OSI LAYERS MODEL

 

WHAT IS OSI MODEL?

The OSI model, created by the International Organization for Standardization (ISO) in 1984, was meant to demonstrate the flow of data from one software program on one computer to another on another computer. Physical layer, Data link layer, Network layer, Transport layer, Session layer, Presentation layer, and Application layer are the layers that make up the seven-layer architecture of the Internet. It is the responsibility of each layer to carry out its job in a self-contained manner.

There have 7 layers in OSI Model

PROJECT SCENARIO

"The scenario of this project can be referred to Figure 1. Figure 1 shows the logical network flow from a home network to UTeM facilities. In this figure, a UTeM’s student is trying to access the Ulearn system located in the UTeM network facilities. The student is using her modem to connect to the public network before able to connect to the UTeM facilities. From the UTeM network, the data then was forwarded to IT Center where the Ulearn System server is located."

Application Layers

The application layer is typically mainly responsible for connecting with host-based and user-facing apps, rather than performing any other functions. When compared to this, the TCP/IP protocol groupings together two layers below the application layer, namely, the Session layer and the Presentation layer, to form a simpler single layer that is responsible for performing various functions, such as controlling the dialogues between computers, establishing and maintaining as well as ending a particular session, as well as providing data compression and encryption, among other things.

Application layer protocols are implemented in the same way on both the source and destination hosts in order to guarantee seamless communication. A few of the characteristics that application layer protocols give include the following:

Application Layer Protocols give the following features:

• The Application Layer protocol specifies the procedure that must be followed by both parties participating in the connection.
• These protocols specify the sort of message that may be transmitted or received from either side of the channel (either source host or destination host).
• These protocols also describe the fundamental grammar of the message that is being transmitted or retrieved.
• These protocols specify how a message should be sent and what kind of response should be anticipated.

Application Layers Based on Project scenario:

According to what we know about the OSI Model, the Application Layer is at the top of the stack. Following the project scenario, students' computers are able to directly interact with the U-Learn System server, which is located in the UTeM IT Center, by typing the link (https://www.utem.edu.my/) into any supported browsers, such as Safari, Google Chrome, or Mozilla Firefox, on their computers. In this scenario, we can categorically state that the application layer is comprised of software that is made available for end-users to interact with.

Presentation Layer

Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model. This layer is also known as Translation layer, as this layer serves as a data translator for the network. The data which this layer receives from the Application Layer is extracted and manipulated here as pert the required format to transmit over the network. The main responsibility of this layer is to provide or define the data format and encryption. The presentation layer is also called as Syntax layer since it is responsible for maintaining the proper syntax of the data which it either receives or transmits to other layer(s).

Functions of Presentation Layer :

• Presentation layer format and encrypts data to be sent across the network.
• This layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data efficiently and effectively.
• This layer manages the abstract data structures and allows high-level data structures (example- banking records), which are to be defined or exchanged.
• This layer carries out the encryption at the transmitter and decryption at the receiver.
• This layer carries out data compression to reduce the bandwidth of the data to be transmitted (the primary goal of data compression is to reduce the number of bits which is to be transmitted).
• This layer is responsible for interoperability (ability of computers to exchange and make use of information) between encoding methods as different computers use different encoding methods.
• This layer basically deals with the presentation part of the data.
• Presentation layer carries out the data compression (number of bits reduction while transmission), which in return improves the data throughput.
• This layer also deals with the issues of string representation.
• The presentation layer is also responsible for integrating all the formats into a standardized format for efficient and effective communication.
• This layer encodes the message from the user-dependent format to the common format and vice-versa for communication between dissimilar systems.
• This layer deals with the syntax and semantics of the messages.
• This layer also ensures that the messages which are to be presented to the upper, as well as the lower layer, should be standardized as well as in an accurate format too.
• Presentation layer is also responsible for translation, formatting, and delivery of information for processing or display.
• This layer also performs serialization (process of translating a data structure or an object into a format that can be stored or transmitted easily).

Presentation Layer Based on Project scenario:

The presentation layer is the 6th layer of the OSI model, which encrypt the data and sent to the application layer for the end user. Using this layer, the end-user is able to get understandable data output from the system.

Session Layer

The Session Layer is the 5th layer in the Open System Interconnection (OSI) model. This layer allows users on different machines to establish active communications sessions between them. It is responsible for establishing, maintaining, synchronizing, and terminating sessions between end-user applications. In Session Layer, streams of data are received and further marked, which is then resynchronized properly, so that the ends of the messages are not cut initially and further data loss is avoided. This layer basically establishes a connection between the session entities. This layer handles and manipulates data that it receives from the Session Layer as well as from the Presentation Layer.

