Hello Friends, In this blog post(OSI model layers functions), I am going to explain to you the detailed layer-based functioning of the OSI Model. Here we will discuss how the process takes place in each layer of the OSI Model to establish communication between the sender and receiver.
In this blog post(OSI model layers functions), we are going to see What is the function of the session layer in the OSI model. Which network device functions only at layer 1 of the OSI model? What is the function of the OSI session layer?…..
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OSI model layers function:
Physical Layer:
In the OSI Model, the physical layer coordinates the functions required to transmit a bitstream over a physical medium.
It deals with the mechanical and electrical specifications of the interface and transmission medium.
It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur. The physical layer is related to the following –
Responsibilities of bits:
In the OSI Model, the physical layer data have a stream of bits(sequence of 0s and 1s) without any interpretation.
For transmission, bits must be encoded into the signal – electrical and optical. The physical layer defines the type of encoding.
Synchronization of bits:
The sender and receiver must be synchronized at the bit level. In another way, the Sender and receiver clocks must be synchronized.
Transmission mode:
The direction of transmission between two devices- simplex, half-duplex, or full-duplex- is defined by the physical layer.
In the simplex mode, one device can only send and another device can only receive.
Simplex mode is one-way communication. In the case of half-duplex mode, two devices can send and receive but not at the same time.
Whereas using a full-duplex mode device can send and receive signals or information at the same time.
Physical characteristics of interfaces and media:
The characteristic of the interface between the devices and transmission medium is defined by the physical layer.
The physical layer also defines the type of transmission medium.
Data rate:
The physical layer also defines the number of bits sent each second which is known as the transmission rate.
In other words, the duration of a bit is defined by the physical layer, which is how long it lasts.
Physical topology:
The physical topology defines the connection of devices to make a network. Devices can be connected by using a star topology(devices are connected through a central device), a ring topology(every device is connected to the next, making a ring), a mesh topology(every device is connected to every other device), or a bus topology(every device on a common link).
Line configuration:
The physical layer is related to the connection of devices to the medium. In the point-to-point configuration, two devices are connected by a dedicated link.
In the multipoint configuration, a link is shared between many devices.
Datalink layer:
In the OSI Model, the data link layer transforms the physical, raw transmission facility, into a reliable link and is responsible for node-to-node delivery.
It makes the physical layer appear error-free to the upper layer. The main functions of the data link layer are as follows:
Framing:
The data link layer divides the stream of bits received from the network layer into manageable data units known as frames.
Physical addressing :
When the frames are to be distributed to various systems on the network, the data link layer adds a header to the frame to define the physical address of the sender(Source address) and/or receiver(destination address) of the frame.
When the frame is intended for a system outside the sender’s network, the receiver address is the address of the device that one network to the next.
Flow control:
When the rate at which the data are absorbed by the receiver is less than the rate produced by the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver.
Error control:
By adding mechanisms to detect and retransmit damaged or lost frames, the data link layer adds reliability to the physical layer.
It also uses a mechanism to prevent the duplication of frames.
Access control:
If two or more devices are connected to the same link, data link layer protocols are required to decide which device has control over the link at any given time.
Network Layer:
In the OSI Model, the network layer is responsible for the source-to-destination delivery of a packet across multiple networks(Links).
If two systems are connected to the same link, there is no need for a network layer. However, if the two systems are attached to different networks(links) with connecting devices between the networks, there is a need for the network layer to accomplish source-to-destination delivery. Specific functions of the network layer include the following:
Routing:
When independent networks or links are connected to develop an internetwork(a network of networks) or a large network, the connected devices(Known as routers or gateways) route the packets to their final destination.
Logical Addressing:
The addressing problem is locally handled by the physical addressing implemented by the data link layer.
If a packet passes the network boundary, another addressing system is required to help distinguish the source and destination system.
A header is added by the network layer to the packet coming from the upper layer.
Among other things, including the logical addresses of the sender and receiver.
Transport layer:
In the OSI Model, the transport layer is responsible for source to destination(end-to-end) delivery of the entire message.
