The purpose of signaling in NGN is twofold. The first one
is to reserve required communication resources and to establish
an end-to-end connection between a pair of hosts in the system.
This signaling is performed at the network layer in so-called
transport stratum. Second type of signaling is performed at the
application layer (service stratum). Service stratum signaling is
in general an application specific signaling (e.g. SIP signaling)
the aim of which is to configure distributed modules of an
application and to process information necessary to send to
transport stratum a request for communication resources
reservation. Signaling can be also viewed as a middleware
which separates the networking layer functionalities and
application domain-specific functionalities. Additionally, such
a decoupling of transport and application layers allows to treat
communication services provided by the network as building
blocks for composition of personalized applications according
to the SOA paradigm.