Site home page
Get alerts when Linktionary is updated
Book updates and addendums
Get info about the Encyclopedia of Networking and Telecommunicatons, 3rd edition (2001)
Download the electronic version of the Encyclopedia of Networking, 2nd edition (1996). It's free!
Contribute to this site
Electronic licensing info
Telecommunications and Telephone Systems
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
"Telecommunications" is derived from the Greek "tele" (distant) and "communicate" (sharing). In modern terms, telecommunication is the electronic transmission of sound, data, facsimiles, pictures, voice, video, and other information between systems using either analog or digital signaling techniques. Transmissions may take place over guided media (copper cables and fiber-optic cables) or unguided media (wireless radio, microwave, and infrared).
The voice telephone systems are generally referred to as the PSTN (public-switched telephone network). You will also hear the phone system referred to as POTS (plain old telephone system). The system was designed from the ground up for voice. It is a circuit-switching system that sets up voice circuits across a hierarchy of digital switching systems connected by copper and optical cables.
Most of the telecommunication systems around the world are regulated by governments and international organizations. In the United States, the interstate telecommunication industry is regulated by the FCC. Regulation is a controversial topic, and one that affects nearly all aspects of metropolitan and wide area voice and data networking.
The telecommunication environment is now populated with a number of carriers and service providers, include the ILECs, IXCs (interexchange carriers), CAPs (competitive access providers), CLECS (competitive LECs), ISPs (internet service providers), and ICPs (integrated communications providers). These are discussed under "Service Providers and Carriers." The changes in the regulatory environment that spawned their creation are discussed under "Telecommunications Regulation."
As mentioned, the FCC regulates telecommunications in the United States. On a global scale, several organizations recommend telecommunication standards and develop policies that encourage cooperation. The primary standards organizations are the ITU (International Telecommunication Union), ISO (International Standards Organization), and the IEC (International Electrotechnical Commission). These organization are discussed elsewhere.
Structure of the Telephone Network
The public-switched telephone network (PSTN) consists of transmission components, switching components, and facilities for maintenance equipment, billing systems, and other internal components. Transmission components (links) define the cable or wireless infrastructure for transmitting signals. Switching components (nodes) include transmitters and receivers for setting up voice circuits.
The existing telecommunication systems in the Unites States consist largely of copper twisted-pair wiring in the local loop (the wiring from the phone company to homes and offices) and fiber-optic cable or microwave systems for backbone trunks and long-distance lines. The local loop still uses analog transmission methods for voice calls.
Figure T-5 illustrates service areas and facilities. LECs (local exchange carriers) operate within specific franchised service areas (basically service monopolies) called LATAs (local access and transport areas). LATAs were defined during the split-up of AT&T. The border of a LATA defines where local service ends and long-distance service begins. A LATA is associated with one or more telephone area code. LECs may be one of the incumbent carriers and a competitive access provider. The local carrier typically has several switching offices (called central offices, or COs) within the same LATA.
Figure T-5 (see book, page 1226)
An IXC (interexchange carrier) is any long-distance provider, such as AT&T, MCI, or US Sprint, that provides services between the LECs. The LECs are required to provide a point of presence (an interface) for the IXCs. Communication facilities employed by the IXCs include fiber-optic cable, ground-based microwave towers, and satellite-based microwave systems.
The telephone system was originally designed as a hierarchy of switches that set up calls across COs, across LATAs, or across long-distance connections. This hierarchy is pictured in Figure T-6.
Figure T-6 (see book, page 1227)
The hierarchy can be traced back to the first phone systems. In the late 1800s, when telephones were first introduced, people would buy a pair of phones and run a wire between the phones. Soon, cities were enmeshed in telephone cables running in all directions. Savvy entrepreneurs like Alexander Graham Bell built telephone companies so customers could run their wires to a single location and let operators connect them with other phones via a manually operated switching system. At first, customers could only connect with other customers at the switch, but soon, trunk lines were established between phone companies and everybody could call everybody else in the same local area. This grew into the hierarchy of switches that eventually extended to outlying areas and other cities. See "Network Core Technologies" for a description of how this system developed into an optical system.
Note in Figure T-6 that calls within the same CO do not need to be switched any further up the hierarchy than the local CO. A call attached to another CO within the same LATA may go directly to that CO if a line exists, or go through a tandem or long-distance carrier. InterLATA calls are handled by IXCs such as AT&T or MCI. The calls circuits are set up to the IXC's point of presence within the LATA, then out across the long-distance lines and into a point of presence at the other end. The point of presence may be within the local carriers CO or a separate building that may be just next door.
