JANN VINCENT PAUL C. LAGMAY BSIT-1A Open System Interconnection (OSI) The Open System Interconnection Reference Model or OSI Reference Model or OSI Model is essentially a conceptual description for layered communications and computer network protocol design. It was created as part of the Open Systems Interconnection or OSI initiative. In its most fundamental form, it separates network architecture into seven distinct layers: the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. As a result it is frequently referred to as the OSI Seven Layer Model. A layer is a collection of theoretically similar functions that provide services to the layer directly above it and obtains service from the layer beneath it. On each single layer an instance provides services to the instances at the layer above and requests service from the layer beneath. An example of this relates to a layer that provides error-free communications across a network which then provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that comprise the contents of the path. SEVEN LAYERS OF OPEN SYSTEM INTERCONNECTION (OSI) Layer 7: Application Layer Defines interface to user processes for communication and data transfer in network Provides standardized services such as virtual terminal, file and job transfer and operation Layer 6: Presentation Layer Masks the differences of data formats between dissimilar systems Specifies architecture-independent data transfer format Encodes and decodes data; Encrypts and decrypts data; Compresses and decompresses data Layer 5: Session Layer Manages user sessions and dialogues Controls establishment and termination of logic links between users Reports upper layer errors Layer 4: Transport Layer Manages end-to-end message delivery in network Provides reliable and sequential packet delivery through error recovery and flow control mechanisms Provides connectionless oriented packet delivery Layer 3: Network Layer Determines how data are transferred between network devices Routes packets according to unique network device addresses Provides flow and congestion control to prevent network resource depletion Layer 2: Data Link Layer Defines procedures for operating the communication links Frames packets Detects and corrects packets transmit errors Layer 1: Physical Layer Defines physical means of sending data over network devices Interfaces between network medium and devices Defines optical, electrical and mechanical characteristics Information Technology Fundamentals
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