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The PSTN Hierarchy
The U.S. PSTN is a collection of switching offices or call centers all over the country.
The original PSTN used a hierarchy of six different types of call centers (listed in
order from the customer's premise):
• Class 5—This is the local switching office or local CO between the PSTN and
its subscribers. The subscribers are connected to the local CO over 24 or 26
AWG copper twisted-pair cable. Most residential homes have between two
and four twisted pairs, whereas a standard office building can have
anywhere from 25 pairs and up. Only a single twisted-pair is needed to
provide a basic 56/64Kbps (4KHz bandwidth) PSTN circuit. Depending on the
geographical area being served by the Class 5 CO, it can support one or more
local dial exchanges. A local dial exchange is limited to 9,999 subscribers.
The Class 5 office also has trunk lines that lead to other local offices or toll
offices.
• Class 4—There are two types of Class 4 offices: tandem and toll. A tandem
office acts as an interconnect point between Class 5 offices. It provides no
local loop service, only interconnections between Class 5 offices and, in
some cases, a Class 4 toll office. A Class 4 toll office provides
interconnections between long distance or interexchange carriers and
different local area exchanges.
• Class 3—This primary toll center terminates Class 4 (and sometimes Class 5)
centers. It can also offer spillover termination services for overcrowded
Class 4 and 5 centers. The center's main function is to perform call routing
between different local area exchanges and long distance exchanges
through Class 1 and 2 call centers.
• Class 2—This sectional toll center can terminate Class 5, 4, and 3 call centers.
The center routes toll level service for a BOC's area of operation, and
performs gateway services for Class 1 centers.
• Class 1—This regional toll center performs the same function as a Class 2
center except that it also connects to international gateway offices. These
centers are operated by long distance carriers.
• International gateway—These offices interconnect with other international
gateway offices in other countries and Class 1 call centers. They perform
international call exchange and operator and support assistance.
In 1974, the U.S. government filed an antitrust suit against AT&T that resulted in
the breakup of AT&T and the BOCs. Under the divestiture agreement, AT&T retained
its long distance and equipment manufacturing businesses (the manufacturing
business is now Lucent Technologies). The BOCs were spilt up into seven Regional
Bell Operating Companies (RBOCs). The divestiture agreement segmented the
country into different local area transport areas (LATAs). The RBOCs and other
independent telephone companies (ITC) were given jurisdiction over different