Internet protocol (IP) is being touted everywhere as today's unifying
technology. Poor Signaling System 7 (SS7) languishes in contrast. If you
walk through a public network convergence tradeshow you might not even hear
SS7 mentioned. It would be easy to conclude that SS7 is boring and out of
the picture -- but it's not. In fact, SS7 has a huge role to play in IP
telephony. This established and evolved technology will enable the public
switched telephone network (PSTN), wireless networks, and IP networks to
INTEGRATING MULTIPLE NETWORKS
The traditional PSTN isn't going away just because IP telephony is a
cool new technology. The PSTN exists, it works, and it represents a
tremendous investment. Convergence may be the latest buzz word, but the new
IP networks will actually interoperate with the PSTN -- giving us multiple
networks to manage as we roll out new services. SS7 has evolved from its
simple original purpose, requesting voice circuits between two offices.
Today it is a primary unifying technology and enabler for enhanced IP
services. Wireless services also depend upon SS7 for most of their
In the changing network, the Internet dominates because of the rich
content it provides. But while the content is rich, the IP network's
delivery -- or lack of delivery -- for data, video, and voice applications
clearly shows it is missing something as a reliable communications
technology. Conversely, reliability and quality of service are the hallmarks
of the traditional PSTN -- at least in most parts of the world. The simple
content this reliable network delivers is voice, along with fax and slow
Integrating these worlds opens up a whole new set of opportunities,
allowing the industry to deliver innovative end-user services based on the
IP infrastructure. It also makes it possible to leverage the huge installed
based of traditional telephony users and services and the growing wireless
The challenge is to integrate three separate paradigms -- one based on
delivering rich content through an unreliable network, one based on
delivering simple content through a reliable network, and one based on the
ability to roam freely without wires -- while communicating with everything
else. SS7 will enable this integration.
HOW DID SS7 BECOME SO IMPORTANT?
Originally designed to carry end-to-end messaging to set up switched
circuit connections, SS7 largely drives the circuit-switched PSTN world. SS7
has evolved to include capabilities that make the network increasingly
- The ability to access remote databases to support toll-free or free
- Management, location identification, and call delivery through
- Number portability; and
- Caller ID display.
SS7 has defined the intelligent network architecture, providing the
ability for distributed nodes to have specific functionality that enables
enhanced, intelligent services. It is not without problems, notably the fact
that there are many different variants in different countries and regions.
At the same time, the SS7 software has continued to add capabilities that
allow more intelligent and wireless networks to rely on SS7 for both basic
and enhanced functionality.
To build on these existing services, and to leverage today's huge
infrastructure investment, we need a method for seamless communication
between the PSTN and IP worlds. By allowing SS7 traffic to be carried over
IP networks, it's possible to extend today's architecture and services
without having to replace existing equipment. Providers can host existing
telephony applications in the IP world -- including routing voice calls
through IP networks. This opens up the possibility of better tariffs to
existing circuit-switched networks.
Also, in the IP world it's possible to host the databases required for
wireless operation or intelligent networking services like number
portability. This enables a gradual integration of the two technologies and,
ultimately, service sets. IP also adds the capability of back-hauling
signaling traffic for processing far away from the home network. Services
can be operated remote from the end user. For example, a remote network
without such a capability can use spare capacity on an existing local number
portability (LNP) database.
WHERE DO WE GO FROM HERE?
In recent years there have been numerous proposals for conveying SS7
information through IP networks. These generally replace the lower SS7 layer
(message transport layer) with an IP transport. The most successful and
widely adopted standard is being defined by the Internet engineering task
force (IETF) signaling transport (Sigtran) working group. The IETF has
defined an end�to-end protocol known as stream transmission control
protocol (SCTP), which exists within the IP protocol stack alongside
transmission control protocol (TCP) and user datagram protocol (UDP). SCTP
provides reliable, non-blocking communication between two IP end points.
This is modified by adaptation layers to provide an SS7 protocol-like
interface, allowing the higher SS7 protocol layers to seamlessly
intercommunicate through an underlying IP backbone.
Using such protocols enables different vendors to offer standard
equipment for deploying converged SS7 and IP networks, like SS7-to-IP
routers or gateways. These -- when combined with media gateways that
packetize the TDM voice and media gateway controllers that provide a
softswitch function -- can be used to deploy voice-over-IP solutions built
from different suppliers' components. It's also possible to deploy a public
network-type system with a lower initial investment -- ideal for areas with
low population or for small-scale deployments such as a corporate private
wireless network based on public network principles or a competitive local
exchange carrier (CLEC).
Access to SS7 network resources enables emerging IP-based services to
route to existing wireline or wireless users. Wireline subscribers can be
managed through local number portability (LNP) databases containing
subscriber information such as text strings for caller display. Wireless
users are connected through either Interim Standard 41 (IS-41) or Global
System for Mobile Communications (GSM) networks, both based on SS7.
Connecting to these resources gives IP service providers access to
information on subscribers' locations and the ability to contact them by
voice or text messaging, whether the subscribers are in a fixed location or
on the move. From the user's perspective, IP integration promises greater
flexibility and control, plus the possibility of programming a personal
subscriber profile to route calls to home, a mobile phone, or the office
through a Web interface.
UNIFYING THE WORLD
SS7 is the enabler that will allow IP and PSTN service providers to
unify the PSTN, wireless, and IP networks. The huge installed base of PSTN
customers will be able to use IP-based enhanced services. The growing base
of IP network customers will be able to access enhanced PSTN services.
Wireless customers will be able to communicate with everyone -- and,
someday, to roam the globe. In fact, SS7 will enable competitive tariffing,
low-cost service rollouts, and communication between disparate networks. In
short, it will unify the world.
Jim Machi is director, product management, CT Server and IPT Products,
for Dialogic Corporation (an Intel
company). Dialogic is a leading manufacturer of high-performance,
standards-based computer telephony components. Dialogic products are used in
fax, data, voice recognition, speech synthesis, and call center management
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