OSI MODEL (Open Systems Interconnection model )

To Day I Explain OSI MODEL, Open Systems Interconnection model (OSI) is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into abstraction layers. The model is a product of the Open Systems Interconnection project at the International Organization for Standardization (ISO), maintained by the identification ISO/IEC 7498-1.

The model groups communication functions into seven logical layers. A layer serves the layer above it and is served by the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of that path. Two instances at one layer are connected by a horizontal connection on that layer.

 The Below Shown Diagram is helpful to understand the OSI seven Layer Model.

OSI MODEL EXPLANATION

The Physical Layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. Ex: this layer defines the size of Ethernet coaxial cable, the type of BNC connector used, and the termination method.  This includes the layout of pins, voltages, line impedance, cable specifications, signal timing, hubs, repeaters, network adapters, host bus adapters. So Physical Connectors are in Layer 1 called as Physical Layer.

The Data Link Layer describes the logical organization of data bits transmitted on a particular medium. Ex: this layer defines the framing, addressing and check summing of Ethernet packets. Data Link Layer divided in two sub layers:-Media Access Control (MAC) layer - responsible for controlling how computers in the network gain access to data and permission to transmit it.Logical Link Control (LLC) layer - control error checking and packet synchronization.The Point-to-Point Protocol (PPP) is an example of a data link layer in the TCP/IP protocol stack.  

The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network. Ex: this layer defines the addressing and routing structure of the Internet. Routing information is there in Layer 3, Routing Protocols like RIP, IGRP, EIGRP, OSPF are Layer 3 Protocols.

The Transport Layer describes the quality and nature of the data delivery. Ex: this layer defines if and how retransmissions will be used to ensure data delivery.

Feature name                                   TP0 TP1 TP2 TP3 TP4

Connection-oriented network   Yes Yes Yes Yes Yes

Connectionless network                    No No No No Yes

Concatenation and separation    No Yes Yes Yes Yes

Segmentation and reassembly   Yes Yes Yes Yes Yes

Error recovery                            No Yes Yes Yes Yes

Reinitiate connectiona                    No Yes No Yes No

Multiplexing / demultiplexing over 

single virtual circuit                    No No Yes Yes Yes

Explicit flow control                    No No Yes Yes Yes

Retransmission on timeout            No No No No Yes

Reliable transport service            No Yes No Yes Yes

a If an excessive number of PDUs are unacknowledged.

The Session Layer describes the organization of data sequences larger than the packets handled by lower layers. Ex: this layer describes how request and reply packets are paired in a remote procedure call or you can say 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.

The Presentation Layer describes the syntax of data being transferred. Ex: this layer describes how floating point numbers can be exchanged between hosts with different math formats. The presentation layer establishes context between application-layer entities, in which the application-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the TCP/IP stack.

This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.

The Application Layer describes how real work actually gets done. Ex: this layer would implement file system operations.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 exists. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer