WiMax Operator's Manual

CHAPTER 6 ■ BEYOND ACCESS 149
outdoors and utilizing hardened enclosures. The analog voice signals carried by the copper
lines will then be digitized and will be conveyed back to a central office over optical fiber. Dialup
Internet connections will undergo a similar transformation inasmuch as they are actually
carried within analog audio signals.
DSL lines will terminate at a DSL access multiplexer (DSLAM) and will generally also run
over fiber from the aggregation point to the central office. A DLC for ordinary phone lines and
a DSLAM can and often do occupy the same enclosure today.
Today the central office itself is a facility that is occupied by a device known as a class 5
switch whose purpose is to manage traffic over circuit voice channels. The switch’s primary
function is to establish a path to the call’s intended destination utilizing available network
resources, but it must also be capable of handling a multitude of special features such as call
waiting, call forwarding, conferencing, and so on.
Other special calling features, such as caller ID and one-number portability, are handled
by special applications servers that are dedicated to those functions coming under the rubric
of Advanced Intelligent Network (AIN). Basic switching functions are compromised when the
switch’s control plane is invoked to perform unrelated tasks; hence, the use of separate platforms
to support special features has nothing directly to do with traffic handling.
Telephone switches, incidentally, are enormously complex devices embodying millions of
lines of code. Interestingly, most of the functionality enabled by the code occurs beneath the
surface. Such complexity is reflected in the price, which runs into the millions of dollars.
The class 5 switch is involved in directing calls within the local exchange itself and outside
the local exchange to other local exchanges, which could be located anywhere in the world.
The immediate path that the signal takes is most likely to be onto what is known as a fiber ring,
however. The class 5 switch itself does not handle call routing to other class 5 switches. That
function is performed by a class 4 or tandem switch, described next.
Fiber rings emerged in the 1980s and followed the SONET standard in the United States
and followed the closely related synchronous digital hierarchy (SDH) standard in most other
places in the world. The rings themselves always take the form of closed loops, but they are not
necessarily rings in the strictest sense and may in fact meander over the landscape to a considerable
degree. The reason a closed-loop architecture is used is to provide an alternative path
back around the loop if a break occurs in the fiber.
Although some fiber rings encompass only a single metropolitan area and primarily distribute
data services to businesses within an urban core, the larger rings extend out over a
greater metropolitan area or even several such areas; the area may enclose thousands of square
miles. These link many local exchanges and ultimately off-load their traffic onto larger
switches known as class 4 switches that handle calls going to other area codes or country codes.
In some cases, long-distance carriers own the larger rings, but in other cases independents
such as American Fiber own them. The rise of independents in this area constituted a major
trend in telecommunications during the late 1980s and early 1990s. Some cable operators also
own fiber rings—generally the smaller, metro variety.
Fiber rings themselves are evolving, and one tendency that is beginning to manifest
itself—though more in the East Asian markets than in the United States—is the adoption of
packet protocols intended to replace SONET. Chief among these is Resilient Packet Ring (RPR),
which combines the fast restoration capabilities of SONET with the spectral efficiency of IP and
Ethernet. Such packet rings are capable of handling all types of traffic, including voice.
Class 4 switches located within the rings send traffic out over long lines, which in the past
were copper or microwave links but today are almost exclusively optical fiber. The long lines in