Question

What the difference between the two Upper layers and lowest layer in the OSI model?

Answer 1

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.

It divides network communication into seven layers. Layers 1-4 are considered the lower layers and mostly are concerned with data around. Layers 5-7, the upper layers, contain application-level data. Therefore each layer has a specific task to do. This model works on the principle of ‘pass it on”. As one layer passes the data by adding the functionalities of that particular layer. The next layer then works on the existing data and passes on to the next. The last layer of OSI model is Application layer.

OSI model is a layered server architecture system in which each layer is defined according to a specific function to perform. All these seven layers work collaboratively to transmit the data from one layer to another.

• The Upper Layers: It deals with application issues and mostly implemented only in software. The highest is closest to the end system user. In this layer, communication from one end-user to another begins by using the interaction between the application layer. It will process all the way to end-user.
• The Lower Layers: These layers handle activities related to data transport. The physical layer and datalink layers also implemented in software and hardware.

The 7 layers of OSI model are as follows

1. Physical layer
2. Data link layer
3. network layer
4. transport layer
5. presentation layer
6. Session layer
7. Application layer

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.

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 occurr, then the receiver sends the acknowledgment for the retransmission of the corrupted frames.
• Access Control: 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.

Functions of Network Layer:

• Internetworking: An internetworking is the main responsibility of the network layer. It provides a logical connection between different devices.
• Addressing: A Network layer adds the source and destination address to the header of the frame. Addressing is used to identify the device on the internet.
• Routing: Routing is the major component of the network layer, and it determines the best optimal path out of the multiple paths from source to the destination.
• Packetizing: A Network Layer receives the packets from the upper layer and converts them into packets. This process is known as Packetizing. It is achieved by internet protocol (IP).

Functions of Transport Layer:

• Service-point addressing: Computers run several programs simultaneously due to this reason, the transmission of data from source to the destination not only from one computer to another computer but also from one process to another process. The transport layer adds the header that contains the address known as a service-point address or port address. The responsibility of the network layer is to transmit the data from one computer to another computer and the responsibility of the transport layer is to transmit the message to the correct process.
• Segmentation and reassembly: When the transport layer receives the message from the upper layer, it divides the message into multiple segments, and each segment is assigned with a sequence number that uniquely identifies each segment. When the message has arrived at the destination, then the transport layer reassembles the message based on their sequence numbers.
• Connection control: Transport layer provides two services Connection-oriented service and connectionless service. A connectionless service treats each segment as an individual packet, and they all travel in different routes to reach the destination. A connection-oriented service makes a connection with the transport layer at the destination machine before delivering the packets. In connection-oriented service, all the packets travel in the single route.
• Flow control: The transport layer also responsible for flow control but it is performed end-to-end rather than across a single link.
• Error control: The transport layer is also responsible for Error control. Error control is performed end-to-end rather than across the single link. The sender transport layer ensures that message reach at the destination without any error.

Functions of Session layer:

• Dialog control: Session layer acts as a dialog controller that creates a dialog between two processes or we can say that it allows the communication between two processes which can be either half-duplex or full-duplex.
• Synchronization: Session layer adds some checkpoints when transmitting the data in a sequence. If some error occurs in the middle of the transmission of data, then the transmission will take place again from the checkpoint. This process is known as Synchronization and recovery.

Functions of Presentation layer:

• Translation: The processes in two systems exchange the information in the form of character strings, numbers and so on. Different computers use different encoding methods, the presentation layer handles the interoperability between the different encoding methods. It converts the data from sender-dependent format into a common format and changes the common format into receiver-dependent format at the receiving end.
• Encryption: Encryption is needed to maintain privacy. Encryption is a process of converting the sender-transmitted information into another form and sends the resulting message over the network.
• Compression: Data compression is a process of compressing the data, i.e., it reduces the number of bits to be transmitted. Data compression is very important in multimedia such as text, audio, video.

Functions of Application layer:

• File transfer, access, and management (FTAM): An application layer allows a user to access the files in a remote computer, to retrieve the files from a computer and to manage the files in a remote computer.
• Mail services: An application layer provides the facility for email forwarding and storage.
• Directory services: An application provides the distributed database sources and is used to provide that global information about various objects.