Structure of the PSTN

At the very beginning of the telephony age, telephones were sold in pairs; for a call to be made, the two telephones involved had to be connected directly. So, in addition to the grounding wire, if there were 20 telephones you wanted to call (or that might call you), each would be connected to your telephone by a separate wire. At certain point, it was clear that a better long-term solution was needed, and such a solution came in the form of the first Bell Company switching office in New Haven, Connecticut. The office had a switching board, operated by human operators, to which the telephones were connected. An operator’s job was to answer the call of a calling party, inquire as to the name of the called party, and then establish the call by connecting with a wire two sockets that belonged to the calling and called party, respectively. After the call was completed, the operator would disconnect the circuit by pulling the wire from the sockets. Note that no telephone numbers were involved (or needed). Telephone numbers became a necessity later, when the first automatic switch was built. The automaton was purely mechanical—it could find necessary sockets only by counting; thus, the telephones (and their respective sockets in the switch) were identified by these telephone numbers. Later, the switches had to be interconnected with other switches, and the first telephone network—the Bell System in the United States—came to life. Other telephone networks were built in pretty much the same way.

Many things have happened since the first network appeared—and we are going to address these things—but the structure of the PSTN in terms of its main components remained unchanged as far as the establishment of the end-to-end voice path is concerned. The components are:

• Station equipment [or customer premises equipment (CPE)]. Located on the customer’s premises, its primary functions are to transmit and receive signals between the customer and the network. These types of equipment range from residential telephones to sophisticated enterprise private branch exchange systems (PBXs).

• Transmission facilities. These provide the communications paths, which consist of transmission media (atmosphere, paired cable, coaxial cable, light guide cable, and so on) and various types of equipment to amplify or regenerate signals.

• Switching systems. These interconnect the transmission facilities at various key locations and route traffic through the network. (They have been called switching offices since the times of the first Connecticut office.)

• Operations, administration, and management (OA&M) systems. These provide administration, maintenance, and network management functions to the network.

Until relatively recent times, switching boards remained in use in relatively small organizations (such as hotels, hospitals, or companies of several dozen employees), but finally were replaced by customer premises switches called private branch exchanges (PBXs). The PBX, then, is a nontrivial, most sophisticated example of station equipment. On the other end of the spectrum is the ordinary single-line telephone set. In addition to transmitting and receiving the user information (such as conversation), the station equipment is responsible for addressing (that is, the task of specifying to the network the destination of the call) as well as carrying other forms of signaling [idle or busy status, alerting (that is, ringing), and so on].

As Figure 1demonstrates, the station equipment is connected to switches. The telephones are connected to local switches (also interchangeably called local offices, central offices, end offices, or Class 5 switches) by means of local loop circuits or channels carried over local loop transmission facilities. The circuits that interconnect switches are called trunks. Trunks are carried over interoffice transmission facilities. The local offices are, in turn, interconnected to toll offices (called tandem offices in this case). Finally, we should note that in all this terminology the word office is interchangeable with exchange, and, of course, switch. It is very difficult to say which word is more widely used.

Figure 1: Local and tandem offices.

The trunks are grouped; it is often convenient to refer to trunk groups, which are assigned specific identifiers, rather than individual trunks. Grouping is especially convenient for the purposes of network management or assignment to transmission facilities. (A trunk is a logical abstraction rather than a physical medium; a trunk leaving a switch can be mapped to a fiber-optic cable on the first part of its way to the next switch, microwave for the second part, and four copper wires for the third part.)

In the original Bell System, there were five levels in the switching hierarchy; this number has dropped to three due to technological development of nonhierarchical routing (NHR) in the long-distance network. NHR was not adopted by the local carriers, however, so they retained the two levels—local and tandem—of switching hierarchy.

