PSTN Architecture

Introduction
In 1876, Alexander Graham Bell patented the telephone and envisioned telephony’s
eventual triumph over the dominant communications network of his day: the telegraph
network. Over the past decade, similar pronouncements have been made
about VoIP and the Public Switched Telephone Network (PSTN) as IP-based communication
becomes more pervasive. In both cases, the overall prediction has proven
correct, even if the path for each was far more gradual and the result more integrated
than originally anticipated. Case in point:Western Union (as a unit of First
Data Corp.) did not discontinue its telegram service in the United States until
January 27, 2006, even as numerous phone-to-telegram and web-to-telegram gateways
continue to operate in conjunction with telegraph, cable, telex, and radio messaging
networks worldwide.
With that in mind, it’s essential to include the PSTN and its associated risks
when examining VoIP security. Don’t forget that today’s Internet hacking community
can trace its roots directly to the “phone phreak” subculture of the 1970s that
first broadly exposed and exploited weaknesses in switch signaling protocols. Ever
since automated long-distance switches were introduced by AT&T in the 1950s,
people have been trying to figure out ways to bypass the toll services and get voice
services for free. And the first known instances of eavesdropping by phone predate
even the Bell System itself.The PSTN has evolved considerably in recent years, but
the addition of VoIP services also has created new and novel vulnerabilities for both
data and voice.
PSTN: What Is It,
and How Does It Work?
Today, the PSTN is the most broadly interconnected communications system in the
world, and is likely to remain so for at least another decade or more. For voice, it has
no equal.VoIP services like Skype have banked on this fact; their business model
depends on a steady flow of PSTN interconnect charges. But the PSTN provides
FAX, data, telex, video, and hundreds of other multimedia services as well. And for
many decades, the PSTN has enjoyed a universal numbering scheme called E.164.
When you see a number that begins with “+” and a country code, you are seeing an
E.164 number. In most of the world, connectivity to the PSTN is considered as
essential as electricity or running water. Even the Internet itself depends on the
PSTN to deliver dedicated access circuits as well as dial-up.
In the early days following Bell’s invention, wired communications at its most
advanced meant two (or more) devices sharing a single iron wire, whether you were
using a telegraph or telephone. A grounded wire to earth completed the circuit running
between phones, each with its own battery to generate the current necessary to
transmit. It was noisy and lines couldn’t run very far, and it would be many decades
before it could truly be called a global network, much less a national one.
To fully define today’s PSTN, we’ll need to focus on several areas in turn. First,
the physical “cable plant” required for signal distribution, from twisted-pair copper
and coaxial electric to the latest fiber-optic cabling. Second, its signal transmission
models, combining analog and digital signal processing and transmission over electrical,
optical, and radio interfaces.This directly affects the kinds of content it can
carry.Third, the increasing sophistication of associated signaling (control) protocols
and “intelligent network” design introduced with the Integrated Services Digital
Network (ISDN). And finally, its associated operational and regulatory infrastructure
on international, national, state, and local levels.