UNIT 1: COMPUTER NETWORKS AND THE OSI MODEL

What is the OSI Model?

• OSI stands for Open Systems Interconnection. It has been developed by ISO – ‘International Organization of Standardization‘, in the year 1984.
• It is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer.
• OSI consists of seven layers, and each layer performs a particular network function.
  The OSI model is now considered as an architectural model for the inter-computer communications.
• OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task.
• The upper layer of the OSI model mainly deals with the application related issues, and they are implemented only in the software. The application layer is closest to the end user. Both the end user and the application layer interact with the software applications. An upper layer refers to the layer just above another layer.
• The lower layer of the OSI model deals with the data transport issues. The data link layer and the physical layer are implemented in hardware and software. The physical layer is the lowest layer of the OSI model and is closest to the physical medium. The physical layer is mainly responsible for placing the information on the physical medium.

LAYERS OF THE OSI MODEL

Application (Layer 7)

OSI Model, Layer 7, 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)

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.

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.

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.

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.

NETWORK TOPOLOGY

• Topology refers to the way the computers (and other devices) in a network are linked together. It is the layout or map of a network. 
• Physical topology describes where the cables are run and where the workstations, nodes, routers, and gateways are located.
• Logical topology refers to the paths that messages take to get from one user on the network to another.

Client-Server Communications

• A server is a software or hardware device that accepts and responds to requests made over a network.
• The device that makes the request, and receives a response from the server, is called a client.
• On the Internet, the term “server” commonly refers to the computer system which receives a request for a web document, and sends the requested information to the client.
• In client/server computing, a server takes requests from client computers and shares its resources, applications and/or data with one or more client computers on the network, and a client is a computing device that initiates contact with a server in order to make use of a shareable resource.
• Servers are used to manage network resources. For example, a user may set up a server to control access to a network, send/receive e-mail, manage print jobs, or host a website.
• They are also proficient at performing intense calculations. Some servers are committed to a specific task, often referred to as dedicated. However, many servers today are shared servers which can take on the responsibility of e-mail, DNS, FTP, and even multiple websites in the case of a web server.

CLASSIFICATION OF CLIENT/SERVER SYSTEMS

• Broadly, there are three types of Client/Server systems in existence.
• (i) Two-tier
• (ii) Three-tier
• (iii) N-Tier

• Two–tier. Refers to client/server architectures in which the user interface runs on the client and the database is stored on the server. The actual application logic can run on either the client or the server.
• A three–tier client/server is a type of multi-tier computing architecture in which an entire application is distributed across three different computing layers or tiers. It divides the presentation, application logic and data processing layers across client and server devices.
• N-tier architecture is a client-server architecture concept in software engineering where the presentation, processing and data management functions are both logically and physically separated. These functions are each running on a separate machine or separate clusters so that each is able to provide the services at top capacity since there is no resource sharing
• This separation makes managing each separately easier since doing work on one does not affect the others, isolating any problems that might occur.

The Future of Data Communication and Networking

• The Internet will continue to play an increasing role in communication. Voice over Internet protocol (VoIP) already plays a large role in several communication products and services

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