OSI for CCNA Layer 4: Transport
On the CCNA exam, you'll work with hosts. The transport layer manages the transport of data between two hosts communicating over a network. In a nutshell, the transport layer does the following:
Slices up the data to be transmitted into small chunks called data segments that can be easily sent over the network medium.
Reassembles the data in order on the receiving host:
Some transport protocols send the segments in order.
Other protocols, such as Transmission Control Protocol (TCP), send the data segments in any order and reassemble them in the correct order on the receiving host.
Provides some error-detection mechanisms.