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blocked 4. Also in November 1988,
a computer virus infiltrated the Internet, shutting down hundreds
of workstations 5.
Several well-publicized SS7 outages occurred in 1990 and 1991
due to software bugs 6, 7. The first
had a nationwide impact and involved the loss of 65,000,000 calls.
Others involved entire cities and affected 10,000,000
In response to a massive outage in September 1991, the mayor of
New York established a Task Force on Telecommunications Network
Reliability. The task force noted that "the potential for
telecommunications disasters is real, and losses in service can be
devastating to the end user" 8.
Lessons Learned that are Applicable to
Network infrastructure architects and designers have used
redundancy and extensive testing to build integrity into
telecommunications networks. They have recognized the critical role
that such infrastructure plays in society and are mindful of the
consequences of network failure. Techniques such as extensive
software testing, hardware duplication, protection switching,
standby power, alternate routing, and dynamic overload control have
been used throughout the network to enhance integrity.
A 1989 report published by the National Research Council
identified trends in infrastructure design that have made networks
more vulnerable to large-scale outage
9. Over the past 10 years, network evolution has been paced by
changes in technology, new government regulations, and increased
customer demand for rapid response in provisioning voice and data
services. Each of these trends has led to a concentration of
network assets. Although additional competitive carriers have been
introduced, the capacity of the new networks has not been adequate
to absorb the traffic lost due to a failure in the established
carrier's network. End-user access to all carriers has been limited
by this lack of familiarity with use of access codes.
Economies of scale have caused higher average traffic cross
sections for various network elements. Fiber optic cables can carry
thousands of circuits, whereas copper cables carried hundreds.
Other technologies such as microwave radio and domestic satellites
have been retired from service in favor of fiber. When a fiber
cable is rendered inoperable for whatever reason, more customers
are affected unless adequate alternate routing is provided. The
capacity of digital switching systems and the use of remote
switching units have reduced the number of switches needed to serve
a given area, thus providing higher traffic cross sections. More
customers are affected by a single switch failure.
In signaling, the highly distributed multifrequency approach has
been replaced by a concentrated common channel signaling system.
Also, call processing intelligence that was once distributed in
local offices is now migrating into centralized databases.
Stored program control now exists in virtually every network
element. Software technology has led to increased network
flexibility; however, it has also brought a significant challenge
to overall network integrity because of its "crash" potential.
Along with accidental network failures, there have been a number of
malicious attacks, including the theft of credit cards from network
databases and the theft of cellular electronic security
In regulation, the Federal Communications Commission has
mandated schedules for the introduction of network features such as
equal access. For carriers to meet the required schedules, they
chose to amalgamate traffic at "points of presence" and modify the
software at a small but manageable number of sites to meet the
imposed schedules. Hinsdale was one such site and, unfortunately,
the fire's impact was greater than it would have been without such
regulatory intervention because of the resulting traffic
In my opinion, the most important lesson learned in the recent
past regarding telecommunications infrastructure integrity is that
we must not be complacent and assume that major failures or network
intrusions cannot happen. In addition to past measures, new metrics
must be developed to measure the societal impact of network
integrity and bring the scientific method of specification and
measurement to the problem 10.
Another lesson learned is that design for "single-point
failures" is inadequate. Fires cause multiple failures, as do
backhoe dig-ups, viruses, and acts of God. There has been too much
focus on individual network elements and not enough on end-to-end