Local switches in the United States are grouped into local access and transport areas (LATAs). You can find a current map of LATAs at www.611.net/NETWORKTELECOM/lata_map/index.htm. A LATA may have many offices (on the order of 100), including tandem offices. Service within LATAs is typically provided by local exchange carriers (LECs). Some LECs have existed for a considerable time (such as original Bell Operating Companies, created in 1984 as a result of breakup of the Bell system), and so are called incumbent LECs (ILECs); others appeared fairly recently, and are called competitive LECs (CLECs). Inter-LATA traffic is carried by inter-exchange carriers (IXCs). The IXCs are connected to central or tandem offices by means of points of presence (POPs).

Figure 1 depicts an interconnection of an IXC with one particular LATA. The IXC switches form the IXC network, in which routing is typically nonhierarchical. Presently, IXCs are providing local service, too; however, since their early days IXCs have had direct trunks to PBXs of large companies to whom they provided services like virtual private networks (VPNs).^[3] We should mention that IXCs in the United States can (and do) interconnect with the overseas long-distance service providers by means of complex gateways that perform call signaling translations, but the IXCs in the United States are typically not directly interconnected with each other.

Figure 2: The interconnection of LATAs and IXCs.

The Interexchange Carriers (IXCs)

There are four main long-distance players in the United States: AT&T, MCI, Verizon, and Sprint. These carriers face a number of seemingly insurmountable problems including rapidly increasing customer counts, growing minutes of use (MoU) per subscriber, and a rapidly declining dollars-per-transported-bit-per-mile figure, the combination of which is deadly. Needless to say, this explains their compelling argument for accelerated local service and broadband entry.

One bone of contention for the IXCs is that more than 40 percent of all IXC revenues are paid to ILECs as access charges. Access charges are the fees paid to ILECs by long-distance carriers for the right to use local exchange facilities for the origination or termination of traffic transported to or from one exchange to another by an interexchange carrier. And while some access charges are billed directly to the end user, most of them are paid by interexchange carriers—not an insignificant amount of money. IXCs hope that future regulatory decisions will address the magnitude of this fee. In fact, one reason that VoIP has become so popular among those deploying the service is that it is currently classified by the FCC as an information service rather than as a telecom service, which means that VoIP carriers are exempt from many of the regulatory tethers that bind traditional service providers.

International Interconnection (Inter Exchange Networks)

IXC backbone carrier facilities primarily use microwave and fiber transmission lines. Microwave systems offer medium capacity of up to several hundred Mbps communications with a range of 20-30 miles between towers. Fiber optic communication systems offer data transmission capacity of over one million Mbps (million million bits per second).

Microwave transmission systems transfer signal energy through an unobstructed medium (no blocking buildings or hills) between two or more points. In 1951, microwave radio transmission systems became the backbone of the telecommunications infrastructure. Microwave systems require a transducer to convert signal energy of one form into electromagnetic energy for transmission. The transducer must also focus the energy (using an antenna dish) so it may launch the energy in the desired direction. Some of the electromagnetic energy that is transmitted by microwave systems is absorbed by the water particles in the air.

Although the extensive deployment of fiber optic cable has removed some of need for microwave radio systems, microwave radio is still used in places that are hard to reach or not cost effectively served by fiber cable such as in developing countries.

Figure 1 shows a terrestrial microwave system-connecting IXC switches in Philadelphia and New York City. The microwave signals are moved between the two switching offices through a series of relay microwave systems located approximately 30 miles apart. Microwave is a line-of-sight technology that must take the earth’s curvature into consideration. Also note that microwave towers are not limited to only facing one or two directions. A single tower can be associated with several other towers by positioning and aiming additional transceiver antennas at other microwave antennas on other towers.


Figure 1: Long Haul Microwave

Fiber optic transmission is the transfer of information (usually in digital form) through the use of light pulses. Fiber optic transmission can be performed through glass fiber or through air. Fiber optic transmission lines are capable of extending up to 1200 km without amplifiers. Each fiber optic strand can carry up to 10 Gbps optical channels and a fiber can have many optical channels (called DWDM). Each fiber cable can have many strands of fiber.

