How The Reliable Transmission Of Data Is Ensured In OSI Model?

OSI (Open Systems Interconnection) model is the model for designing network architecture. A 7 layer OSI objective is to separate the work for each from each other.

In designing the packet based network, one must be careful that reliability is not affected. For this purpose OSI carefully designed its layers and the function of transmitting data reliably is given to the fourth layer which is called the transport layer.

In transport layer, TCP (Transmission Control Protocol) works to provide the reliable data transmission. To be able to do that, the TCP first establishes a connection using a 3 way handshake method. Once the connection is established, the data is transmitted. But is it enough for providing a reliable data transmission? Probably not!

TCP provides the function of retransmission if the data is lost while being transmitted. For doing this an acknowledgment receipt is required before the sender can send more data. Since the data is sent using packets using different routing paths, the TCP header is added with a segment number which can help the packers to be assembled in order once they are received at the receiving end.

TCP also provides checksum technique and congestion control to reduce errors. All these techniques help to reduce errors and increase the reliability.

The fourth layer of the OSI (open system interconnection) model is responsible for making the end to end data transmission reliable. At the sending end of network communication, transport layer takes the data from session layer and transport it to the network layer.
It splits that data into smaller pieces if data size is too large to be carried over the transmission media. This layer ensures that the data is efficiently and correctly reach its destination.

The services provided by this layer are connection oriented; it means that the data sent by this layer must be acknowledged by the destination device. These data acknowledgments are used to ensure that the data is received correctly by the destination device.
The transport layer adds the message headers and control messages to the session data that is sent to the destination. These headers and control messages help to detect the errors in the data transmission. These are also used to control the data flow and sequence of packets.

The common connection services provided by this transport layer are segment sequencing, flow control and error control. As the packets routed to the destination may arrive in different orders. Thus it is objective of transport layer to re-sequence these packets in correct order.