While this system can be explained as a hierarchical structure, today, almost all switching offices are now interconnected to avoid congestion. In addition, a nonhierarchical dynamic routing system is used, as explained later. In Figure T-6, the dotted lines indicate the addition of these trunks. Still, hierarchical terminology is used to describe switching equipment.
The original switches were manually configured by operators. Later, mechanical switches were developed, such as the rotary wiper switches that moved through contacts were arranged in a circle. ESS (electronic switching systems) started to appear in the 1960s. At first, the switches were electromechanical, with the electronic components being added to reduce the number of mechanical parts. Eventually, all-electric switches were developed using solid-state components and no mechanical parts. Today, switches are programmable digital systems that support unique services such as call forwarding and caller ID, as described later under "IN (Intelligent Network)."
A Class 5 switch is an end office switch that is located at a CO. Customers are attached to these switches. They provide POTS (plain old telephone service), local numbering, emergency services, and other services. COs connect to tandem office, and the Class 5 switches interconnect with Class 4 switches in the tandem office. Tandem switches provide connections among COs and connections into higher-level switches. There is little differentiation among Class 1 through Class 3 switches. They are often called regional, sectional, or intercity switched, respectively.
In the 1980s, AT&T replaced the static hierarchical network scheme with a dynamic routing scheme called DNHR (Dynamic Non-Hierarchical Routing). DNHR is similar to IP routing. A paper by Greg Trangmoe (see related entries page) describes DNHR.
The telephone system now uses digital signaling except in the local loop. When an analog voice signal reaches a central office, it is digitized and multiplexed into a digital trunk line that connects to another CO or a tandem office.
Analog voice is digitized using PCM (pulse-code modulation), a sampling technique in which the voltage level of an analog signal is read 8,000 times per second. This 8,000-Hz sampling rate is two times the highest frequency of the range used to carry voice (300 to 4,000 Hz) and produces a relatively good digital representation. Each voltage level sample is converted to an 8-bit value that is transmitted across the line. A single digitized voice call requires 64,000 bits/sec of bandwidth (8,000 samples/sec × 8 bits/sample).
The carriers use TDM (time division multiplexing) to transmit multiple voice calls over a single line. A single call has a data rate of 64 Kbits/sec as just described. This is called a DS-0 in NADH (North American Digital Hierarchy). A total of 24 DS-0s are multiplexed into a T1 circuit. A T3 line, which is called a DS-3, consists of 28 T1 lines or 672 DS-0 channels. At higher levels, DS signal are multiplexed into SONET neworks, which use the OC (optical carrier) scheme. See "NADH (North American Digital Hierarchy)" and "OC (Optical Carrier)."
DLC (Digital Loop Carrier)
DLC is a system that lets the telephone companies extend telephone services to outlying areas. Picture a small town with a single telephone company central office. All the copper wires for all the phones extend back to this central office. Now, suppose a subdivision is built outside town. To provide service, the telephone company installs a digital loop carrier system near the subdivision. All the subscribers in the subdivision connect to the DLC system, which itself is connected back to the central office via a trunk line (T1/E1) or fiber-optic connection.
With DLC, it is not necessary to run copper cable for every subscriber back to the central office. A DLC basically terminates the copper loop in local neighborhoods. The outlying DLC systems may be either remote offices that house DLC equipment to support entire neighborhoods or they may be small remote terminals, which are typically installed in office buildings and support approximately 100 subscribers.
DLC poses problems for CLECs that want to reach remote customers and offer DSL services. The CLEC must run a cable out to the remote terminal in order to access the copper loops. They may have trouble establishing a presence at the site and ILECs have not been accommodating to their needs.
PBX Systems and Multichannel Lines
The bottom of the hierarchy in Figure T-6 seems to indicate that all lines from COs terminate at a single phone. In fact, the phone company extends multichannel digital lines (T1 and T3 lines) into businesses that have multiple phones. The business sets up a PBX (private branch exchange) that essentially provides an extension of the telephone company's switching system into the local business. The telephone company can then route all calls for phones within a business to the PBX and rely on the PBX to distribute those calls. Centrex is a PBX that the carrier maintains at its own facility. Subscribers lease Centrex services rather than buy their own PBX. In either case, a digital trunk extends from the carrier to the customer site. See "PBX (Private Branch Exchange)" and "Centrex (CENTRal Exchange)."
