New features in the slave devices are useless unless they are also supported
by the master. This limits the ability to rapidly develop new services and fea-
tures, since extensions to the package must be defined. In comparison to SIP,
new headers to implement new services and features can be implemented by
endpoints only, in most cases, without the knowledge or support in the SIP
network. SIP system designers can choose to put only the absolutely minimum
required features in SIP servers: user registration and the proxy function.
Master/slave protocols, such as MEGACO, only succeed in reducing infra-
structure costs if the simplification of the extremely “dumb” terminals offsets
the increased costs of additional protocols and of new intelligent network ele-
ments. However, when a requirement is added to the “dumb” terminals to be
able to act autonomously under certain circumstances (for example, complete
an E911 call when no controller is available), most of the assumed lower-cost
benefits of a master/slave protocol will be lost.IP Telephony Gateways
The earliest implementers of IP telephony gateways used monolithic and
highly proprietary approaches for auxiliary functions such as tone announce-
ments and IVR functions or, for example, for credit card number input. Small
gateways can be built using application programming interfaces (APIs),
depending on the particular product and operating system. However, such
monolithic designs proved to be undesirable for both vendors and service
providers, because they tried to scale the systems in size and across the net-
work, and to add various new services.
The abundance of services and features in the competitive marketplace led
service providers to search for unbundled systems, so as to benefit from prod-
ucts by multiple vendors, specialized to be the best of the breed.
A first attempt to provide unbundled IP telephony gateways was the
decomposition of the gateway into a gateway controller (GC) and one or more
media gateways (MG), as shown in Figure 19.1a.
The initial MEGACO protocol has been made obsolete in the IETF and
replaced by the Gateway Control Protocol is described in [2]. The link between
the GC and the MG has undergone numerous developments, starting with
APIs and later giving birth to protocols with names such as IP Domain Control
(IPDC), Simple Gateway Control Protocol (SGCP), MGCP, MEGACO, and
H.248. Initially, the de facto industry standard was the MGCP. The IETF and the
ITU have coordinated the development of the protocol, called MEGACO in the
IETF and H.248 in the ITU. These standards were developed with some broader
aims, such as to accommodate both SIP and H.323, and to be used for the control320 Chapter 19