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The Seven Layer Osi Model

Paper Type: Free Essay Subject: Information Technology
Wordcount: 4994 words Published: 28th Apr 2017

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Seven-layer OSI model is a logical layer and developed by the ISO international standardization organization. The purpose of this layer is to understand the transmission of data flows from source to destination and to identify problems during data transmission. The seven layers include Application, Presentation, Session, Transport, Network, Data link and Physical layer. The sequence of layer names to be remembered by this phrase “All people seem to need data processing”. The following is a general description of each layer. OSI reference model is now being considered as the main model for the internetworking and inter computing. The seven layers divide the task of data transmission in sub tasks and then complete the data transmission cycle.


In the 1980s, the European-dominated International Standards Organization (ISO) began to develop its Open Systems Interconnection (OSI) networking suite. OSI has two major components: an abstract model of networking (the Basic Reference Model, or seven-layer model), and a set of concrete protocols.

Briefly explain each of the seven OSI layer included their function and usage in data communication and networking.

OSI Reference Model

OSI reference model (Open Systems Interconnection) illustrates how information from a software application on a computer moves through a network medium to a software application on another computer. OSI reference model is conceptually divided into 7 layers where each layer has a specific network function, as described by figure 1.1 (with no physical media). This model was created based on a proposal made by the International Standards Organization (ISO) as a first step toward international standardization of protocols used at various layers. �This model is called ISO OSI (Open Systems Interconnection) Reference Model because this model is intended for sharing open system�.[1] Open Systems can be interpreted as an open system to communicate with other systems. For his brief, we will call this model as the OSI model only.

Figure 1.1. OSI Reference Model

OSI model has seven layers. The principles used for the seven layers are:

1. A layer must be made when needed different levels of abstraction.

2. Each layer shall have certain functions.

3. The function of each layer should be chosen carefully in accordance with the provisions of international protocol standards.

4. Layer boundaries labored to minimize the flow of information through the interface.

5. The number of layers must be quite a lot, so that different functions do not need to be united in a single layer outside needs. However, the number of layers must also be cultivated as little as possible so that the network architecture does not become difficult to use.

Below discuss about each layer of the OSI model sequentially, starting from the bottom layer. It should be noted that the OSI model itself is not network architecture, because this model does not explain the exact services and protocols to be used on any layer. OSI model only explains about what should be done by a layer. However, ISO has also established standards for all layers, although these standards do not constitute a reference model itself. Each layer has been declared as a separate international standard.

Characteristics of the OSI Layers

To seven layers of OSI reference model can be divided into two categories, namely upper and lower layers. The top layer of the OSI model deals with application issues and generally implemented only in software. The highest layer (application layer) is the cover prior to the user (users), both users and application layer processes interact with software applications that contain a communication component. The term upper layer is sometimes used to refer to some of the top layer of another coating layer in the OSI model. The bottom layer of the OSI model to control data transport issues. Physical layer and data link layer implemented in hardware and software. The layers below the others are generally only implemented in software. �The lowest layer, the physical layer is a layer of cover for the physical network medium (cable networks), and as responsible for the placement of information on the network media�.[2] The following table shows the separation of the two layers in the layers of the OSI model.

Application Application Overlay



Transport Data Transport Undercoat


Data Link


Table 1.2 Separation of top layer and bottom layer of the OSI model


OSI model provides a conceptual framework for communication between computers, but this model is not a method of communication. In fact communication can occur due to use of communication protocols. In the context of data networks, a protocol is a formal rules and agreements that determine how computers exchange information over a network medium. A protocol implements one or more of the layers of the OSI. A wide variation of the communication protocol, but all maintains the flow in one group: the LAN protocols, WAN protocols, network protocols, and routing protocols. LAN protocols operate at the physical and data link layer of the OSI model and define communication over various LAN media. WAN protocols operate at the third lowest layer of the OSI model and define communication over various WAN. Routing protocol is a network layer protocol that is responsible for determining the road and traffic arrangements. Finally, the network protocol is different from the upper layer protocols that exist in a series of protocols.

