How VoIP Works

Voice over IP, or VoIP, is simply the transfer of voice conversations as data over an IP
network. Unlike traditional circuit-switched calls on the PSTN, in VoIP calls, the telephone
connection is “packet-switched.” In a packet-switched environment, multiple computer
devices share a single data network. They communicate by sending packets of data to one
another, each packet containing addressing information that specifies the source and target
computers. The packets within a single transmission can take different paths from end to
end across a data network.
With a VoIP call, the call setup portion of the calling sequence has to be simulated—dial
tone, ringing, busy signals. The audio portion of the call itself needs to be converted from
analog to digital, cut into packets, sent across the network still in packet format, reassembled,
and converted from digital back to analog. Codecs at either end do the conversion from
analog to digital and back. We’ll explain how they work a bit later.
Here’s what happens when a call is made using VoIP:
1) The caller picks up the telephone handset and hears a dial tone.
2) The caller enters a telephone number, which will be mapped to the IP address of the
callee.
3) Call setup protocols are invoked to locate the callee and send a signal to produce a ring.
4) The destination phone rings, indicating to the callee that a call has arrived.
5) The callee picks up the telephone handset and begins a two-way conversation. The
audio transmission is encoded using a codec and travels over the IP network using a
voice streaming protocol.
6) The conversation ends, call teardown occurs, and billing is performed.