Over the past decade, the telecommunications industry has witnessed rapid
changes in the way people and organizations communicate. Many of these changes
come from the explosive growth of the Internet and from applications based on
the IP. The Internet has become a popular means of communication, and the total
amount of packet-based network traffic has quickly surpassed traditional voice,
or circuit-switched, network traffic.
Voice traffic and services is
expected to become one of the next major application areas to take full
advantage of IP. This expectation is based on the impact of VoIP technologies,
which are sometimes referred to as IP telephony. Figure
illustrates some
of the ways that VoIP can be used.
VoIP offers many benefits, which
include the following:
-
Cost savings – By moving voice traffic to IP networks, companies can
reduce or eliminate the toll charges associated with transporting calls over
the Public Switched Telephone Network (PSTN). Service providers and end users
can also conserve bandwidth by investing in additional capacity only when it is
needed. This is made possible by the distributed nature of VoIP and by reduced
operations costs as companies combine voice and data traffic onto one network.
-
Open standards – By adopting open standards, multi-vendor
interoperability is ensured. Both businesses and service providers can purchase
equipment from multiple vendors and eliminate their dependency on proprietary
solutions.
-
Integrated voice and data networks – When voice becomes another IP
application, companies can build truly integrated networks for voice and data.
These integrated networks provide the same quality and reliability as the PSTN,
while enabling companies to quickly and flexibly take advantage of new
opportunities within the changing world of communications.
In 1995, the first commercial VoIP products began to emerge in the
market. These products were targeted at companies that wanted to reduce their
telecommunications expenses by moving voice traffic to packet networks. Early
adopters of VoIP networks built toll-bypass solutions to take advantage of the
favorable regulatory treatment of IP traffic. Without any established
standards, most early implementations were based on proprietary technology.
As these packet telephony networks grew and interconnection dependencies
emerged, it became clear that the industry needed standard VoIP protocols.
There are four different standardized signaling and call-control protocols used
for VoIP:
- H.323
- Media Gateway Control Protocol (MGCP)
- Session Initiation Protocol (SIP)
- H.248/Megaco
Other protocols that work with these signaling and control protocols
include Real-time Transport Protocol (RTP), Real-time Transport Control
Protocol (RTCP), and Resource Reservation Protocol (RSVP).
Each of these
protocols will be discussed in the following sections.