Layers of the OSI Model

Physical Layer

Physical Layer functions in the delivery of raw bits into the channel of communication. Design issues that must be considered here is to ensure that when one side sends data 1 bit, the data must be accepted by the other side as a bit too, and not 0 bit. The question that arises in this case is: how many volts should be used to declare a value of 1? And how many volts is also required for the number 0?. It takes a bit how microsecond will be exhausted? Is the transmission can be processed simultaneously in both directions? How many pins are owned by the network and what is the use of each pin? In general, design problems are found here related to the mechanical, electrical and procedural interfaces, and physical media which is under the physical layer.

Data Link Layer

The main task of the data link layer is as raw data transmission facility and transforms the data to a channel that is free from transmission errors. Before forwarded who work layer, data link layer perform this task by allowing the sender meme cage breaking input data into a number of data frames (usually numbering in the hundreds or thousands of bytes). Then the data link layer transmits these frames in sequence, and process the acknowledgment frames sent back by the recipient. Because the physical layer send and receive bit stream without regard to the meaning or architectural frame, then depending on the data link layer was to create and recognize frame boundaries it. This can be done by affixing a special bit to the beginning and end of the frame. If incidental bit patterns can be found in the data, it is necessary to pay special attention to ensure that the pattern is not incorrectly considered as the boundaries of the frame.

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The occurrence of noise in the channel can damage the frame. In this case, the data link layer software on the source machine to send back the broken frame. However, transmission of the same frame over and over again can cause duplication of frames. Duplicate frames need to be sent if an acknowledgment frame from the receiver who returned to the sender has been lost. �Depending on the layer is to overcome the problems that caused the destruction, loss and duplication of frames. The data link layer provides some service classes for the network layer. This service class can be distinguished in terms of quality and price�.[4]

Other problems that arise in the data link layer (and also most of the layers above it) are seeking the smooth process of data transmission from the sender to the recipient a slow speed. Mechanism of regulation of traffic data should allow the sender to know the amount of buffer space owned by the recipient at a certain moment. Often setting flow and error handling is done in an integrated manner. Channel that can send data in both directions also can be problematic. So therefore need to be considered for the data link layer software. Problems that can arise here is that the frames acknowledgement flowing from A to B compete with each other ahead of the flow from B to A. Settlement of the best (piggy backing) could have been used, then we shall discuss it in depth. Broadcast networks have an additional problem in the data link layer. The problem is in terms of controlling access to a shared channel. To fix this you can use a special sub layer of data link layer, called the medium access sub layer.

Network Layer

Network layer serves to control operation of the subnet. Important design problem is how to determine the route packets from source to destination. Route can be based on static tables are “linked to” network. Route can also be determined at the beginning of a conversation such as a terminal session. Finally, the route can also be very dynamic; it can be different for each package. Therefore, the delivery route of a packet depends on network load at that time. If at the same time in a subnet there are too many packets, then there is the possibility of the packages arrive at the same time. This can lead to bottlenecks. Congestion control as it is also the task of the network layer. Because the sub net operators expect the good fortune of his job duties. There is often some accounting function built at the network layer. To make billing information, at least the software should calculate the number of packets or characters or bits that are sent by each customer. Accounting becomes more complicated, when a packet crosses a country that has a different rate.

The move packets from one network to another can also cause problems that are not small. Addressing mode used by a network can be different from the methods used by other network. A network may be unable to receive the package at all because the packet size is too big. Protocol was be different, as well as others. Network layer has been given the task to solve all these problems, allowing different networks interconnected to each other.

Transport Layer

The basic function is the transport layer receives data from the session layer, split the data into pieces smaller if necessary, forwards the data to the network layer, and ensure that all pieces of data can arrive at the other side correctly. Moreover, all this must be done efficiently, and aims to protect the upper layers of the changes in hardware technology that can not be avoided. Under normal circumstances, the transport layer makes the network connection is different for each transport connection which is required by the session layer. When the transport connection requires a high throughput, then the transport layer can make a lot of network connections. Transport layer divide sending data to a number of networks to increase throughput. On the other hand, if the manufacture or maintenance of the network connection is quite expensive, transport layer can combine multiple transport connections to the same network connection. This is done to make this merger is not seen by the session layer.

