TCP/IP Tutorial and Technical Overview - IBM Redbooks

20.1 Voice over IP (VoIP) introduction
As the number of services implemented over IP-based networks continues to
grow, the inclusion of voice communication services in this progression is only
natural. This convergence manifests itself through Voice over Internet Protocol
(VoIP). In traditional implementations, communication services occur across the
circuit protocols of the public switched telephone network (PSTN). Conversely,
VoIP works by encoding voice information into a digital format, which can be
carried across IP networks in discrete packets. Additional distinctions between
VoIP and PSTN exist and we discuss these side-by-side.
By nature, analog voice signals are perceived as a sequence of sound vibrations.
Because vibrations cannot be directly represented in a digital format, modern
telecommunications must replicate voice signals by sampling human sound.
These samples are then encoded in digits to replicate the sample’s sounds. This
digitized voice signal can then be carried by a transport network, using either a
carrier network or a public internet.
Traditional telephone networks allocate dedicated circuits for voice
communications. This occurs whenever someone picks up the phone and dials
someone. Because the circuits are dedicated to the user, the quality of the voice
signals can be guaranteed. However, for this same reason, the connection’s
potential bandwidth is wasted whenever there are periods of silence during the
communication.
This wasted bandwidth does not occur when using VoIP, because packet
networks allow sharing of transport facilities. However, the guarantee of service
is lost as well. To counteract this, many carriers are in the process of
transforming their network into IP packet-switching facilities to increase
efficiency, while retaining control of the overall network resources. This ensures
that transport resources will be available during a phone call.
Note: Sending voice in packet format through IP network is only one of many
steps in making Voice over IP a possibility. There are additional functions
necessary in setting up a call (signaling) when converting from an IP network
to a telephone network (gateways).
20.1.1 Benefits and applications
The key drivers in VoIP are cost benefits, because cost savings are the primary
short-term reason to converge voice, data, and video onto a single IP network
(with everything over same infrastructure). Corporations have deployed
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