The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system. The OSI model characterizes computing functions into a universal set of rules and requirements in order to support interoperability between different products and software. In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.
The OSI was published in 1984 by the International Organization for Standardization (ISO). Though it does not always map directly to specific systems, the OSI Model is still used today as a means to describe Network Architecture.
1. Physical Layer
The physical layer is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next. When receiving the data, this layer will get the signal received and convert it into the form of 0 and 1 and send them to the Data Link Layer, which will put the frame back together.
Function of Physical Layer
- Bit Synchronization
- Bit rate control
- Transmission mode
- Representation of Bits
- Devices must be connected using topologies
- Physical characteristic of interface and media
2. Data Link Layer
Data link layer is the second layer in OSI Model. This layer is the protocol layer in a program to handles the moving of data into and out of the physical link in the network. Data link layer of the most complicated layer because has complex functionalities and liabilities. Data link layer has worked between two hosts that are directly connected in some sense. The direct connection could be a point to point or broadcast. Data link layer also be responsible for converting data stream to signals bit by bit then send over the underlying hardware.
Functions of the data link layer
- To handles problems that occur as a result of bit transmission error
- To ensures the data flows at a pace that does not overwhelm sending and receiving devices
- Permits the transmission data to the network layer where it addressed and routed
Sublayer of data link
Also, we should be known that the data link layer has two sub-layer which is:
- Logical Link Control (LLC): It deals with protocols, flow-control, and error control
- Media Access Control (MAC): It deals with actual control of media
Standard of data link layer
The data link standards are:
Formatting Data for Transmission
The figure below shown formatting data for transmission
3. Network Layer
The network layer is the third layer in OSI Model. The network layer can define the communication over of the network with four basic processes which are addressing, routing, encapsulation and decapsulation. The role of the network layer is to move packets from sending hosts to receiving hosts.
Network Layer Protocols
Network layer protocols are:
4. Transport Layer
The layer that provides the means of transferring data sequences (packets) from the source to a destination (from a client to a server or vice versa) and while maintaining quality of service functions (i.e priority queuing and bandwidth management).
Transport layer is also responsible for the reliability of a link through the methods of flow control, segmentation, desegmentation, and error control. Some protocols are state (stateless= http, stateful= tcp) and connection oriented (connectionless= udp, connection-oriented= tcp). The transport layer must keep track of segments that is moving through and is able to retransmit segments that are not able to be delivered. The transport layer also provide acknowledgement of the successful data transmission and sends the next data if no errors occurred (refer to tcp protocol).
Critically, the transport layer creates segments out of the message received from the application layer so that it is easier to transport and manage.
Transport layer has it classes which ranges from class 0 to class 4. Class 0 is the least complicated but provides the least features, and class 4 is complicated but has the most features and usually suitable for unreliable networks. Class 0 contains no error recovery and is usually used on networks that is error-free. Looking at the feature set of a class 4, it closely resembles a tcp protocol. Although tcp contains functions such as graceful close.
5. Session Layer
The session layer controls the connections between hosts, or computers. It establishes, manages and terminates the connection between the local and the remote application (servers). It provides full-duplex(input and output at full speed) , half-duplex(only input or output one at a time) or simplex (only receive or transmit i.e walkie-talkies). Session layer is also responsible for procedures such as checkpointing, suspending, restarting and terminating a session. It is also responsible for gracefully closing a session.
Nowadays, it is replaced by tcp/ip system and the session is non existent and part of the tcp protocol.
6. Presentation Layer
This layer is primarily responsible for preparing data so that it can be used by the application layer; in other words, layer 6 makes the data presentable for applications to consume. The presentation layer is responsible for translation, encryption, and compression of data.
Two communicating devices communicating may be using different encoding methods, so layer 6 is responsible for translating incoming data into a syntax that the application layer of the receiving device can understand.
If the devices are communicating over an encrypted connection, layer 6 is responsible for adding the encryption on the sender’s end as well as decoding the encryption on the receiver's end so that it can present the application layer with unencrypted, readable data.
Finally the presentation layer is also responsible for compressing data it receives from the application layer before delivering it to layer 5. This helps improve the speed and efficiency of communication by minimizing the amount of data that will be transferred.
7. Application Layer
This is the only layer that directly interacts with data from the user. Software applications like web browsers and email clients rely on the application layer to initiate communications. But it should be made clear that client software applications are not part of the application layer; rather the application layer is responsible for the protocols and data manipulation that the software relies on to present meaningful data to the user. Application layer protocols include HTTP as well as SMTP (Simple Mail Transfer Protocol is one of the protocols that enables email communications).
In this section, you are expecting to explain on how the data travels from the student’s computer
to reach the Online Learning System server which is in the OUM IT Center. The explanation should
be related the OSI Layer Model. Explain the possible network components involves throughout
the communications. Illustrate the diagram to support your explanation.
In sequence for human-readable information can be transferred from one device to end device but the data should travel down using a seven-layer in OSI Model on sending the device. Then travel up using seven-layer be on the receiving end. Figure as shown below is the possible network components involved throughout the communication.
For example, the student uses Online Learning System to submit the assignments to lecturers. The procedure began on the student's personal computer (PC). The assignments, or raw data, will be passed to the application layer by the file application when the students upload the assignments. The data format and file transfer protocol (FTP) will be defined by the application layer, and the data will be passed to the presentation layer.
In presentation layer, the data format will be standardized, and the encryption process will be completed if necessary - refer to the communication connection type whether it is encrypted or not. Then, the presentation layer will compress the data and pass it to the session layer. The compression will help improve the speed and efficiency of the data uploaded.
During the session layer, the communication session will be initialized by sending request and responses between the application layer and the transport layer. It determines whether the requested session is legitimate or not. If yes, then the data will be pass to the next layer.
Next, the data is passed to the transport layer. During this stage, the transport layer it provides multiplexing, enables the host to send and receive error corrected data through the implementation of TCP and UDP protocols. The flow and error control will be performed, and the data will be segmented then broken up into packets at the network layer.
Then the network layer will facilitate the data transferred and ensure whether the network layer is necessary or not by checking the device communicating on the same network or not. Then, the network layer will route the packets to reach its destination using the best physical path. This process is called routing and occurred at the router.
When the packets passed to the data link layer, it will be broken down into frames and delivered to the physical layer. The frames will be converted to a bitstream of 0s and 1s and send through a physical medium which WLAN. After that, the data will be stored in the Online Learning System Server that is located in the IT Centre.
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