enterprise networks. The GC is controlled by a call agent (CA), where the ser-
vice logic resides, and has access to the necessary database to control call
setup.
The central control, the proprietary controller, and the control protocol now
have produced a network that is neither the TDM network nor an IP network,
but, in effect, a third type of some proprietary network hybrid—the ones that
are the most difficult to operate. This new network can provide voice services
only. Service providers have enough work cut out for them to manage existing
circuit switched networks and the IP network, and need not trouble them-
selves with the managing a third type of network. Such a PSTN or PBX bypass
network cannot support any services that do not exist already on the circuit
switched side, thus taking away the main rationale for such third, new net-
works to operate.
Central control of distributed media gateways, as shown in Figure 19.1c,
may be useful, however, in such cases where many smaller IP telephony gate-
ways from an ISP have to interface with the PSTN using Signaling System 7
(SS7) signaling. Since SS7 interconnection points are quite expensive, and no
other services than voice are possible anyway over the PSTN side of the call, a
central controller combined with an SS7 interconnect point makes good sense.
However, ISPs have to be careful not to have any service features provided by
the central controller, since such services would be difficult to extend across
the rest of the IP network, where multivendor compatibility will be required.
This example is an exception to the rule, in our opinion, to avoid central con-
trol-type IP telephony gateway networks.
The residential gateway (RG) shown in Figure 19.1d is another example of
the use of master/slave protocols such as MGCP and MEGACO/H.248. This
time, it is the end user who is deprived of three main benefits available on the
Internet:
■■ Free choice of any other service, as is the case on the Web
■■ Free choice of any communication application, since all applications
reside in the central office of the service provider
■■ Telephony that is integrated with other services (telephony is segre-
gated from all other Internet applications)Residential gateways for voice as shown in Figure 19.1d are negating the
requirements for equal access to service providers, since competing service
providers cannot have access to control the phones or IP-phone adapters
behind the residential gateway.
We will show by contrast how these problems can be avoided by using an
Internet and Web-centric architecture for the application environment.322 Chapter 19