Fiber cable is relatively light, low cost, and can be easily installed in a variety of ways. It does not experience distortion from electrical interference and this allows it to be installed on high voltage power lines or in other places that have high levels of electromagnetic interference.

Figure 2 shows common installations of fiber optic cable. This diagram shows that fiber transmission systems are installed along railway and natural gas pipelines, under water, and along high voltage lines.


Figure 2: IXC Fiber Optical Cable Installation Options

High Speed Switching Systems
IXCs use high speed switching systems to interconnect transmission lines. The key high speed switching system used in IXC networks is asynchronous transfer mode (ATM). ATM is a fast packet switching technology that transports information through the use of small fixed length packets of data (53 byte cells).

The ATM system uses high-speed transmission facilities (155 Mbps/OC-3 and above). OC-3 is the entry-level speed for commercial ATM. Higher speeds (such as OC-192) are used in backbone networks of IXC’s and other specialized service providers. ATM service was developed to allow one communication technology (high-speed packet data) to provide for voice, data and video service in a single offering.

International Interconnection
International interconnection issues include converting transmission line and control signaling formats, transcoding different types of digital voice signals, and rating billing records.

IXC networks must be capable of converting transmission line formats. These include digital signaling standards (e.g., T1 to E1), different optical standards (SONET and SDH), and command signaling protocols differences such as ISDN signaling differences.

Transcoding is the conversion of digital signals from one coding format to another. Transcoding is necessary because the digital signal companding process that is used for encoding/decoding signals is different throughout the world. This companding process increases the dynamic range of a binary signal by assigning different weighted values to each bit of information than is defined by the binary system. The A-law encoding system is an international standard and the uLaw standard is used in the Americas.

IXC systems must be capable of creating billing record in different formats. Billing systems in different countries use different rating systems (e.g., flat rate compared to time usage). It may be necessary for IXCs to receive and pay in different currencies and currency exchange rates for different countries rapidly vary. The payment or receipt of payments for calls routed through the IXC must be settled through clearinghouse companies that have relationships with many IXC, LEC, and PTT operators.

Several independent companies have installed or operate international transmission lines. These international circuits may be leased to IXCs or to independent corporations. Companies that operate these international transmission lines are often called international carriers (IC’s) or international record carriers (IRC’s).

Figure 3 shows an IXC network that has many international interconnections. This diagram shows that various transmission systems are used for interconnection. There are several high capacity switching points in these networks with redundant links between them. Some of the interconnection lines are operated by satellite and transoceanic cable/fiber carrier services provided IC/IRCs.


Figure 3: International IXC Interconnections

Inter-Exchange Carriers (IXC)

Inter-Exchange Carrier (IXC) networks are used to link telephone networks within geographical service area to each other. AT&T, Sprint, MCI, and Qwest are examples of well-known IXCs.

In order to provide the bandwidth necessary to carry the volume of long-distance voice and data traffic at reasonable cost, most IXCs have deployed large bundles of fiber-optic cables that interconnect their switching systems. Burying thousands of miles of fiber cable is costly. However, each pair of fibers is capable of providing many Gbps of bandwidth.

The explosion of the Internet and the demand for advanced multi-media services continues to drive the demand for increased bandwidth at low cost. To increase the capacity of fiber cables, new fiber optic technology has emerged. By utilizing a technology known as dense wavelength division multiplexing, DWDM, each fiber can carry 80 or more separate light-waves. As of 2001, some DWDM technologies were capable of providing over 1 Tbps (1,000 Gbps) of bandwidth, enough to transmit in one second the contents of 150,000 encyclopedias. Advances in optical networking equipment and light-wave amplification technologies will continue to add bandwidth the fiber networks.

Picture on top shows the typical inter-exchange carriers (IXCs) connections. This diagram shows that there are many different IXCs. Each of these IXCs must interconnect to the local telephone companies at a defined point of presence(POP)...