The Intelligent Network (IN),

The model drawn up in the 1980s and 1990s for advanced network functionality is
called the Intelligent Network (IN). Services such as 8XX-number lookups as well
as Calling Cards, Private Integrated Services Network (PISNs), and many other
advanced services are all made possible through SS7, ISDN, and IN capabilities.
PISNs are geographically disparate networks that are connected via leased lines that
allow for enhanced services such as multivendor PBX deployments,Voice VPNs
(don’t get these confused with data VPNs, they are a true private network for voice
just like that provided by a PBX), and even certain kinds of VoIP. A Private
Integrated service Network Exchange (PINX) lives within a PISN. Another application
is integration with the QSIG protocol, which allows PBX products from other
vendors be able to be used transparently to integrate all voice networks.
QSIG (a Q.931 ISDN extension) as a protocol has been around since the early
to mid 1990s.We will talk about ISDN in the next section, but QSIG can be used
to integrate systems even without ISDN. QSIG also leverages DPNSS, which was
developed prior to when the final QSIG protocol was agreed upon. Not used much
in U.S. networks, DPNSS had much of its life in the United Kingdom. Modern networks
are using QSIG as the means to interconnect voice channels between PBXs
while preserving critical information about caller and call state in the process.
ISDN is a common-channel signaling (CCS) solution that works with media or
data traveling down one pair of wires while signaling control is handled over
another. Remembering back to our earlier discussions of the channels of 64 kbps in
size, a typical ISDN will hold 23 bearer (B) channels that carry voice and data and
one data (D) channel that carries signaling information. All channels are 64kbps, so
we have 24, 64-kbps channels totaling 1536 Mbps, or equivalent to a T1 and 30 B
channels plus a D channel on an E-1, but in each case we lose one channel for signaling.
Not only was distance from the central office a new issue with ISDN trunks,
but the customer also had to implement new equipment.This Customer Premise
Equipment (CPE) required ISDN terminators in order to access the network.Today
the use of ISDN in the provisioning and delivery of broadband Internet access via
DSL and cable services keep pricing competitive and affordable. Besides its use in
the DSL services, ISDN still has an active share in providing redundant and emergency
data network access to critical servers and services when higher speed lines or
primary access has been disrupted.
Over the last 100 years, signaling has moved from operator-assisted modes to
loop and disconnect modes, from single frequency to multifrequency signaling, and
now to common channel signaling using the ISDN signaling channel.