer 1 – The Physical Layer
- The physical layer of the OSI model is the first layer
- It refers to connector and interface specifications, as well as the medium (cable) requirements.
- Electrical, mechanical, functional, and procedural specifications are provided for sending a bit stream on a computer network.
- Components of the physical layer include:
- Cabling system components,
- Adapters that connect media to physical interfaces
- Connector design and pin assignments
- Hub, repeater, and patch panel specifications
- Wireless system components
- Parallel SCSI (Small Computer System Interface)
- Network Interface Card (NIC)
Layer 2 – Data Link Layer
- The data link layer is the second layer of the OSI model
- It provides the following functions
- Allows a device to access the network to send and receive messages
- Offers a physical address so a device’s data can be sent on the network
- Works with a device’s networking software when sending and receiving messages
- Provides error-detection capability
- Common networking components that function at layer 2 include
- Network interface cards
- Ethernet and Token Ring switches
- Bridges
Layer 3 – The Network Layer
- Layer 3, the network layer of the OSI model
- It provides an end-to-end logical addressing system so that a packet of data can be routed across several layer 2 networks (Ethernet, Token Ring, Frame Relay, etc.).
- The network layer addresses can also be referred to as logical addresses.
- Initially, software manufacturers, such as Novell, developed proprietary layer 3 addressing.
- It should be noted that the networking industry has evolved to the point that it requires a common layer 3 addressing system.
- The Internet Protocol (IP) addresses make networks easier to both set up and connect with each other.
- The Internet uses IP addressing to provide connectivity to many number of of networks around the world.
- To make it easier to manage the network and control the flow of packets, numerous organizations divide their network layer addressing into smaller parts known as subnets.
- Routers takes the help of the network or subnet portion of the IP addressing to route traffic between different networks.
- Each router must be configured with high accuracy for the networks or subnets that will be connected to its interfaces.
- Routers communicate with one another using routing protocols, such as Routing Information Protocol (RIP) and Open version of Shortest Path First (OSPF), to learn of other networks that are present and to calculate the best way to reach each network based on a variety of criteria (such as the path with the fewest routers). Routers and other networked systems make these routing decisions at the network layer.
Layer 4 – The Transport Layer
- Layer 4, the transport layer of the OSI model
- It offers end-to-end communication between end devices through a network.
- Depending on the application, the transport layer either offers reliable, connection-oriented or connectionless, best-effort communications.
- Some of the functions offered by the transport layer include
- Application identification
- Client-side entity identification
- Confirmation that the entire message arrived intact
- Segmentation of data for network transport
- Control of data flow to prevent memory overruns
- Establishment and maintenance of both ends of virtual circuits
- Transmission-error detection
- Realignment of segmented data in the correct order on the receiving side
- Multiplexing or sharing of multiple sessions over a single physical link
Layer 5 – The Session Layer
- Layer 5, the session layer of OSI
- It provides various services, including tracking the number of bytes that each end of the session has acknowledged receiving from the other end of the session.
- The session layer allows applications functioning on devices to establish, manage, and terminate a dialog through a network.
- Session layer functionality includes
- Virtual connection between application entities
- Synchronization of data flow
- Creation of dialog units
- Connection parameter negotiations
- Partitioning of services into functional groups
- Acknowledgements of data received during a session
- Retransmission of data if it is not received by a device
Layer 6 – The Presentation Layer
- Layer 6, the presentation layer
- It is responsible for how an application formats the data to be sent out onto the network.
- The presentation layer basically allows an application to read or analyze the message.
- Examples of presentation layer functionality include
- Encryption and decryption of a message for security
- Compression and expansion of a message so that it travels efficiently
- Graphics formatting
- Content translation
- System-specific translation
Layer 7 – The Application Layer
- Layer 7 is the application layer
- It gives an interface for the end user operating a device connected to a network.
- This layer is what the operator sees, in terms of loading an application (such as Web browser or e-mail)
- The application layer is the data the user views while using these applications.
- Examples of application layer functionality include
- Support for file transfers
- Ability to print on a network
- Electronic mail
- Electronic messaging
- Browsing the World Wide Web
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