OSI Model Layer

 

There are few network components that involved throughout the communication. First, message. Message is the information or data to be sent by sender or received by receiver. For examples, text, numbers, pictures, audio, and video. Next, the sender which is the device that sends data whereas the receiver is the device that receives the data. In this case, the Lecturer’s Computer is the Sender and the ULearn System Server is the receiver. The transmission medium is the physical path between the sender and the receiver. For examples, twisted-pair wire, fiber-optic cable, and radio waves. A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating. In this case, the HTTP/S are the protocols that are used to perform the lecturer activities to access the ULearn System.

The OSI Model

            OSI stands for Open System Interconnection. It is a reference model that helps users to visualize how data are being transferred from one device to another device through physical medium in the networking system. OSI Model consists of seven layers and each layer carry out different network function.

            The top layer is the Application Layer and it is the closest to the end-user which is the lecturer. This layer will receive any forms of data like text, image or video from the lecturer. In order to access the ULearn System, the lecturer need to use web browsers like Chrome. Web browsers is a network application and all network application are depending on Application Layer to function as it provides protocol package. HTTP/S is the protocols that being used for web surfing. Essentially, this application layer provides services for the network application to perform any user activities with the help of protocols.  

  

            The next layer is Presentation layer. This layer will receives data from the Application Layer. The data can be in the form of characters and numbers. This layer will converts those character to machine-understandable binary format. This layer will also compressed the data to reduce the number of bits of the original data. Thus, it can reach the server faster since the size is reduce. Before the data being transferred, the data will be encrypted to maintain the integrity of the data. On the sender side, the data is encrypted before reaching the destination and the data will be decrypted after reach the destination. SSL (Secure Socket Layer) is use at the presentation layer for encryption and decryption.

            In session layer, it will helps in establishing or terminating connections to enable sending and receiving process. Session layer used APIs (Application Programming Interfaces) like NETBIOS that allows applications on different computer to communicate with each other. Before a session or connection is established, ULearn System Server will performs authentication to verify who is accessing system. In this scenario, the lecturer will need to enter username and password. Once the username and password are matched and verified, a session is establish between the lecturer’s computer and ULearn System Server. After Authentication is done, Authorization is being perform to determine if the lecturer have permission to access the system or not. If not then the lecturer will get message "You are not authorized to access this page".

Transport layer controls the reliability of communication through flow control, error control and segmentation. In segmentation the data received from the session layer will be divided into small data unit called segments. Each segments contain a source and destination port number to make sure each data reach correct destination and sequence number to make sure the data are being send in correct order. Flow control means it will controls the amount of data that are being transferred. For example, if the lecturer wants to download a file from the ULearn System Server and the server starts sending data in 50 Mbps which greater than the lecture’s computer can processed. With the help of transport layer, lecturer’s computer can tell the server to slow down the data transmission rate to 10 Mbps and vice versa. For error Control. If some data does not arrive the destination transport layer uses Automatic Repeat Request scheme to re-transmit the lost or corrupted data. A group of bits called checksum is added to each segment to find out the received corrupted segment.

Network layer received the data segments from the transport layer. The function of network layer is logical addressing. Lecturer’s computer and the ULearn System Server have their own unique IP Address. Network layer will assigned both sender and receiver IP address to each segments to form an IP Packet. This is to ensure that each data packet reach correct destination. Routing is a method of moving data packet from source to destination and it is being done in this layer. Choosing the best possible path for the data delivery from source to the destination is called path determination. The lectures’s computer can be connected to the ULearn System Server in many ways. This layer uses protocols such as BGP (Border Gateway Protocol) and IS-IS (Intermidiate System to Intermediate System) to determine the best path.

Data link layer receives data packets that contains IP address of the lecturer’s computer and ULearn System Server from the network layer. Physical layer is done at data link layer where MAC addresses of the sender and receiver devices are assigned to each data packets to form a frame which is a sequence of binary 0’s and 1’s.

At the bottom of the OSI model is the physical layer that represents the electrical and physical representation of the system which includes cable type, radio frequency link for the wireless systems and so on. In this layer, the sequence of binary bits from the data link layer will converted into signals and transmit it over a transmission medium. For example, Copper cable if the signal is an electrical signal or air in case of radio signal. The signal is generated based on the type of transmission medium used to connect the lecturer’s computer and the ULearn System Server. In this case, since it is wireless, the signal is radio signal.