Following are some of the functions which are performed by Session Layer –

• Session Layer works as a dialog controller through which it allows systems to communicate in either half-duplex mode or full-duplex mode of communication.
• This layer is also responsible for token management, through which it prevents two users from simultaneously accessing or attempting the same critical operation.
• This layer allows synchronization by allowing the process of adding checkpoints, which are considered as synchronization points to the streams of data.
• This layer is also responsible for session checkpointing and recovery.
• This layer basically provides a mechanism for opening, closing, and managing a session between the end-user application processes.
• The services offered by Session Layer are generally implemented in application environments using remote procedure calls (RPCs).
• The Session Layer is also responsible for synchronizing information from different sources.
• This layer also controls single or multiple connections for each-end user application and directly communicates with both Presentation and transport layers.
• Session Layer creates procedures for checkpointing followed by adjournment, restart, and termination.
• Session Layer uses checkpoints to enable communication sessions that are to be resumed from that particular checkpoint at which communication failure has occurred.
• The Session Layer is responsible for fetching or receiving data information from its previous layer (transport layer) and further sending data to the layer after it (presentation layer).

Session Layers Based on Project scenario:

Session Layer works as a dialog controller through which it allows systems to communicate in either half-duplex mode or full-duplex mode of communication. In Utem they are using the full-duplex mode of communication.

Transport Layer

This Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation and error control. Some protocols are state- and connection-oriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred. As an simple example, the transport layer is responsible for delivering a message from one user to another but processing it.

Typical examples of layer 4 are the :

• Transmission Control Protocol (TCP) 
• User Datagram Protocol (UDP)

The duties of this layer:

• Port addressing 
• Segmentation & Reassembly
• Connection control
• Flow control
• Error control

Network Layer

The network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing. For an example; The selection of the shortest path to transmit the packet , from the number of routes available. The sender and receiver's IP addresses are placed in the header by the network layer.

• Routing : - The network layer protocols determine which route is suitable from source to destination. The function of the network layer is known as routing.

• Logical Addressing : -  In order to identify each device on internetwork uniquely network layer defines an addressing scheme. The sender and receiver's IP addresses are placed in the header by the network layer. Such an address distinguishes each device uniquely and universally.

Data Link Layer

At the data link layer, directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. The data link layer also corrects errors that may have occurred at the physical layer. The data link layer encompasses two sub-layers of its own. The first, media access control (MAC), provides flow control and multiplexing for device transmissions over a network. The second, the logical link control (LLC), provides flow and error control over the physical medium as well as identifies line protocols.

• This layer is responsible for the error-free transfer of data frames.
• It defines the format of the data on the network.
• It provides a reliable and efficient communication between two or more devices.
• It is mainly responsible for the unique identification of each device that resides on a local network.

• It contains two sub-layers:
• Logical Link Control Layer
• It is responsible for transferring the packets to the Network layer of the receiver that is receiving.
• It identifies the address of the network layer protocol from the header.
• It also provides flow control.
• Media Access Control Layer
• A Media access control layer is a link between the Logical Link Control layer and the network's physical layer.
• It is used for transferring the packets over the network.

Functions of the Data-link layer

• Framing: The data link layer translates the physical's raw bit stream into packets known as Frames. The Data link layer adds the header and trailer to the frame. The header which is added to the frame contains the hardware destination and source address.

• Physical Addressing: The Data link layer adds a header to the frame that contains a destination address. The frame is transmitted to the destination address mentioned in the header.
• Flow Control: Flow control is the main functionality of the Data-link layer. It is the technique through which the constant data rate is maintained on both the sides so that no data get corrupted. It ensures that the transmitting station such as a server with higher processing speed does not exceed the receiving station, with lower processing speed.
• Error Control: Error control is achieved by adding a calculated value CRC (Cyclic Redundancy Check) that is placed to the Data link layer's trailer which is added to the message frame before it is sent to the physical layer. If any error seems to occur, then the receiver sends the acknowledgment for the retransmission of the corrupted frames.
•  When two or more devices are connected to the same communication channel, then the data link layer protocols are used to determine which device has control over the link at a given time.

Based On the scenario:

The Data link layer adds the header and trailer to the frame. The header which is added to the frame contains the hardware destination and source address. For the Utem portal its have Mac address and Ip address the destination suppose to travel. In flow control from server Utem to personal pc the data rate flow maintain certain constant. Does not get the data corrupted.

Physical Layer

 The lowest layer of the OSI Model is concerned with electrically or optically transmitting raw unstructured data bits across the network from the physical layer of the sending device to the physical layer of the receiving device. It can include specifications such as voltages, pin layout, cabling, and radio frequencies. At the physical layer, one might find “physical” resources such as network hubs, cabling, repeaters, network adapters or modems.

Functions of a Physical layer:

• Line Configuration: It defines the way how two or more devices can be connected physically.
• Data Transmission: It defines the transmission mode whether it is simplex, half-duplex or full-duplex mode between the two devices on the network.
• Topology: It defines the way how network devices are arranged.
• Signals: It determines the type of the signal used for transmitting the information

Based On scenario:

The physical layer from pc to Utem server, transfer data bits from pc to router and switch. Then the data bits converted to signal. The bit stream grouped into code words or symbols and converted to a physical signal that is transmitted over a transmission medium. The physical layer consists of the electronic circuit transmission technologies of a network.