Computer networks uses different port in order to provides different services and protocols. The port 20 is a popular port which is used by the FTP as data transfer. The TCP and UDP port 20 is dedicated for the FTP protocol data transfer. The FTP protocol uses a pair of connections to manage and transmit data. The FTP protocol uses the port 21 in order to start communcation, authentication and starting transmission but the data is transmitted over the port 20.
Port 20 (Data Channel)
As stated previously the FTP data is transferred over the port number 20. So the port 20 is also named as the data channel as the data is transferred over this port. The port 20 is both used to send data which is called as source port and receive data which is called as destination port on both FRP server and FTP client.