Transport layer also determines the type of service for the session layer, and in turn the type of service for users of the network. Type of the most popular transport layer is a channel error-free point to point that forward messages or bytes in accordance with the order delivery. However, there are also other types of transport services. The services are transport isolated messages that do not guarantee order of delivery, and broadcast messages to a number of purposes. Type of service determined when the connection starts.

Transport layer is a layer end to end in fact, from source to destination. In other words, a program on the source machine to bring the conversation with the same program on the target machine. In the lower layers, there are protocols between the two machines and other machines that are close by. Protocol is not located in the outer or the source machine’s outer purpose machine, which may be separated by a router. The differences between layers 1 to 3 are intertwined and layer 4 to 7 are end to end. This can be explained as shown in Figure 1-1.

In addition to merging multiple streams of messages into a single channel, transport layer must be careful in defining and deciding on the network connection. This process requires a naming mechanism, so that a process on a machine has a way to explain with whom the machine wants to talk. Also there should be a mechanism to regulate the flow of information, so that the flow of information from a fast host does not overwhelm a slow host. Such mechanism is called flow control and plays an important role in the transport layer (also in other layers). Flow control between hosts with different hosts by controlling the flow of the router with the router. We’ll find out later that the same principles used for both types of the control.

Session Layer

Session layer allows users to establish sessions with other users. A session in addition allows ordinary data transport, as did the transport layer, also provides special services for specific applications. A session is used to allow one user logs into a remote timesharing system or to move files from one machine to another to machine. A session layer service is to implement the dialogue control. Sessions can allow traffic to move in two directions at one time or only in one direction only. If at one time in traffic only in one direction only analogous to a single railway line, the session layer helps to determine who is entitled to use the channel’s turn at an instant.

Service sessions are described as token management. For some protocols, it is important to ensure that both parties concerned do not perform surgery at the same time. To regulate this activity, the session layer provides tokens that can turn. Only a party that holds the token is allowed performs critical operations.

Another session is a synchronization service. Take for example that can occur when trying to transfer files that lasted 2 hours from one machine to another machine with the possibility of having an interval of 1 hour between the two crashes that can occur. After each transfer is canceled, the transfer may need to be repeated again from scratch, and may experience another failure. To reduce the possibility of this problem, the session layer can insert a specific sign to the flow of data. Therefore, if a crash, only the data that is after the sign is to be transferred back.

Presentation Layer

Presentation layer performs certain functions that are required to ensure the discovery of a common solution for certain problems. Presentation Layer does not allow users to solve their own problems. Unlike the layers underneath are only moving the bits from one place to another place, presentation layer attention to syntax and semantics of information transmitted. One example is the encoding of data presentation service. Most users do not move a random string of binary bits. The users exchange data just as the name, date, amount of money, and bills. The items are expressed in the form of a string of characters, integer numbers, floating point numbers, a data structure formed from some simpler items. There is a difference between one computer to another computer in a given code to declare a string of characters (for example, ASCII and Unicode), integer (for example, complement and complement one two), and so forth. To allow the two computers that have different presentation to communicate, data structures to be exchanged can be expressed by means of abstract, in accordance with standard encoding to be used “on line”. Presentation layer-data set and convert this abstract structure of representation that is used on a computer into a network standard representation, and vice versa.

Application Layer

Application layer consists of a variety of protocols. For example there are hundreds of incompatible terminal types in the entire world. Take the situation where full-screen editor is expected to work on a network with a variety of terminals, which each have different screen layouts, have a way different sequence of keystrokes for the insertion and deletion of text, move the sensor and so forth. One way to overcome such problems in data is to define an abstract network virtual terminal, so editors and other programs can be written for each corresponding. To handle each terminal type, one part of the software must be written to map the network virtual terminal functions to the actual terminal. For example, when moving the cursor editor virtual terminal into the left corner of the screen, the software should issue the appropriate command sequence to achieve the cursor. All virtual terminal software is in the application layer.

Another application layer function is file transfer. File systems from one another have different naming conventions, how to express the lines of text are different, and so forth. Transfer files from a system different to other systems that require treatment to overcome the lack of this compatible. The task is also a job application layer, such as electronic mail, remote job entry, directory lookup, and various general purpose facilities and other special purpose facilities.


