A. The open system interconnection was developed by international standard organization in nineteen eighty four it was designed for a specific purpose to allow a channel of communication between two computers using some set of rules and regulation. The developers divide the function of networking into layers as these decrease complexity. The open system interconnection is made out of seven layers and every layer provide a set of service specific (header) to the upper layers. Therefore when communication took place every specific layer only communicated to each other added its set of rules and passes the data to the upper layer. The lower four layers (Physical, Datalink, Network and Transport) focus on the flow of data from end to end through a network whereas the upper layers (Application, Presentation and Session) are set more towards service and application.
Some of the benefits of using layered model is listed below:
• Simplifies the network architecture as every layer has its set of task does what it is required and forwards the data to the upper layer.
• Easy to debug the network application as it is in layers.
• The layer sets a standard of communication between the hosts.
• Determines essential feature for data communication between devices.
I. The Transport layer is responsible for message delivery it makes sure that the information is error free and is in an order with no loss or duplication between two processes in different host. As the message data is sent it’s broken down into segments and rearranges when is delivered to the host it also provides a message acknowledgment. Controls traffic by messaging if a message is sent by another host when there is no data buffer it notifies the host to wait host is not available. It is assigned with port address such as SMTP (25) and POP3 (110) so that the message is delivered at the right end incoming and outgoing once the message is sent it ends the connection.
II. The Network layer receives frames for lower layers and converts it to packets.in this layer ip address is assigned