This layer is responsible for the end-to-end communications between two decives.
This layer also ensures flow and error control. This means it makes sure someone with a
fast connection does not overwhelm the reciever and ensuring the data recieved is complete. In this layer the data of the session layer is divided into smaller pieces: segments.

The transport layer controls the reliability of communications with segmentation, flow control and error control.

In segmentation the data received from the session layer is divided into small data units called segments. Each segments contains a source and destination port number and a sequence number. The port number helps to direct each segment towards the right application. The sequence number helps to reassemble segments in the correct order.

In flow control the transport layer controls the amount of data that is transmitted. For example, take a mobile that is connected to a server. The server might be able to transmit data at 100Mbps while the mobile can only process at 10Mbps. If the mobile is downloading a file from the server it can with the help of the transport layer tell the server to slow down data transmission to 10Mbps so that no data gets lost. Similarly, if the server is sending data at 5Mbps the mobile tells the server to increase the data transmission to 10Mbps.

In error control, if some data does not arrive at the destination the transport layer uses the Automatic Repeat Request schemes (ARQ) to retransfer the lost or corrupted data. In order to do so, a group of bits called checksum is added to each segment of the transport layer to identify received corrupted segments.

List of protocols running in the presentation layer

  • TCP - Transmission Control Protocol
  • UDP - User Datagram Protocol

The transport layer provides two different services:

  • Connection Oriented Transmission
  • Connectionless Transmission

The former is done via TCP, while the latter is performed with UDP.
UDP is faster than TCP because it does not provide any feedback if data is lost. TCP on the other side, provides feedback for lost data so that it can be retransmitted. A practical scenario of UDP is when streaming a movie for example, here it does not matter if data gets lost and speed of transmission is preferred. TCP is used for reliable tranmission of data such as in email. Here it is required to receive the full data in correct order.

Exam Question

Transport layer provides ______ communication

  • A. End to End
  • B. Host to Host
  • C. Process
  • D. Both: A & B

The correct answer is D.

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