What is the OSI model? 7 Layers And 7 Layers Remember






7 Layers Remember 

A convenient aid for remembering the OSI layer names is to use the first letter of each word in the phrase:
  All People Seem To Need Data Processing












What is the OSI model?

The Open Systems Interconnection (OSI) model is a conceptual model created by the International Organization for Standardization which enables diverse communication systems to communicate using standard protocols. In plain English, the OSI provides a standard for different computer systems to be able to communicate with each other.

The OSI model can be seen as a universal language for computer networking. It’s based on the concept of splitting up a communication system into seven abstract layers, each one stacked upon the last.




Why does the OSI model matter?

Although the modern Internet doesn’t strictly follow the OSI model (it more closely follows the simpler Internet protocol suite), the OSI model is still very useful for troubleshooting network problems. Whether it’s one person who can’t get their laptop on the Internet, or a web site being down for thousands of users, the OSI model can help to break down the problem and isolate the source of the trouble. If the problem can be narrowed down to one specific layer of the model, a lot of unnecessary work can be avoided


The Seven Layers




The Standards




OSI Model:

Open Systems Interconnection (OSI) is a set of internationally recognized, non-proprietary standards for networking and for operating system involved in networking functions.
It is nothing tangible. Means that it does not perform any role in real communication.
It just defines which work will be done by which part of the network.




7 Layers





Tasks involved in sending  letter:



LAYER 7 – The APPLICATION Layer:

The top layer of the OSI model 

Provides services to user for sending and receiving data.
Allows user to gain access to network services, such as: file transfer, message handling, email reading and database query processing
All application programs work on this layer e.g. web browser and email software.



The application layer is responsible for 
providing services to the user.

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  1. Application (Layer 7)-OSI Model, Layer 7, supports application and end-user processes. Communication partners are identified, quality of service is identified, and user authentication and privacy are considered. This layer provides application services for file transfers, e-mail, and Layer 7 Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, and FTP
    Presentation (Layer 6)-This layer provides independence from differences in data representation by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems.
    Session (Layer 5)-This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination. Layer 5 Session examples include NFS, NetBIOS names, RPC, SQL.
    Transport (Layer 4)-OSI Model, Layer 4, provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. Layer 4 Transport examples include SPX, TCP, UDP.
    Network (Layer 3)-Layer 3 provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing. Layer 3 Network examples include AppleTalk DDP, IP, IPX.
    Data Link (Layer 2)-At OSI Model, Layer 2, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking. Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay.
    Physical (Layer 1)-OSI Model, Layer 1 conveys the bit stream - electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

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