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.
Basically, there are the following file transfer protocols around: FTP â the plain old FTP protocol that has been around since 1970s. The acronym stands for File Transfer Protocol. It usually runs over TCP port 21. SFTP â another, completely different file transfer protocol that has nothing to do with FTP. SFTP runs over an SSH session, usually on TCP port 22. It has been around since late 1990s.