Originating on the
This chapter discusses the first of two crossover services; the events for
this type of service originate in the Internet, but the service itself resides
in and is executed on the public switched telephone network (PSTN).
The intrinsic value of a computer network is measured by the services it
provides to its users. As the number of such networks increases, so do
the chances that services residing on one network will need to be accessed
by users on a different network. As discussed in previous chapters, the
two networks that make up the communication network — PSTN and the
Internet — are converging, which necessitates access to services residing
in one of these networks from the other.
This cross-service access poses a number of problems to be solved.
To formulate the problem set consistently, we will define some terms first.
We call the network on which the service runs natively the local network
(or local domain); alternatively, a foreign network (or foreign domain) is
one from which a request to execute the service is made. The service
and its associated data reside on the local network.
© 2007 by Taylor & Francis Group, LLC
64 Architecting the Telecommunication Evolution
The first problem in accessing services from a foreign domain is that
of differing network protocols. The PSTN and Internet use dissimilar
network protocols and, in fact, are designed with different goals. While
the PSTN is a highly tuned network to transport voice, the Internet is a
generalized network that can transport any type of payload — voice, video,
or data (text). Second, a request for service arriving from a foreign domain
will start service execution in the local domain; as such, the entities in the
local and foreign domains need to be synchronized. Third, when a service
is accessed from a foreign domain, the semantics of the service must be