of IP gateways to ATM networks and, last but not least, for the control of ATM
circuit switches for voice. MEGACO and H.248 are, thus, considerably more
complex than MGCP, without offering any more functionality.
All of the preceding protocols have one feature in common: They are mas-
ter/slave protocols, where an “intelligent” central master controls every action
in detail of the “dumb” slave devices, such as media gateways, media servers,
and slave telephones.
The gateway controller is also sometimes called a “softswitch.” Various
designs have started out with the model in Figure 19.1a and have added pro-
prietary APIs for third-party developers to add new services and also APIs to
control the MG itself, as shown in Figure 19.1b. Since each system has its own
APIs, third-party developers would have to learn all the APIs for all the vari-
ous proprietary designs. Full-featured multivendor interoperability between
the MG and GC is more difficult to achieve, the more APIs there are. Complete
interoperability has not been accomplished in the industry, to our knowledge,
as of this writing, and there are companies that have found a niche in writing
code to for GCs to interoperate with various MGs.
As the number of required services increases, the need for separate service
platforms becomes evident. Figure 19.1b shows the decomposition of the ser-
vice platform between a service controller and media servers using one of the
previous master/slave protocols. This decomposition has, however, the well-
known drawbacks of central control, such as the following:
■■ Single point of failure (if there is only one geographic location).
■■ Proprietary service logic.
■■ Heavy control traffic between master and slaves leads to very lengthy
and complex call flows.
■■ Details in implementations by vendors and APIs make interoperability
unlikely.
■■ Bundled services inhibit third-party application providers.
■■ New services are difficult to introduce because of tight coupling of
features.
■■ Integration with Web, e-mail services, presence, and IM is very difficult.We believe the last item to be the most restrictive for the architecture shown
in Figure 19.1b for the master slave approach.
The decomposition using master/slave protocols (such as MGCP or
MEGACO/H.248) has constraints for service providers. Figure 19.1c shows a
network composed of IP telephony gateways used to bypass the PSTN long-
distance and international networks, or to avoid PSTN trunking for PBXs in
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