PSTN Hierarchy

The PSTN hierarchy is implemented differently in the United States and the United Kingdom. The following sections provide an overview of the PSTN hierarchy and its related terminology in each of these countries.

PSTN Hierarchy in the United States

In the United States, the PSTN is generally divided into three categories:

• Local Exchange Networks

• InterExchange Networks

• International Networks

Local Exchange Carriers (LECs) operate Local Exchange networks, while InterExchange Carriers (IXCs) operate InterExchange and International networks.

The PSTN hierarchy in the United States is also influenced by market deregulation, which has allowed service providers to compete for business and by the divestiture of Bell.

Local Exchange Network

The Local Exchange network consists of the digital switching nodes (EOs) that provide network access to the subscriber. The Local Exchange terminates both lines and trunks, providing the subscriber access to the PSTN.

A Tandem Office often connects End Offices within a local area, but they can also be connected directly. In the United States, Tandem Offices are usually designated as either Local Tandem (LT) or Access Tandem (AT). The primary purpose of a Local Tandem is to provide interconnection between End Offices in a localized geographic region. An Access Tandem provides interconnection between local End Offices and serves as a primary point of access for IXCs. Trunks are the facilities that connect all of the offices, thereby transporting inter-nodal traffic.

InterExchange Network

The InterExchange network is comprised of digital switching nodes that provide the connection between Local Exchange networks. Because they are points of high traffic aggregation and they cover larger geographical distances, high-speed transports are typically used between transit switches. In the deregulated U.S. market, transit switches are usually referred to as carrier switches. In the U.S., IXCs access the Local Exchange network at designated points, referred to as a Point of Presence (POP). POPs can be connections at the Access Tandem, or direct connections to the End Office.

International Network

The International network consists of digital switching nodes, which are located in each country and act as international gateways to destinations outside of their respective countries. These gateways adhere to the ITU international standards to ensure interoperability between national networks. The international switch also performs the protocol conversions between national and international signaling. The gateway also performs PCM conversions between A-law and m-law to produce compatible speech encoding between networks, when necessary.

Service Providers

Deregulation policies in the United States have allowed network operators to compete for business, first in the long-distance market (InterExchange and International) beginning in the mid 1980s, and later in the local market in the mid 1990s. As previously mentioned, LECs operate Local Exchange networks, while IXCs operate the long-distance networks. Figure 5-2 shows a typical arrangement of LEC-owned EOs and tandems interconnected to IXC-owned transit switches. The IXC switches provide long-haul transport between Local Exchange networks, and international connections through International gateway switches.

Figure 5-2. Generic U.S. Hierarchies

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Over the last several years, the terms ILEC and CLEC have emerged within the Local Exchange market to differentiate between the Incumbent LECs (ILECS) and the Competitive LECs (CLECS). ILECs are the incumbents, who own existing access lines to residences and corporate facilities; CLECs are new entrants into the Local Exchange market. Most of the ILECs in the United States came about with the divestiture of AT&T into the seven Regional Bell Operating Companies (RBOC). The remainder belonged to Independent Operating Companies (IOCs). Most of these post-divestiture companies have been significantly transformed today by mergers and acquisitions in the competitive market. New companies have experienced difficulty entering into the Local Exchange market, which is dominated by ILECs. The ILECs own the wire to the subscriber's home, often called the "last mile" wiring. Last mile wiring is expensive to install and gives the ILECs tremendous market leverage. The long-distance market has been easier for new entrants because it does not require an investment in last mile wiring.

Pre-Divestiture Bell System Hierarchy

Vestiges of terminology relating to network topology remain in use today from the North American Bell System's hierarchy, as it existed prior to divestiture in 1984. Telephone switching offices are often still referred to by class. For example, an EO is commonly called a class 5 office, and an AT is called a class 4 office. Before divestiture, each layer of the network hierarchy was assigned a class number.

Prior to divestiture, offices were categorized by class number, with class 1 being the highest office category and class 5 being the lowest (nearest to subscriber access). Aggregation of transit phone traffic moved from the class 5 office up through the class 1 office. Each class of traffic aggregation points contained a smaller number of offices. Table 5-1 lists the class categories and office types used in the Bell System Hierarchy.

Table 5-1. Pre-Divestiture Class Categories and Office Types

Class	Office Type
1	Regional Center
2	Sectional Center
3	Primary Center
4	Toll Center
5	End Office

Local calls remained within class 5 offices, while a cross-country call traversed the hierarchy up to a regional switching center. This system no longer exists, but we included it to give relevance to the class terminology, which the industry still uses often.

PSTN Hierarchy in the United Kingdom

Figure 5-3 shows the PSTN topology used in the United Kingdom. End Offices are referred to as Digital Local Exchanges (DLE). A fully meshed tandem network of Digital Main Switching Units (DMSU) connects the DLEs. Digital International Switching Centers (DISC) connect the DMSU tandem switches for international call connections.