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The Working Principle Of Network OSI Layers

Paper Type: Free Essay Subject: Information Technology
Wordcount: 2048 words Published: 1st May 2017

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A stand-alone personal computer means a computer having an operating system, user programs and data. It has the means to put information on a floppy diskette, on a hard disk, or on paper. Information from such a computer to passes to another user only by first taking a copy. The copy can be in a form of a printout or it can be information in a floppy disk, a removable hard disk, or on a compact disk.

Analysis of data

1. What is computer network?

A computer network, often simply referred to as a network, is a collection of computers and devices interconnected by communications channels that facilitate communications among users and allows users to share resources. Networks may be classified according to a wide variety of characteristics. A computer network allows sharing of resources and information among interconnected devices.

2. Elements of computer networks

End devices, also referred as hosts: The sources and destinations of the communication. The devices the end users are most familiar with. These devices act as the interface between the end users and the underlying network.

Intermediary devices: Devices that give network access to the attached end devices and transport the messages between hosts. Usually it is transparent to the end users. Also, these devices accomplish communication functions in order to ensure the success of the communication process. Examples: Hubs, switches, routers, modems, firewalls, etc.

Transmission media: The physical media that connects the devices, enabling the exchange of messages between them. It may be wired, that is, some copper cable or optical fiber cable, or wireless, that is, some radio link.

Services: Network, aware software applications that request network resources in order to enjoy the end user of the application some provided service.

Processes: Software that runs on network devices in order to support the communication functions in accordance with the established, also in software, communication rules or protocols and facilitate the provision of services to the end users.

Messages: Well-known applications. Include telephone calls, e-mail, web pages, etc.

Devices and media are the physical elements or hardware of the network, whereas the services and the processes are the computer programs or software of the network.

We have to note that modern computer networks are converged data or information networks, meaning that can carry any type of information between wide ranges of devices, and provide previously separate and distinct conventional communication methods and services, as well as new, alternative communication forms onto a common platform.

3. Explanation about seven OSI layers in clear

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, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. 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, and 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 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 sub layers: The Media Access Control layer and the Logical Link Control 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)

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.

4. Relationship between all the layers

The OSI model originally distinguishes between service, interval and protocols. 

The TCP/IP model doesn’t clearly distinguish between service, interval and protocol.

The OSI model is a reference model.

.The TCP/IP model is an implementation of the OSI model.

In OSI model, the protocols came after the model was described.

In TCP/TP model, the protocols came first, and the model was really just a description of the existing protocols.

In OSI model, the protocols are better hidden.

In TCP/IP model, the protocols are not hidden.

The OSI model has 7 layers.

The TCP/IP model has only 4 layers.

The OSI model supports both connectionless and connection-oriented communication in the network layer, but only connection -oriented communication in transport layer.

The TCP/IP model supports both connectionless and connection-oriented communication in the transport layer, giving users the choice.

Explain all the layers with example network?

Application Layer 7: It is employed in software packages which implement client-server software. When an application on one computer starts communicating with another computer, then the Application layer is used. The header contains parameters that are agreed between applications. This header is often only sent at the beginning of an application operation. Examples of services within the application layer include:




SMTP gateways

Web browser

Network File System (NFS)

Telnet and Remote Login (rlogin)



Database software

Print Server Software

Presentation Layer 6: This provides function call exchange between host operating systems and software layers. It defines the format of data being sent and any encryption that may be used, and makes it presentable to the Application layer. Examples of services used are listed below:








Session Layer 5: The Session layer defines how data conversations are started, controlled and finished. The Session layer manages the transaction sequencing and in some cases authorization. The messages may be bidirectional and there may be many of them, the session layer manages these conversations and creates notifications if some messages fail. Indications show whether a packet is in the middle of a conversation flow or at the end. Only after a completed conversation will the data is passed up to layer 6. Examples of Session layer protocols are listed below:



NetBIOS names

AppleTalk ASP

Decent SCP

Transport Layer 4: This layer is responsible for the ordering and reassembly of packets that may have been broken up to travel across certain media. Some protocols in this layer also perform error recovery. After error recovery and reordering the data part is passed up to layer 5. Examples are:




Network Layer 3: This layer is responsible for the delivery of packets end to end and implements a logical addressing scheme to help accomplish this. This can be connectionless or connection-oriented and is independent of the topology or path that the data packets travel. Routing packets through a network is also defined at this layer plus a method to fragment large packets into smaller ones depending on MTUs for different media. Once the data from layer 2 has been received, layer 3 examines the destination address and if it is the address of its own end station, it passes the data after the layer 3 headers to layer 4. Examples of Layer 3 protocols include:

Appletalk DDP




Data Link Layer 2: This layer deals with getting data across a specific medium and individual links by providing one or more data link connections between two network entities. End points are specifically identified, if required by the Network layer Sequencing. The frames are maintained in the correct sequence and there are facilities for Flow control and Quality of Service parameters such as Throughput, Service Availability and Transit Delay. Examples include:

IEEE 802.2

IEEE 802.3

802.5 – Token Ring


Frame Relay




The Data link layer performs the error check using the Frame Check Sequence in the trailer and discards the frame if an error is detected. It then looks at the addresses to see if it needs to process the rest of the frame itself or whether to pass it on to another host. The data between the header and the trailer is passed to layer 3. The MAC layer concerns itself with the access control method and determines how use of the physical transmission is controlled and provides the token ring protocols that define how a token ring operates. The LLC shields the higher level layers from concerns with the specific LAN implementation.

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Physical Layer 1: This layer deals with the physical aspects of the media being used to transmit the data. The electrical, mechanical, procedural and functional means this defines things like pin outs, electrical characteristics, modulation and encoding of data bits on carrier signals. It ensures bit synchronization and places the binary pattern that it receives into a receive buffer. Once it decodes the bit stream, the physical layer notifies the data link layer that a frame has been received and passes it up. Examples of specifications include:













To conclude with a computer network provides connectivity between computers like autonomous systems, networks or nodes. It permits sharing of various resources among all, or several, or one of those computers that are associated with the network.


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