Previous Table of Contents Next


Switches

As was established in Chapter 2, switches are devices which establish connections over circuits or paths. In a traditional PSTN environment, such switches are circuit switches, which include Central Office Exchanges, tandem exchanges, access tandem exchanges, and International Gateway Facilities.

Central Offices (COs)

Also known as exchanges, COs typically are owned by the Local Exchange Carriers (LECs). COs provide local access services to end users via local loop connections and within a relatively small area of geography known as an exchange area. In other words, the CO lets a subscriber within that neighborhood to connect to another subscriber within that neighborhood. COs also provide a number of services, such as custom calling features (e.g., call waiting, call transfer and three-way calling) and Centrex. It also is through the CO that a subscriber typically gains access to the LEC metropolitan calling area, which does not involve a long distance (toll) charge. Finally, most subscribers also gain access to the various long distance networks through the CO, as well.

COs also are known as end offices, as they are at the terminal ends of the network. In other words, they are the first point of entry into the PSTN and they are the last point of exit. They also are known as Class 5 offices, the lowest class in the switching hierarchy. Manufacturers of COs include Lucent Technologies (5ESS); Nortel (DMS), which previously was Northern Telecom; Siemens (EWSD); and Ericsson (AXE).

Tandem Switches

These are network switches which serve in partnership with lesser switches, linking them together. Tandem switches typically serve to link together CO switches over dedicated interoffice trunks, forming a fully interconnected and toll-free metropolitan calling area in the process.

Access Tandem Switches

These switches act to connect the Local Exchange Carriers (telephone companies) to the Interexchange Carriers (long distance carriers), also over dedicated interoffice trunks, known as access trunks. It is in this manner that the local service providers typically are connected to the long distance providers.

International Gateway Facilities (IGFs)

These are the switches owned by the international carriers. Located at landing points on each end of the international connection, they provide connectivity between the international carriers, and the national and local carriers on each end.

Transmission Facilities

As was explored in Chapter 3, transmission facilities are the physical media and associated electronics which are used to provide the circuits in all domains of the PSTN. Oftentimes a combination of media is employed, perhaps including twisted pair, coaxial cable, microwave, satellite, infrared, and fiber optic cable.

Signaling and Control

These systems and networks are used to signal within the network and to control its operation. Examples include on-hook and off-hook indication, dial tone provision, call routing control, busy indication, and ringing.

Access

Access is best expressed as the local loop, the function of which is to provide access to the Wide Area Network (WAN). Access services extend from the customer premise to the LEC CO exchange, with the physical point of separation being known as a demarcation point or demarc. While generally provided by the LEC, access services increasingly are provided by competitors, such as IntereXchange Carriers (IXCs) and Competitive Access Providers (CAPs), also known as Alternative Access Providers (AAVs). Transmission facilities, switches, and signaling and control systems are all employed in an access environment.

Transport

Transport is information transportation over the PSTN. Transport is the responsibility of LECs, IXCs, and CAPs/AAVs, either individually or in concert, and depending on the geographic scope of the communication. Transmission facilities, switches, and signaling and control systems are all employed in a transport environment.

Services

Services include a wide variety of options provided by LECs, IXCs and CAPs/AAVs. Such services include various discounted calling plans, custom calling services, Centrex services, and toll-free (800/888) calling.

Regulatory Domains

There exists a complex set of domains that address regulatory and standards areas of responsibility. While it would be much simpler if these domains were discrete, there are overlapping domains with which one must contend:

International

Here regulation is largely nonexistent, although it is heavily influenced by the International Telecommunications Union-Telecommunication Standardization Sector (ITU-T), which is an agency of the United Nations (UN). Formed in 1865, as the Consultative Committee for International Telephone and Telegraph (CCITT), the ITU-T is responsible for setting standards recommendation intended to ensure the interconnectivity of national networks. The International Telecommunications Union-Radiocommunication (ITU-R), previously known as the Consultative Committee for International Radio (CCIR), is responsible for over-the-air communications. Intelsat and other consortia are responsible for allocating and managing satellite orbital slots.

The International Standards Organization (ISO) comprises a group of various national standards-setting organizations, including the American National Standards Institute (ANSI) from the United States. The ISO has great influence over international standards, and is best known for the development of the OSI (Open Systems Integration) Reference Model.

Regional

Regional authorities, although few in number, have the same responsibility within a more compact region. For example, DG XIII (Directorate General XIII) is responsible for such issues within the EC (European Community) of Western Europe. The EC has real authority in the areas of regulation and standards, establishing and enforcing policy matters such as network competition within a member nation’s network.


Previous Table of Contents Next