For other uses, see RF (disambiguation) . Radio frequency (RF): any of the electromagnetic wave frequencies that lie in the range extending from around 7003300000000000000â 3 kHz to 7011300000000000000â 300 GHz, which include those frequencies used for communications or radar signals.owever, since the antenna will pick up thousands of radio signals at a time, a radio tuner is necessary to tune into a particular frequency (or frequency range). This is typically done via a resonator â in its simplest form, a circuit with a capacitor and an inductor form a tuned circuit.
How the TCP Port 21 Works With FTP. The File Transfer Protocol Uses Port 20 and 21. The File Transfer Protocol (FTP) provides a means to transfer information online, much like how the Hypertext Transfer Protocol (HTTP) is used through a web browser. FTP, however, operates on two different TCP ports: 20 and 21.
FTP is an underling of TCP and shuttles files back and forth between FTP server and FTP client. Because FTP requires that two ports be open--the server's and the client's--it facilitates the exchange of large files of information. First, you as client make a TCP control connection to the FTP server's port 21 which will remain open during the transfer process. In response, the FTP server opens a second connection that is the data connection from the server's port 20 to your computer.