The process of sending a data to be sent to the recipient. Sender processes submitting data to the application layer, which then add the user application header, the AH (which may also be empty), the front end and submit the results to the presentation layer. Presentation layer can form these data in various ways and may just add a header at the front end, which is given by the session layer. It is important to remember that the presentation layer is not aware of the data which are marked AH by application layer is the data users really are.

The process of granting this header repeated until the data reaches the physical layer, where data will be transmitted to other machines. On these machines, all headers were be removed one by one until reaching the admissions process.

Figure1 .3 Examples of how the OSI model used

The key here is that although the actual data transmission is each layer is programmed as though the transmission in question took place horizontally. For example, when transport layer delivery get a message from the session layer, transport layer will then affix the transport layer header and sends it to the receiver transport layer.


Computer networks are a tool, known also as nodes, relating to media liaison. Node may consist of a computer, printer or other device to send and receive data generated by other nodes, nodes in the network. Media Liaison, also known as communication media.

On this day, there are various types of networks, including local area network (LAN) and wide area network (WAN). The largest network in the world is the Internet.

Edit Development. Network started with a small building, the connection about ten computers and a printer. The technology that exists at the time the computer is connected to limit the number and length of communications cable that can be allowed on the network. In the early nineties the communication cable can be pulled so far about 600 feet to the extension of approximately 30 computers only. This network is suitable for a small organization known as local area network (LAN). LANs typically connect devices in a building or a building. A LAN can be built only with two computers and a printer or can be extended only in one building to include audio and video software. LAN size is limited to a distance of several kilometers only. LAN with data rates from 4 Mbps to 100 Mbps. Network connections from different countries has established the development of LAN to the wide area network (WAN). WAN can be sending data, audio and video at distances up to cover a country or continent or around the world.


Briefly explain at least four (4) topologies of networks and suggest how we implement these topologies in networking.


Network can be of any of the following three types:

Local Area Network:

Figure: 2.1 Local Area Network

Local Area Network (LAN) is a limited range of geographical barriers such as a laboratory, a school or building. In a typical LAN configuration, one of the computers used as file server file server that functions to keep all the software that controls the network. All computers are connected to the server file known as a workstation. Computer workstation comprising a desktop computer specifications less than the server and also may have software on the disk severity. Cable used to connect the LAN via network card installed on each computer.

Metropolitan Area Network:

Figure : 2.2 Metropolitan Area Network

Town Area Network (MAN) covers a wider geographical area as a region or city. Widespread area network connecting a number of Local Area Network in a place to enable sharing of information carried. Universities and government agencies Wide spread Area Network to communicate with the other communities and “private industries.”

One example is the MAN MIND Network, located in Pasco County, Florida which enables communication between the Central Pasco Media Center Main Frame using a telephone line, coaxial cable, and wireless communication providers.

Wide Area Network:

Figure : 2.3 Wide Area Network

Widespread Area Network (WAN) covers a wider area as a state, nation and world. To create a WAN requires systems across the sea or the use of microwave or satellite.

Use WAN enables communication between the states can be implemented quickly.

Configuring the WAN is complex and requires a hardware multiplexing multiplexer for LAN and MAN links for access to a comprehensive communications network such as the Internet.

Network Topology:

Figure : 3.1 Network Topology

�A computer network system using communication line that connects computers to each other�.[5] There are various methods that can be used for the communication and is known as topology. Topology can be considered as a virtual shape or structure of a network. This shape actually does not correspond to the actual physical design of the devices on the computer network. The computers on the home network can be arranged in a circle shape but it does not necessarily mean that it presents a ring topology.

There are few types of topologies for computer networks, namely: –

Star Network Topology

Figure :3.1.1 Star Network Topology

Star topology is a network where all computers or other devices connected to the central distribution system, known as the Hub or Switch. Hub or Switch is responsible for managing the computer network. All messages or data that is sent to a central control hub for avoid collisions from occurring. There are two operations that used by the hub or switch that is operating broadcast and the operations of the transition switching. �Distribution operations, hub or switch will make the delivery of data received from one station to all the routes connecting them. In transition operations, the hub or switch will save the data received and sends the data through a receiving station�.[3] Each node can be connected to the network and can be removed at any time without affecting the whole system of directly by. Furthermore, if one node does not work, it will not affect other computer networks. Any damage to the connection between the hub nodes will not affect the system as a whole, but if the damage occurred on the hub, the entire system will also be affected. The main weakness of this topology is that it requires a fairly high cost in view of all systems connected to it computers, scanners, printers, etc. requires its own cable.