Whereas the network layer oversees the end-to-end delivery of individual packets, it does not recognize any relationship between these packets.
The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level.
Specific responsibilities of the transport layer include the following:
Segmentation and reassembly:
The message is divided into transmittable segments, each segment having a sequence number.
These numbers enable the transport layer to reassemble the message accurately upon arriving at the destination and to identify and replace packets that were lost in the transmission.
Error Control:
Similar to the data link layer, the transport layer is responsible for error control.
However, error control at this layer is done end-to-end rather than across a single link.
Sending the transport layer ensures that the whole message arrives at the receiving transport layer without error.
Usually, error correction is obtained through retransmission.
Service point addressing:
A computer often runs many programs at the same time. Due to this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process(running program) on one computer to a specific process(running program) on the other.
Therefore, the transport layer header must include a type of address known as a service point address(or port address).
The network layer receives each packet to the correct computer.
The transport layer receives the complete message to the correct process on that computer.
Flow Control:
Similar to the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is done end-to-end rather than across a single link.
Connection control:
The transport layer can be either connectionless or connection-oriented.
A connectionless transport layer deals with each segment like an independent packet and delivers it to the transport layer at the destination machine.
A connection is established by the connection-oriented transport layer with the transport layer at the destination machine first before delivering the packets. The connection is terminated after all the data are transferred.
Session layer:
In the OSI Model, the session layer is the network dialog controller.
It establishes, maintains, and synchronizes the interaction between the communication system. Specific responsibilities of the session layer include the following:
Synchronization:
The session layer permits a process to add checkpoints(synchronization points) into stream data.
For example, at the time of sending a file of 2000 pages by a system, it is advisable to insert a checkpoint after every 100 pages to make sure that each 100-page unit is received and acknowledged independently.
Dialog Control:
The session layer permits two systems to enter into a dialog. It permits communication between two processes to take place either in a half-duplex(one way at a time) or a full-duplex(two ways at a time).
Presentation Layer:
In the OSI Model, the presentation layer is connected with the syntax and semantics of the information exchanged between the two systems.
The presentation layer has the following specific responsibilities.
Encryption:
A system must be able to assure privacy for carrying sensitive information.
Encryption means that the sender transforms the original information into another form and sends the resulting message out over the network.
The original process is reversed by the decryption to transform the message back to its original form.
Translation :
Usually, the processes(running programs) in two systems exchange information in the form of character strings, numbers, and so on.
Before being transmitted, the information should be converted into bitstreams.
Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods.
Presentation at the sender converts the information from its sender-dependent format into a common format.
At the receiving machine, the presentation layer converts the common format into its receiver-dependent format.
Compression:
Data compression minimizes the number of bits to be transmitted.
Data compression is particularly important in the transmission of multimedia such as text, audio, and video.\
Application Layer:
In the OSI Model, the application layer enables the user to access the network.
It provides the user interface and support for services such as electronic mail, remote file access, transfer, etc.
The following specific services are provided by the application layer.
File Transfer, access, and management(FTAM):
This application permits a user to access files on a remote computer(to make changes or read data), retrieve files from a remote computer, and manage or control files on a remote computer.
Network Virtual Terminal:
A network virtual terminal is a software version of a physical terminal and permits a user to log on to a remote host.
The user’s computer talks to the software terminal, which in turn, talks to the host, and vice versa.
Directory service:
Distributed database sources and access to global information about different objects and services are provided by the application.
Mail Services:
This application provides the basis for email forwarding storage.
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Conclusion:
Using this blog(OSI model layers functions) post we have gone through What is the function of the session layer in the OSI model. Which network device functions only at layer 1 of the OSI model? What is the function of the OSI session layer? What is the function of the presentation layer in the OSI model? OSI model layers explanation, OSI layer protocols, and functions, 7 layers of the OSI model, OSI model layers functions, transport layer in the OSI model, and the physical layer of the OSI model. OSI model is the hypothetical communication model where communication is based on the functionality of each layer.
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