Understanding VoIP Protocols

Application programs build their own families of “higher-layer” protocols on top of the
lower-layer protocols they use for transport and other tasks. Implementing a VoIP telephone
call on a data network involves the call setup—that is, the VoIP equivalent of getting
a dial tone, dialing a phone number, getting a ring or a busy signal at the far end, and picking
up the phone to answer the call—and then the telephone conversation itself. VoIP
protocols are required during both phases:
Several higher-layer protocols can accomplish call setup and takedown, including H.323,
SIP, MGCP, and Megaco. The programs that implement the call setup protocol use
TCP and UDP to encapsulate the data exchanged during the call setup and takedown
phases.
The exchange of actual encoded voice data occurs after the call setup (and before the
call takedown), using two data flows—one in each direction—to let both participants
speak at the same time. Each of these two data flows uses a higher-layer protocol called
Real-time Transport Protocol (RTP), which is encapsulated in UDP as it travels over the
wire.