The OSI Model Layers from Physical to Application – The Open system Interconnection (OSI) is the layered model for the network systems designed by ISO (International Organization for Standardization). OSI enables devices in the network to communicate with each other.
The working of the OSI layers of the OSI model appears as shown in below figure.
These seven layers are related to each other for their functioning. Each layer call for the service of the layer below it.
At the Physical layer, the communication is direct. The communication flows from the highest layer of the sending computer to its lowest layer. This communication then flows to the lowest layer of the receiving computer till it reaches to the highest layer.
The layers at the sending computer add some information to the original message received from a layer above it. This information is in the form of headers and trailers. The header and trailer information represents the control data appended at the beginning and end respectively. The control data enables the message to be transmitted at the appropriate destination.
Application Layer – The application layer is the top most layer of the OSI model. This layer provides and interface for the user to interact with the application such as electronic mail, remote access transfer and shared database management. It also issues request to the presentation layer.
The function of the application layer are:
- Authentication – Authenticates the sender or receiver of the message or both.
- File access, transfer and management – Allows the user at a remote site to access files on another host.
- Directory Services – Provides access to global information and database sources.
Presentation Layer – The presentation layer is the sixth layer of the OSI model. This layer responds to the requests sent by the application layer. It is responsible for delivering information to the application layer. It defines the forms of text, graphics, video or audio information that is to be sent to the user. The presentation layer encrypts and adds a header to the data.
The function of the presentation layer are:
- Data compression – Provides efficiency while transmitting data. It refers to a process of encoding data using less number of bits.
- Encryption – Ensures security by using different algorithms for coding, password and log-in-codes.
Session Layer – The session layer is the fifth layer of the OSI model. It controls the communication between the applications on the hosts. The session layer is responsible to validate and establish a connection between the user.
The function of the session layer are:
- Session management – Divides the session into sub sessions by inserting checkpoints.
- Synchronization – Selects the order in which dialog unit must pass to the transport layer. It also gets confirmation from the receiver machine.
- Dialog control – Controls which user will send data and at what time.
- Closing the Session – Ensures that the data transfer is completed before the session closes.
Transport Layer – The transport layer is the fourth layer of the OSI model. The transport layer is responsible for source-to-destination delivery of the entire message. The transport layer ensures that the entire message arrives at the destination computer.
The function of the transport layer are:
- Service-point Addressing – Transport layer includes service-point address (also referred to as port address) in the header. Using these port addresses, transport layer delivers the packet to the correct process.
- End-to-end message delivery – Ensures that the entire message is transmitted to the destination.
- Segmentation and reassembly – Divides each message into segments and assigns a sequence number to these segments. This helps to reassemble the message if some error occurs during message transmission.
- Connection control – Decides whether all the packets will be sent using a single path or not.
Network Layer – The network layer is the third layer of the OSI model. This layer is responsible for routing and logical addressing. A message is divided into several packets.
The network layer provides services such as assigning IP address and routing, for delivering the packets. Routing is the process of selecting the best path of the available paths in order to send the packets. Address Resolution Protocol (ARP) is a common protocol used in the network layer.
The function of the network layer are:
- Source-to- destination delivery – Transfers packets from the source to its destination.
- Logical Addressing – Adds the source and destination addresses in the header.
- Routing – Selects the optimal path out of the multiple paths so that a packet can follow.
- Address Transformation – Interprets the logical address.
- Multiplexing – Utilizes one physical line for transferring data between several devices at a time.
Data Link Layer – The network layer is the second layer of the OSI model. It performs five functions:
- Physical Addressing
- Error detection
- Identifying encapsulated data
The data link layer works with Ethernet as per the IEEE specification. When the data link layer accepts the data from the network layer, it adds meaningful bits at the beginning and end of the data. These bits are known as the header and the trailer respectively. The header contains bits that alert the receiver about the arrival of the frame. When the data is received by the client router, the header is removed and only the data is transmitted. The IEEE Ethernet Data Link Layer works with two sub layers, Media Access Control (MAC) and Logical Link Control (LLC).
Physical Layer – The physical layer is the first layer of the OSI model. It is responsible to transmit a bit over the physical medium. A bit is a short form for binary digit and is the smallest unit of information on the computer. The bit can hold either a ‘0’ or ‘1’ value.
The responsibilities of the physical layer are:
- Line configuration – Defines the way in which two or more devices can be connected physically.
- Data transmission – Defines the transmission mode between the two devices on the network.
- Topology – Determines the way in which the network devices are arranged.
- Signals – Determines the type of signal that is used for transferring information.
So this was all about “The OSI Model Layers from Physical to Application” I have tried my beat to explain it, in case there is any error or suggestion you can tell me by your comments.