OSI Model

Open System Interconnection (OSI) Model

                        Open system interconnection (OSI) model is a logical layout. It defines network communication with one system to another system. The OSI Model was constructed by the International Organization for Standardization (ISO) in 1978. According to OSI ,There are 7 layers to complete a networking process among PC’s.

OSI 7 layers

7.Application layer

6.Presentation layer

5.Session layer

4.Transport layer

3.Network layer

2.Data Link layer

1.Physical layer

Network layers in detail

 

1.Physical layer

                        Physical layer is the first of the OSI 7 layer model. Physical layer concerned with transmission of the unstructured raw bit stream over an optical /electrical, mechanical, and functional interfaces. Those call as physical medium. Basically a PC can understand only machine language it is 0 and 1 (binary values).

                        Physical layer transmit data with the help of cables from a network to other network using electrical characteristics of the signal. The repeater is a device which we can see in physical layer. Repeater is used to regenerate the signal before signal goes down. And also 10BaseT hubs also in physical layer. This type of hub is a multiport repeaters . those devices does not know what contents included in the packets. They just regenerate the signal of the data packets.     

                                                                                                (Lecture note)

2.Data Link layer

                        When physical layer process successfully completed the transmission goes to layer 2 call data link layer. Data link layer provides error free transfer data frames from one node to another node.  If there is an error that is uncorrectable, the data link standard must find a way to inform the PC about the error.

                        Basically networking devices has an address call MAC address . MAC stands for Media Access Control. The MAC address is inserted to devices by manufacturer and it cannot be change by users. there are two main devices in the data link layer. Those are bridge and switch. These devices are intelligent. Those devices recognizes MAC address to forward data packets to particular node. To send data packets from one node to other without interference data link layer using two main ways. Those are CSMA/CD (Carrier sense multiple access/collision detection and token passing.

3.Network layer

                        The 3^rd layer of OSI model is network layer. Network layer’s task is to forward packets including routing using routers from one node to another. A router’s task is to receive packets from one network and forward it to another. Router recognize the destination PC’s IP address. So router can forward the data packets to particular network and the PC. Router has its own IP address too.

                        The use of IP address appears in network layer. Every network has IP address and also we can set up our own IP address too. An advantage of IP address is router can identify the network and can send the data packet to particular network.

4.Transport layer

                        Transport layer is in the fourth place of OSI 7 layers. Basically the process of transport layer is first accepting the data from layer above. And then split the data into small parts and also pass the data units to the network layer. After sending transport layer will monitor whether all units were successfully arrived successfully at the destination.

                                                                                                            (Authors view)

                                    When in a moment that high speed and efficiency is most important than the reliability, then transport layer uses a connectionless protocol. In this type simple the data will send to the end. And it won’t look for any drops of the data packets while transferring.

                                                                                                            (lecture note)

5.Session layer

                        Basically session layer’s job is to create and maintain a session between different networked devices. That means while communicating different nodes over the network the session layer allows users to establish active communication sessions between those nodes.

                                    The session of between nodes is managed by session layer protocol. The session layer synchronizes the conversations between those nodes. In that case the communicators can avoid the data loss between them. After establishing a session the session will maintain until the nodes terminated the session. We can say that session layer’s main target is to establish, maintain and synchronize the interaction between communicating systems. There are three types of transmission types call simplex , half-duplex and full-duplex.

6.Presentation layer

                       

                        Presentation layer is the 6^th layer of OSI model. We know that the data is coming to the presentation layer from above layer. The presentation layer’s duty is to make the data understandable. Basically most PC’s like Windows, Macintosh, use ASCII code to represent data and also some PC’s like IBM uses EBCDIC code to represent data

                                    The presentation layer need to convert data from one code to another. Basically the presentation layer’s task is encode and decode messages. The presentation layer should change the characters and numbers of the information to bit stream. And also presentation layer is responsible for encryption and decryption. In that case there are three main functions in presentation layer. Those are Translation, Encryption, and Compression

7.Application layer

                        Application layer is the top layer of the OSI 7 layer model. In this layer data will be manipulate in various ways. And also enables user or software to get access to the network. In this layer a protocol call HTTP (Hypertext Transfer Protocol) is used. And also File Transfer Protocol (FTP), Simple Mail Transfer Protocol(SMTP) , Trivial File Transfer Protocol(TFTP), Domain Name System(DNS) are in application layer.

                                    Application layer is end of data transfer , this layer is responsible for error handling and recovery. The application layer need to be provide data which can understand by users.