OSI Model - 2

Open System Interconnection 

(OSI) Model - 2

Summary on what we have studied on last tutorial :-

• Open System Interconnection (OSI) Model was developed by the International Standards Organization (ISO) describes the flow of information from one computer to another.

•  All the layers of the OSI reference model use different protocols.

•  Physical layer is the first or the bottom most layer of the OSI model. This layer is used to establish or terminate a connection to a communication medium. It also defines the electrical and mechanical specifications like cables, connectors and signalling options of the medium.

• Data Link Layer is second layer of the OSI model. It handles data transfer between network and the physical layers.

•  Data a link layer receives the Data from the network layer. It adds Header and Trailer of the data and passes data to the physical layer.

(1.3) Network Layer :-

      This is third layer of the OSI Model. The data unit at this layer is known as packet. There is no need of the network layer if two communicating devices lie on the same network. However, when the two devices are connected on the different networks, network layer is essential for providing delivery of the packets.

      Functioning of the network layer is shown in the figure below :

Functioning of Network layer

      Network layer receives data from transport layer. It adds header to the data and passes data to the data link layer. At the receiver side, network layer receives data from data link layer. It detaches header from data to the transport layer.

      Responsibilities of Network layer are :

• Logical addressing : The data link layer provides physical addressing which is useful for a local network. When the packet is destined from a device outside a network, we require other addressing scheme to identify source and destination. Network layer adds header to the data that includes the logical address ( IP address ) of source and destination. It is 32-bit address that uniquely identifies the device connected to a network.
• Routing : Routing is a process wherein a proper path is defined for the packets to reach destination. Routing can be of two types static or dynamic. In static routing, the route to be followed by a packet id already set by the network administrator. In dynamic routing, the route to be followed by a packet is decided at the time of Transmission of packet The route can be changed based on the available traffic of the network.
• Internetworking :  Internetworking means connecting two or more computer networks together. The internet is best example of Internetworking. There are different types of networks that exist in real world such as LAN, MAN and WAN. These networks are interconnected using various networking devices such as bridges, Routers and Gateways.

(1.4) Transport Layer :-

      This is the fourth layer of OSI model. The data unit at this layer is called Segment. This network layer deals with source-to-destination delivery of individual packets and does not recognize any relationship between them. On the other hand the transport layer ensures that the entire message reaches in order and handles error control and flow control at the source-to-destination level.

   

      Functioning of Transport layer is shown the figure down below :

Transport layer

      Transport layer receives data from the session layer. It divides the data into segment with each segment having its own header and passes these segments to the network layer. At the receiver side, transport layer receives packets from the network layer. It detaches headers from them and then reassembles the segments into original message and passes the message into session layer.

      Responsibilities of the Transport layer :

• Service-Point Addressing : There may be number of program running at the same time on a computer. For this purpose source-to-destination delivery should also include process-to-process delivery. Data must transferred from a specific process (running program) on another computer to a specific and to accomplish this, port address is requires. Thus, transport layer includes service-point address (also referred to as port address) in the header. Using these port addresses transport layer delivers the packets to the correct process.
• Segmentation and Reassembly : A message is divided into segments with each segment having a sequence number. When the message arrives at the destination, with the help of these sequence numbers, the transport layer reassembles the message correctly. Sequence numbers also enable transport layer to determine and replace the segments that were lost in the transmission.
• Connection Control :  Transport layer provides either connection-oriented or connection-less service. In connection-oriented service, transport layer establishes a connection with the transport layer at the destination before transmitting segments and terminates the connection when all the data is transferred. In connection-less service, transport layer treats each segment as individual packet and transmits it to the transport layer at destination.
• Error Control :  Transport layer also performs error checking. Error control takes place end to end. Transport layer at the sending machine ensures that the entire message is delivered at the receiving machine without an error. Segments are re-transmitted to achieve error correction.

Read: OSI model - 1

Read: OSI Model - 3

=================================================

Thanks for reading !

Have a GOOD DAY !