Telecommunications and Data Communications Handbook

Ray Horak

• Publisher: Wiley-Interscience
• Year: 2012
• Print ISBN: 9780470396070
• eBook ISBN: 9781118585801
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

For an accessible and comprehensive survey of telecommunications and data communications technologies and services, consult the Telecommunications and Data Communications Handbook, which includes information on origins, evolution and meaningful contemporary applications. Find discussions of technologies set in context, with details on fiber optics, cellular radio, digital carrier systems, TCP/IP, and the Internet. Explore topics like Voice over Internet Protocol (VoIP); 802.16 & WiMAX; Passive Optical Network (PON); 802.11g & Multiple Input Multiple Output (MIMO) in this easily accessible guide without the burden of technical jargon.

CHAPTER 1

FUNDAMENTALS OF THE TECHNOLOGY: CONCEPTS AND DEFINITIONS

What is the transmitter? It is an electrical ear which receives the shock of the dancing molecules, just as does the membrane of the human ear…. What is the receiver? It is an electric mouth which can utter human sounds.

John Mills, The Magic of Communication: A Tell-You-How Story, Information Department, American Telephone and Telegraph Company

Telecommunications is the transfer of information (communications) from a transmitter or sender to a receiver across a distance (tele). Some form of electromagnetic energy is employed to represent the data, usually through a physical medium, such as a copper wire or a glass fiber. A wireless medium, such as radio or infrared light, also may be employed. Additionally, a number of intermediate devices are typically involved in setting up a path for the information transfer and for maintaining adequate signal strength.

The information transfer must be established and maintained at acceptable levels in terms of certain key criteria such as speed of connection, speed of information transfer, speed of response, freedom from error, and, finally, cost. The information can be voice, data, video, image, or some combination of these—in other words, multimedia. The information can retain its original, or native, form during transmission. Alternatively, the transmission process can alter the data in some way in order to effect compatibility between the transmit and receive devices and with various intermediate network elements. For example, analog voice often is converted into a digital (data) bit stream for transmission over a digital network and is restored to analog form for the benefit of the analog-oriented human being on the receiving end. Additionally, the information can be compressed in order to improve the efficiency of information transfer and can even be encrypted for purposes of security.

The electromagnetic energy employed to carry the data can be in the form of electric impulses or radiated energy in the form of either radio waves or light rays. The media employed can include metallic conductors (e.g., twisted pair or coaxial cable); free space, or airwaves (e.g., radio technologies such as microwave, satellite, or cellular or optical technologies such as free space optics); and glass or plastic fiber (fiber-optic cable). In a network of substantial size that spans a significant distance, a combination of transmission media typically is involved in the information transfer between transmitter and receiver. An intercontinental voice or data call might involve a combination of many media.

Additionally, a wide variety of intermediate devices might be employed to establish and maintain the connection and to support the information transfer. Such devices may include an appropriate combination of modems or codecs, controllers, concentrators, multiplexers, bridges, switches, routers, gateways, and so on.

This chapter examines a number of concepts and defines a fundamental set of elements that apply universally to communications networks. Distinctions are drawn between dedicated, switched, and virtual circuits, with two-wire and four-wire circuits defined and illustrated. The concept of bandwidth is explored in both analog and digital terms, with the advantages and disadvantages of each explained. The concept of multiplexers is discussed in detail, with variations on the theme detailed and illustrated. Finally, this chapter briefly explores the nature and evolution of various types of switches, including circuit, packet, frame, cell, and photonic switches.

1.1 FUNDAMENTAL DEFINITIONS

Developing a solid understanding of communications networking requires that one grasp a number of fundamental definitions, for telecommunications has a language of its own. The following terms, many of which are illustrated in Figure 1.1, are significant and are applied fairly universally across all voice, data, video, and other systems and network technologies. Some of the terms have multiple definitions that can be specific to a technology or application. As this book will use and illustrate these terms many times across a wide variety of technologies and applications, they soon will become part of your everyday vocabulary. (Note: This would be an excellent time to pause and warn your family and friends.)

Figure 1.1 Simple circuit between transmitter and receiver across a network involving multiple links and switches.

