Regulation traditionally has focused on voice network services, as they certainly are more basic and necessary than are data and other services, although this posture now can be argued and particularly so with the advent of the Internet. In fact, voice communications recently has become possible over the Internet through equipping a PC with a high-speed modem, speakers and microphone, and special software. Considered by many as a threat to the traditional PSTN and the concept of universal service, a number of interested parties recently have requested that the FCC examine the issue, with the intent that voice over the Internet be regulated or banned altogether. Interestingly, the major IXCs are not parties to this request. AT&T and others have taken strong positions as Internet service providers and intend to encourage its use. Voice over Frame Relay, an offering primarily intended for data communications, is possible as well, and increasingly is used where excess frame relay bandwidth is available. Although the cost of implementing voice over either the Internet or frame relay networks is a hindrance and although the quality of the voice communication generally is poor in either case, advances in technology promise to improve quality, lower cost and, therefore, threaten the PSTN. The Internet and frame relay will be discussed in length in subsequent chapters.
Local exchange competition, voice-over-the-Internet and voice-over-frame relay all threaten the concept of universal service, which has been a cornerstone of the PSTN since the formation of the FCC in 1934. In order to ensure the universal availability of voice service at affordable cost to the subscriber, a complex structure of settlements (subsidies) developed between IXCs and LECs. Thereby, a subscriber in a high-cost area such as Hackberry, Arizona could gain affordable network access, just as could a subscriber in New York despite the obvious cost differences in the carriers’ providing such service. Unless a universal service fund is established, with all carriers contributing, the concept of universal service may be relegated to a historical footnote.
Carrier Domains and Network Topology
Some years ago, and certainly previous to AT&T’s divestiture of the Bell Operating Companies in 1984, the network was relatively simple in terms of its ownership and topology. Each operating telephone company provided service in its franchised serving areas, and gained access to the AT&T long distance network on a fairly straightforward basis. Beginning in the late 1920s, the network was organized on a layered basis, with five levels of hierarchy, known as classes [6-3].
- Class 5 offices are the local exchange offices, or central offices, serving end users through local loop connections. While the Class 5 offices were interconnected directly where high volumes of traffic flowed between them, they more commonly were interconnected via tandem switches. There exist approximately 19,000 Class 5 offices in the United States.
- Class 4 offices were tandem toll centers, which served to interconnect Class 5 offices not directly connected. As the lowest class of toll center, these also served as the first point of entry to the long distance, or toll, network. Class 4 offices were interconnected within a relatively local toll network and provided access to higher order toll centers. In many instances, a Class 4 office also served as a Class 5 office; in other words, a hybrid switch served as both a central office and a tandem toll office, with the separate functions being provided through logical and physical partitioning within the switch. Approximately 1,500 tandem toll centers existed in North America prior to AT&T’s divestiture of the BOCs.
- Class 3 offices, or primary toll centers, were higher-order toll centers, generally serving to connect Class 4 offices for intrastate toll calling. Class 4 offices typically served to interconnect independent telephone companies and BOCs. Approximately 200 such offices existed prior to divestiture.
- Class 2 offices were known as sectional toll centers, which served to interconnect primary toll centers, largely for interstate calling within a geographic region such as the Northeast or the Southwest. There were approximately 67 sectional toll centers in the AT&T network prior to divestiture.
- Class 1 offices, or regional toll centers, served to interconnect sectional toll centers in support of interregional calling. There were ten regional toll centers in place in the United States prior to divestiture; seven currently exist, with another two in Canada.
As illustrated in Figure 6.1, the offices were interconnected on a hierarchical basis, with end offices being at the bottom of the network food chain. As a user placed a long distance call, the network would examine and analyze the originating and destination telephone numbers and associated geographic areas. Based on that information, the network would process the call. Local long distance calls (e.g., within the San Francisco Bay area) would be handled either by directly connected Class 5 offices, or through a Class 4 tandem toll office. A coast-to-coast call, on the other hand, might involve all 5 classes of the hierarchy. For instance, a call from Turlock, CA to New York, NY originates in the Class 5 switch of Evans Telephone Company, an independent Telco company, and is handed to a nearby AT&T tandem toll center. The call then works its way up the hierarchy until it reaches the Class 1 regional toll center in San Francisco. High-capacity, coast-to-coast intertandem toll trunks carry the call to New York City, where it work its way down an abbreviated hierarchy and is delivered to the target number in Manhattan.
This approach was quite sensible in the days when AT&T dominated the local and long distance networks. As calls worked their way through the network, larger and larger volumes of traffic were aggregated by more and more capable switches and shipped over trunks of ever greater capacity, taking advantage of the economies of scale.
This network topology has been flattened over the years as the cost of transport over fiber optic facilities has dropped, the cost of switches has remained relatively flat, and competition has increased. As a result, there exist fewer, more intelligent and multifunction switches, interconnected by higher capacity transmission facilities in what is known as a sparse network configuration.
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