A protocol is a set of standardized rules that communication devices follow when exchanging data. Just as human conversation has implicit rules (greet, state purpose, say goodbye), machine communication needs defined agreements on data format and response procedures.
Key telephony protocols include SIP (Session Initiation Protocol), which controls VoIP call setup, answering, and termination. RTP (Real-time Transport Protocol) handles actual voice data transmission. SIP and RTP together enable IP telephony.
TCP/IP (Transmission Control Protocol / Internet Protocol) underpins all internet communication. TCP ensures reliable data delivery while IP manages addressing. HTTPS, SMTP, and FTP are all built on TCP/IP.
For security, TLS (Transport Layer Security) provides communication encryption. The "S" in HTTPS means TLS encryption protecting web traffic. VoIP call encryption uses SRTP (Secure RTP). See VoIP basics for protocol operations in practice.