• Transmitter: The transmitter, also known as the sender or source, is the device that originates the information transfer. Transmitters include voice telephones, data terminals, host computer systems, and video cameras.
• Receiver: The receiver, also known as the sink, is the target device, or destination device, that receives the information transfer. Receivers can include telephones, data terminals, host computers, and video monitors. Note that most devices are capable of both transmitter and receiver functions; exceptions include broadcast radio and TV devices.
• Circuit: A circuit is a communications path, over an established medium, between two or more points, from end to end, between transmitter and receiver. Circuit generally implies a logical connection over a physical line. Further, the term circuit often is used interchangeably with path, link, line, and channel, although such usage can be specific to the underlying technology, the overall context, and other factors. Circuits comprising copper twisted wire are either two-wire or four-wire, depending on the requirements of the specific application and the fundamental nature of the network. Circuits also may be for purposes of either access or transport. Access circuits are from the customer premises to the edge of the carrier network, while transport circuits are employed in the core, or backbone, of the network for purposes of long-haul transmission. Circuits may be simplex (one-way), half-duplex (two-way, but only one way at a time), or full-duplex (simultaneous two-way).
• Link: A link is a two-point segment of an end-to-end circuit (e.g., from terminal to switch or from switch to switch). Typically, a circuit comprises multiple links. Also, a circuit may consist of a single link, as often is the case between a host computer and a peripheral, such as a printer. Link sometimes is used interchangeably with line or circuit.
• Line: Line has several definitions, which may result in some confusion. In a Private Branch eXchange (PBX) environment, a station line refers to the connection between the PBX switch and the station user’s terminal equipment, which usually is in the form of telephone, although it could be a computer workstation, a printer, a facsimile machine, or some other device. In rate and tariff terminology, line refers to a local loop connection from the telephone company Central Office (CO) switch to the user premises in support of Customer Premises Equipment (CPE) other than a switch. For example, such CPE may be in the form of a single-line residence or business set, a multiline set, or the common control unit of a key telephone system. In any case, line refers to a voice-grade circuit, in other words, a circuit serving a single physical location where it terminates in a relatively unsophisticated device. Further, a line has a single associated telephone number and generally is single channel in nature (i.e., supports a single transmission at a time). A line may be thought of as a tributary of a trunk. In telephone company (telco) parlance, line describes the user side or local loop side of the central office switch; in other words, the line side is the side of the network to which users connect to access the network. The trunk side involves the high-capacity trunks that serve to interconnect the various telco switching centers in the core of the carrier network.
• Trunk: Trunk comes from the Latin truncus, meaning torso. The trunk is the main body apart from the head or appendages, much as the main channel of a river is apart from its tributaries. In the context of telecommunications, a trunk is a communications circuit, available to share among multiple users, on a pooled basis and with contention for trunk access managed by an intelligent switching device. Therefore, trunks interconnect switches. For example, tie trunks connect PBX switches in a private, leased-line network, central office exchange trunks connect PBXs to telephone company central office exchange switches, and interoffice trunks interconnect central office exchange switches. Trunk groups are groups of trunks serving the same special purpose, with examples including Direct Inward Dial (DID) and tie trunk groups. Trunks are directional in nature, with the options being one-way outgoing (originating), one-way incoming (terminating), or two-way (combination).
• Channel: In formal standards terms, a channel is a means of one-way connection between transmitter and receiver—therefore, a one-way circuit or signal path. In data processing terminology, particularly IBM, a channel is a high-speed two-way connection between mainframe and peripheral. In common usage, a channel is a logical connection over a physical circuit to support a single conversation. You can configure a physical circuit in such a way as to support one or many logical conversations. Multichannel circuits always are four-wire in nature—either physical or logical four-wire.
• Switch: A switch is a device that establishes, maintains, and changes logical connections over physical circuits. Common examples of switches include PBXs and Central Office Exchanges (COs or COEs), both of which are circuit switches. Circuit switches establish connections between circuits (or links) on demand and as available. While developed to support voice communications, circuit switches can support any form of information transfer (e.g., data and video communications). Packet switch is a generic term that actually includes packet, frame, and cell switches. Packet switching evolved in more sophisticated networks, primarily in support of computer-to-computer data and image transfer. In terms of physical placement, there are edge switches and core switches. Edge switches are positioned at the physical edge of a network; the user organization gains access to an edge switch via an access link. Core switches, also known as tandem switches and backbone switches, are high-capacity switches positioned in the physical core, or backbone, of a network and serving to interconnect edge switches. Although some switches are very intelligent in many respects, a pure switch makes connection decisions only at the link level. That is to say that a switch has a very limited view and cannot consider the network as a whole. Therefore, switches operate link by link, that is, hop by hop, generally under the control of a centralized set of logic that can coordinate their activities in order to establish end-to-end connectivity across a multilink circuit.
• Router: A router is a highly intelligent switch capable of making traffic routing decisions based on a view of the network as a whole. This is in contrast to simple switches, which see only an individual link and have no sense of the larger network. Routers are programmable devices that can be quite sophisticated. In determining the route for a given communication, a router can be programmed to consider a number of factors including the addresses of the originating and destination devices, the least-cost route, the least-congested route, and the shortest route. Routers can be capable of connecting dissimilar networks, such as circuit-switched and packet networks, and accomplishing the conversion processes necessary to resolve any issues of incompatibility. Chapter 8 discusses routers in great detail.
• Network: A network is a fabric of elements that work together much as the fabric of a net to support the transfer of information. In the extreme sense, a network includes everything from the transmitters to the receivers, including all links, switches, and other intermediate devices that can be called upon to support a communication.
  • Local Area Network (LAN): A LAN is a local, that is, limited-distance, packet network designed for interconnecting computers, peripherals, storage devices, and other computing resources within a confined area. A LAN may serve an office, a single floor, an entire building, or perhaps a campus of many buildings but generally does not cross a public right-of-way. LANs generally are private networks. LANs can be interconnected, perhaps across a MAN or WAN.
  • Metropolitan Area Network (MAN): A MAN is a public network that serves a metropolitan area or perhaps a portion of a metropolitan area such as a city or a suburb. MANs tend to be data oriented and increasingly serve to interconnect LANs. A number of carriers now offer high speed metropolitan Ethernet services, for example.
  • Wide Area Network (WAN): A WAN is a network that covers a wide geographic area such as a state, province, region, or country. The Public Switched Telephone Network (PSTN) is a voice-oriented WAN that individuals use to connect voice calls. The Internet is a WAN, as are many other data-oriented public networks. WANs can serve to interconnect LANs and MANs. The WAN commonly is depicted as a cloud (Figure 1.1), which originated in sales presentations of the 1970s for data communications networks. The thought behind the cloud simply was that the specific internal workings of the networks could be many and various, change from time to time, and vary from place to place. The cloud served to obscure those internal workings from view. The cloud was the consummate conceptual sale—data simply popped in on one end of the network in one format and popped out on the other side of the network in another format. Interestingly, the data network that gave rise to the cloud never worked, but the cloud lives on. Actually, the cloud is entirely appropriate for depicting the Internet, as the specific internal workings are largely unpredictable for any given call.

