The 7 Layers of the OSI Model

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. Application(Layer 7) This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer. Presentation(Layer 6) This layer provides independence from differences in data representation (e.g., encryption) 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. It is sometimes called the syntax layer. 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. Transport(Layer 4) This layer 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. Network(Layer 3) This layer 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. Data Link(Layer 2) At this layer, 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 sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer 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. Physical(Layer 1) This layer 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.

Javascript Introduction

JavaScript is the most popular scripting language on the Internet, it works with all major browsers, like Internet Explorer, Google Chrome, Mozilla FireFox, Safari, and Opera. JavaScript is a scripting language, this means, it is a lightweight programming language and everyone can use it without purchasing a license. It is usually embedded directly into HTML pages. It was designed to add interactivity to HTML, and it's scripts execute without preliminary compilation. The amazing this about Java and Javascript is that they are two completely different languages in both concept and design, which some of the newbies do not know. JavaScript is a scripting language with a very simple syntax. Almost anyone can put small "snippets" of code into their HTML pages. A JavaScript can be used to detect the visitor's browser, and - depending on the browser - load another page specifically designed for that browser. It can be used to store and retrieve information on the visitor's computer. JavaScript's official name is ECMAScript. The language was invented by Brendan Eich at Netscape (with Navigator 2.0), and has appeared in all Netscape and Microsoft browsers since 1996. The development of ECMA-262 started in 1996, and the first edition was adopted by the ECMA General Assembly in June 1997. ECMA-262 is the official JavaScript standard, which was approved as an international ISO (ISO/IEC 16262) standard in 1998. The development of the standard is still in progress.