Ring Network Topology

Figure :3.1.2 Ring Network Topology

In local area networks where the ring topology is used, each computer is connected to the network in a closed loop or ring. The Messages or data to be transferred in order according to the same path in the same ring. Network technology was first used by IBM systems with Type I in the category of cable shielded Twisted Pair STP. Each data node will be checked by delivery location. If not the same node strike, it will be submitted to the next node and this will continue until the delivery address the same node strike. �As the data is transferred by the same route as the sequence of the potential for violations of data that appears to be similar to the network and can be removed at any time without affecting the whole system of directly by�.[6] However, one problem that arises is if a node fails, the entire network will probably also be affected.

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Arrangement of nodes in this topology is almost the same physically as star topology; only a logical was a ring. The attachment is required in this topology is called Multi Station Access Unit (MSAU). MSAU is installed in a ring-shaped network where it is more than one unit. If only one unit only used the physical topology is similar to the star connection.

Bus Network Topology

Figure :3.1.3 Bus Network Topology

Bus topology, are also known as the Ethernet using a line length where each node in relation to him. Data will be sent by each node in the hope that it does not collide with data transmitted from other nodes. If it happens, the node will try again until successful.

Weakness of the bus topology is in terms of maintenance. It is quite difficult to maintain in view of the connection is in the form of a series. In the event of damage to any line at any node, it will cause the entire system will not work. In addition to accurately locate points or nodes those are damaged. Each node can be connected to the network and can be removed at any time without affecting the whole system directly. Furthermore, if one node does not work nor does it affect other computer networks. The other advantage is there in this topology is that it uses a single cable and requires no additional hardware to build the network. But it does not require a very high cost for maintenance purposes.

Tree Network Topology

Figure :3.1.4 Tree Network Topology

The tree topology is essentially a hybrid of the bus and star layouts. It is also known as a hierarchical topology and has a central root node that is connected to one or more nodes of a lower hierarchy. �In a symmetrical hierarchy, each node in the network has a specific fixed number of nodes connected to those at a lower level�.[3] Apart from these basic types of network topologies, there are hybrid network topologies, which are composed of a combination of two or more basic topologies. These network mappings aim at harnessing the advantages of each of the basic topologies used in them. Network topologies are the physical arrangements of network nodes and wires. What is interesting is that the inanimate nodes and wires turn ‘live’ for the transmission of information.

Mesh Network Topology

Figure :3.1.5 Mesh Network Topology

In a full mesh network, each network node is connected to every other node in the network. Due to this arrangement of nodes, it becomes possible for a simultaneous transmission of signals from one node to several other nodes. In a partially connected mesh network, only some of the network nodes are connected to more than one node. �This is beneficial over a fully connected mesh in terms of redundancy caused by the point-to-point links between all the nodes�.[5] The nodes of a mesh network require possessing some kind of routing logic so that the signals and the data traveling over the network take the shortest path during each of the transmissions.


Topologies are essence of computer networks design. Efficient networks can only be built based on the complete knowledge and understanding of above mentioned topologies. Knowledge of every communication device is of equal importance to help you find the best option for your network requirements. Optimum networks can be built with complete knowledge and understanding of computer network devices and how they are designed, any mistake in choosing inappropriate techniques, device etc can only be the waste of time, resources and efforts.


Figure 1.1. OSI Reference Model

Application Application Overlay



Transport Data Transport Undercoat


Data Link


Table 1.2 Separation of top layer and bottom layer of the OSI model

Figure1 .3 Examples of how the OSI model used

Figure: 2.1 Local Area Network

Figure : 2.2 Metropolitan Area Network

Figure : 2.3 Wide Area Network

Figure : 3.1 Network Topology

Figure :3.1.1 Star Network Topology

Figure :3.1.2 Ring Network Topology

Figure :3.1.3 Bus Network Topology

Figure :3.1.4 Tree Network Topology


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