1.2 DEDICATED, SWITCHED, AND VIRTUAL CIRCUITS

Circuits can be provisioned on a dedicated, switched, or virtual basis, depending on the nature of the application and the requirements of the user organization. Ultimately, issues of availability and cost-effectiven...

Table of contents

• Cover
• Half Title page
• Title page
• Copyright page
• Dedication
• Preface
• Acknowledgments
• About the Author
• Chapter 1: Fundamentals of the Technology: Concepts and Definitions
• Chapter 2: Fundamentals of Transmission Systems: Technologies and Applications
• Chapter 3: Voice Communications Systems: KTS, PBX, Centrex, and ACD
• Chapter 4: Messaging Systems
• Chapter 5: Public Switched Telephone Network
• Chapter 6: Fundamentals of Data Communications
• Chapter 7: Conventional Digital and Data Networks
• Chapter 8: Local Area Networks: Connectivity and Internetworking
• Chapter 9: Broadband Network Infrastructure
• Chapter 10: Broadband Network Services
• Chapter 11: Wireless Networking: Emphasis on Mobility
• Chapter 12: Video and Multimedia Networking
• Chapter 13: The Internet and World Wide Web
• Chapter 14: Network Convergence
• Chapter 15: Regulation: Issues and (Some) Answers
• Appendix A: Acronyms, Abbreviations, Contractions, Initialisms, and Symbols
• Appendix B: Standards Organizations and Special Interest Groups (SIGs)
• Index