OSI Model | OSI 7 Layer Model

What is OSI Model?

The OSI stands for Open Source Interconnection. The OSI model has seven different layers. It is a reference model for how applications communicate over a network.

Each layer of OSI defines a family of functions distinct from those of the other layers. By defining and localizing functionality in this fashion, the designers created an architecture that is both comprehensive and flexible.

The purpose of the OSI (Open Source Interconnection) reference model is to guide vendors and developers so the digital communication products and software programs they create can interoperate, and to facilitate a clear framework that describes the functions of a networking or telecommunication system.

OSI 7 Layer Model:

There are 7 layers of OSI that facilitate a different functions of a networking.

1. Physical Layer:

The Physical Layer coordinates the functions required to transmit a bit stream over a physical medium from one system to another. 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 main functions of the physical layer:

1. Representation of bits:

The physical layer of OSI consists of a stream of bits (sequence of Os or 1s). These bits must be encoded into signals–electrical or optical. The physical layer of OSI defines the type of encoding (how Os and 1s are changed to signals).

2. Data rate:

The data transmission rate of the system i.e. the number of bits sent each second-is also defined by the physical layer.

3. Synchronization of bits:

The physical layer also synchronize the data rates. The sender and receiver of the message not only must use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks of data transmission rate must be synchronized.

4. Physical topology:

The physical topology defines how devices are connected to make a network. Devices can be connected by using a mesh topology, a star topology, a ring topology, a bus topology, or a hybrid topology.

5. Transmission mode:

The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex.

2. Data Link Layer:

The Data link layer is responsible for node-to-node delivery of the message. It makes the physical layer appear error-free to the upper layer i.e. Network layer.

The Data Link Layer Responsibilities:

1. Framing:

The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

2. Physical addressing:

If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender’s network, the receiver address is the address of the device that connects the network to the next one.

3. Flow control:

If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

4. Error control:

The data link layer adds reliability to the physical layer by adding mechanisms to detect and re-transmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.

5. Access control:

When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

3.Network Layer:

The network layer is responsible for the delivery of individual packets from the source host to the destination host possibly across multiple networks (links).

Whereas the data link layer oversees the delivery of the between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination. If two systems are connected to the same link, there is usually no need for a network layer.

The Network Layer Responsibilities:

1. Logical addressing:

The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems.

The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.

2. Routing:

When independent networks or links are connected to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism.

4.Transport Layer:

The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on a host. Whereas the network layer oversees source-to-destination delivery of individual packets, it does not recognize any relationship between those packets.

It treats each one independently, as though each piece belonged to a separate message, whether or not it does. 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.

The Transport Layer Responsibilities:

1. Service-point addressing:

Computers often run several programs at the same time. For 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.

The transport layer header must, therefore, include a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

2. Segmentation and reassembly:

A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

3. Connection control:

The transport layer can be either connectionless or connection-oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine.

A connection-oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.

4. Flow control:

Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.

5. Error control:

Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than across a single link: The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). Error correction is usually achieved through retransmission

5.Session Layer:

The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes. The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.

The session layer responsibilities:

1. Dialog control:

The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either half-duplex (one way at a time) or full-duplex (two ways at a time) mode.

2. Synchronization:

The session layer allows a process to add checkpoints, or synchronization points, to a stream of data. For example, if a system is sending a file of 2000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently.

In this case, if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 523. Pages previous to 501 need not be resent.

6.Presentation Layer:

The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. The following Figure shows the relationship between the presentation layer and the application and session layers.

The Presentation Layer Responsibilities:

1. Translation:

The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information must be changed to bitstreams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods.

The presentation layer at the sender changes the information from its sender-dependent format into a format. The presentation layer at the receiving machine the common format into its receiver-dependent format.

2. Encryption:

To carry sensitive information, a system must be able to ensure privacy. Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.

3. Compression:

Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, Audio, and video.

7.Application Layer:

The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access, and transfer, shared database management, and other types of distributed information services.

Where many application services available, the figure shows only three:

XAOO (message-handling services), X.500 (directory services), and file transfer, access, and management (FTAM). The user in this example employs XAOO to send an e-mail message.

The Application Layer Services:

1. Network virtual terminal:

A network virtual terminal is a software version of a physical terminal, and it allows a user to log on to a remote host. To do so, the application creates a software emulation of a terminal at the remote host.

The user’s computer talks to the software terminal which, in turn, talks to the host, and vice versa. The remote host believes it is communicating with one of its own terminals and allows the user to log on.

2. File transfer, access, and management:

This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local the, and to manage or control files in a remote computer locally.

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