IN (Intelligent Network)
IN is the intelligent portion of the public telephone network that contains the logic for routing calls, establishing connections, and providing advanced features such as unique customer services and custom programming of the network. You will hear about the AIN (Advanced Intelligent Network). It was supposed to provide a way for customers to deploy services, but was never fully developed. For more information, see "Intelligent Network (IN)," a paper written by Telecordia and available at the Web ProForum site listed on the related entries page.
Before the IN, the telephone network consisted of hardwired switching systems. These hardwired systems were difficult to upgrade. As new features and services were requested from customers, new switches had to be designed, manufactured, and installed. That process could take years. In addition, different carriers used switches from different vendors, so services were difficult to implement across carrier service areas.
In the mid-1960s, SPC (stored program control) switches were developed that allowed the carrier to program new services directly into the switch. In the 1970s, the networks were further enhanced with the introduction of CCS (Common Channel Signaling) networks and SS7 (Signaling System 7) protocol. CCS networks have a signaling path that is separate from the actual voice call circuit. Call setup information is handled by SS7 and the information is transferred via packets across an overlay packet-switching network. The components of the network are pictures in Figure T-7.
Figure T-7 (see book, page 1230)
The network consists of a transport plane that provides circuit-switched telephone connections. Calls are multiplexed onto trunk lines via time division multiplexing. These nodes are connected to the SS7 signaling plane via SSPs (service switching points). The signaling plane is a packet-switched network that carries SS7 messages. The STPs (service transfer points) are the switching nodes of this network and the SCPs (service control points) are database servers where service control information is hosted.
The system goes to work when a caller lifts the handset on a telephone. The telephone is connected to a switch at a CO. This switch detects that the phone is "off-hook" and responds with a dial tone. It then listens for the dialed digital tones that represent the destination telephone. The digits are passed up to the SS7 network and the SCPs, which determine the route to the destination CO. A circuit is then set up in the transport layer. When the called party answers, the circuit is completed. Analog speech is then digitized and delivered across the circuit.
The Intelligent Network provides unique services. For example, a service called SRF (special resource function) plays recorded messages and prompts users to respond with inputs from the telephone keypad. The SSPs capture the digits and pass them up to the service layer, where they are routed via SS7 messages to appropriate SCPs. The SCP may use the information to query the service database and provide a suitable response. Calling card services user this feature.
The primary advantage of a separate signaling system is that the phone network becomes more flexible and allows for the introduction of new services, such as the range of three-digit services (800, 888, 900, etc.). For example, with caller ID, the caller's telephone number is transferred across the SS7 signaling path.
The Intelligent Networking Forum was formed in 1995 to further the use of the IN and stimulate global market growth for distributed network intelligence products and services across public telephony, data, and enterprise networks. The Web address is listed on the related entries page.
While the Intelligent Network is a good idea (anybody that uses caller ID will agree) for a voice communication system, David Isenberg argues in his classic essay "Rise of the Stupid Network" that the Intelligent Network is based on assumptions that are detrimental to the deployment of new data-oriented network services. These assumptions include a belief that the infrastructure is limited and bandwidth is scarce, that human voice generates the most traffic, that circuit switching is all that matters, and that the telephone company should control the network. Isenberg notes that the telephone companies developed the Intelligent Network to counter threats to its infrastructure, but that this response is much like the way sailing merchants responded to the threat of steam by inventing faster sailing ships in the mid-1800s!
In contrast, the Internet is built on the assumption that the network should be dumb and fast, and that end systems are smart. The original Internet architects removed as many services from the network as possible to reduce complexity and provide fast packet-switching services. Routers do not keep track of packets or do anything to ensure delivery. They just forward packets.
The assumption was that end systems would have processors and memory and be able to provide reliability services such as detecting errors and recovering lost packets. This design has been profound. It meant that end systems would become the focus of application development for the Internet, not the network provider. The telephone network may be "smart," but telephones are dumb. You can't run applications on your phone like you can on a PC. In fact, you are totally dependent on the phone company to deploy new applications (call waiting and caller ID are examples). Compared to the Internet, the phone system is a dinosaur. Consider that the user interface for the Web is a full-color graphical browser while the interface for the telephone network is a 12-key pad!
The NPN (new public network) is an emerging communication system that converges the PSTN (public-switched telephone network) and the Internet. It is a packet-switched network that will deliver voice with the same reliability now provided by the PSTN. See "NPN (New Public Network)."
Carrier and Service Provider Resources
The following Web sites provide links to telecommunications companies. Due to mergers and acquisitions, some of the links at these sites may redirect you to other sites or fail. Still, these sites are the best source for links. See "Service Providers and Carriers" for additional information.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.