Chapter 2 Fundamentals of the Technology: Concepts and Definitions
I see no reason why intelligence may not be transmitted instantaneously by electricity.
Samuel Morse, 1832
Telecommunications can effectively be used to describe the transfer of information (communications) from a transmitter or sender to a receiver across a distance (tele), typically of quite some significance. Some form of electromagnetic energy is used to represent the data, usually through a physical medium, such as copper or glass wire. Additionally, some number of intermediate devices are involved in order to set up a path for the information transfer and to maintain adequate signal strength.
The information transfer must be at acceptable levels in terms of certain key criteria such as speed of connection, speed of information transfer, freedom from error, speed of response (two-way), and cost. The information can be voice, data, video, image, or some combinationin other words, multimedia. The information can be transmitted in its original, or native, form. Or it can be altered in some way to effect compatibility between the transmitting and receiving devices and/or with the various elements of the network. Examples include analog voice or video converted to a digital (data) bit stream, digitized voice or video converted to analog voice or video, and digital data converted to an analog format. Additionally, the information can beand often iscompressed in order to improve the efficiency of information transfer through what is effectively some form of data shorthand.
The electromagnetic energy used to carry the data can be in the form of electricity, radio, or light. The medium used can include copper wire (e.g., twisted pair or coaxial cable); space or airwaves (e.g., microwave, satellite, cellular, or infrared); and glass or plastic wire (fiber-optic cable). In a network of any size that carries information across any significant distance, a combination of transmission media is typically involved. In fact, an intercontinental voice or data call might involve a combination of all such media.
Additionally, a wide variety of intermediate devices might be used to establish the connection and support the information transfer. Such devices may include an appropriate combination of switches, modems or codecs, controllers, bridges, 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 among 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 being explained. The concept of multiplexers is discussed in detail, with variations on the theme detailed and illustrated. Finally, this chapter explores the nature and evolution of various types of switches, including circuit, packet, frame and cell.
Fundamental Definitions
Developing a solid understanding of communications networking requires a grasp of several fundamental definitions. The following terms are significant and are applied fairly universally across all voice, data, video, and other system and network technologies:
- Transmitter
- The transmitter, also known as the sender or source, is the device that originates the information. Such devices include telephones, any variety of computer systems, and cameras in a videoconferencing application.
- Receiver
- The receiver, also known as the sink, is the device that receives the information. Again, such devices can include telephones and computers; in a videoconferencing application, the display device would be the receiver.
- 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 is often used interchangeably with path, link, line, and channel, although such usage can be specific to the underlying technology, the context, and other factors. Circuits are either two-wire or four-wire, depending on the requirements of the specific application and therefore on the fundamental nature of the network.
- 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). Link is sometimes used interchangeably with line or circuit. A circuit may consist of a single link, as often is the case between a host computer and a peripheral.
- Line
- The term line has several definitions, resulting in confusion. In a PBX environment, a station line refers to the connection between the PBX switch and the station users terminal equipment, whether it be an analog or digital, telephone or workstation. In rate and tariff terminology, it refers to a local loop connection from the telephone company Central Office (CO) switch to the users premises in support of premise equipment other than a switch; for example, a single-line residence or business set, a multiline set, or a key system common control unit. In any event, a line refers to a voice-grade circuit; in other words, a circuit serving a single physical location, with a single telephone number and generally supporting a single transmission. Internal to the telephone companies, line is used to describe the user side or local loop side of the connection. In other words, the line side is the side of the network to which users connect, while the trunk side involves the high-capacity trunks that interconnect the various telephone company switching centers.
- Trunk
- A trunk is a communications circuit that is available to be shared among multiple users on a pooled basis and with contention for trunk access managed by an intelligent switching device. Trunks connect switches. For example, tie trunks connect PBXs in a private, leased-line network, central office exchange trunks connect PBXs to telephone company central office exchanges, and interoffice trunks interconnect central office exchange switches. Trunk groups are groups of trunks serving the same special purpose; examples include WATS (Wide Area Telecommunications Service) and DID (Direct Inward Dial) trunk groups. Trunks are directional; they can be 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, it is a one-way circuit or path. In data processing terminology, particularly IBM, a channel is a high-speed connection between mainframe and peripheral. In common usage, a channel is a logical connection over a physical circuit to support a single conversation. A physical circuit can be treated in such a way as to support one or many logical conversations. Multichannel circuits are always four-wireeither 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 (CO) exchanges. Switching traditionally has been accomplished on a circuit basis, for the establishment of connections between circuits on demand and in support of voice and data communications. Packet, frame, and cell switching have recently evolved in more sophisticated networks, primarily in support of data and image transfer.
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