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      The VACETS Technical Column is contributed by various members , especially those of the VACETS Technical Affairs Committe. Articles are posted regulary on vacets@peak.org forum. Please send questions, comments and suggestions to vacets-ta@vacets.org

The World of Communications

Thao Mong Le

The term "communications" is defined as the act of exchanging information. This information may be in the form of words, numbers, messages or drawings and is transmitted in the form of signals. Such passing of information generally involves three activities: encoding, transmission, and decoding. Encoding is the process of placing the information on a carrier. For example, the vibration of our vocal chords places the code of our voice on air. The air is a carrier, changed to carry information by the code of our vocal chords. Once information has been encoded by modulating a carrier, it is transmitted. Transmission can be via air, copper cables (e.g., twisted cables and coaxial cables), through space to a satellite and back, or through optical fiber. At the end of the transmission path, the receiver separates the information from the carrier in the decoding process. Today, communications represents the vast industry of telecommunications and data processing.

A system of routes or paths through which information travels from one point to another is defined as a communications network. In a data communications network, these paths may interconnect data terminals and computers within a single building or campus-like complex, to large geographically distributed networks covering entire countries. A communications network has become a fundamental part of the computing environment, adding more efficiency in information processing. The oldest and most obvious example of the communications network is the telephone network. Previously, this network carried voice almost exclusively, whereas this network now carries voice, computer data, electronic mail, and video. Our office personal computer or terminal can now communicate with the world outside the office.

Networks are comprised of three basic categories of equipment: input/output devices, transmitter/receiver, and transmission medium. A personal computer or terminal attached in the network becomes a node or a workstation which is referred to as an input/output device or data device. This device is usually connected directly to a transmitter or a receiver which is used to generate or regenerate transmission signals. The devices attached in the network also include communications devices, which have a variety of functions such as routing data, merging several low-speed transmission into one high-speed transmission and interconnecting networks . Front-end Processors, concentrators, multiplexers, bridges and modems are examples of communications devices. The transmission media, that are used to carry transmission signals, play an important role in a communications network. Standard telephone lines, coaxial cables, optical fiber cables, satellite, microwave are examples of transmission media.

Copper cables are often used to transmit electrical signals among data devices and play an important part of the current telephone networks. Communication by fiber optic cables is a new technique which uses pulses of light (as an information agent) to propagate information from source to destination through hair-thin strands of plastic or glass. Communication by satellite begins at a single earth station, passes through a satellite in geosynchronous orbit above the earth, and ends at one or more earth stations. The satellite itself serves as an active relay system consisting of transponders and antennas. Microwave communications involves sending waves of information between a radio transmitter and a radio receiver, each mounted on a tower.

In a communications network, a set of rules and regulations that governs the transmitting and receiving of data is called a communications protocol. Depending on the protocol, networks are capable of providing information sharing, resource sharing, cost reduction, increased productivity, central management of resources and connectivity. Networks are often required to provide the end-to-end connections for a variety of source and destination devices with vastly different characteristics. Shared resources also include the storage attached to the server/mainframe that runs the network operations. Networks may be local or wide area in scope, or they may be combined into one network serving, for example, corporate headquarters and regional offices. Today, organizations may develop their own private networks as an alternative to using telephone company services.

As shared resources, modern wide-area networks must meet diverse objectives of numerous users at acceptable costs. In making the trade-offs that diversity entails, network topology, the pattern of interconnection between nodes, is a fundamental consideration. These topologies fall broadly into three categories: private line, circuit switched and packet switched. Any network may selectively employ a combination of these topologies to meet a particular business or technical requirement. Early networks employing private, leased-line connections from terminal to host equipment represent a prime example of dedicated transmission facilities. Each physical device attached to the network requires its own separate leased line to connect to the host. Circuit switching is used for normal telephone operation, sharing telephone network resources among users. In the packet switching, network data is transmitted in a block, referred to as a packet, that is well defined and limited to a maximum size. Packet switching is suited to interactive traffic. A typical communications protocol for accessing a packet-switched network is X.25. This X.25 provides for sequence numbers in packets, which contain data, to allow a transmitting user and a receiving network node to keep packets in proper order and to provide a way for them to acknowledge their successful receipt.

As networking requirements have increased in complexity and sophistication, the disparity among users' needs has sent designers of networking protocols looking hard for common grounds that were leading to develop network architectures. Today, the significant network architectures include the Open Systems Interconnection (OSI) Reference Model, International Business Machines, Inc. (IBM)'s Systems Network Architecture (SNA), Transport Control Protocol/Internet Protocol (TCP/IP) and Digital Equipment Corporation (DEC)'s Digital Network Architecture (DNA). The OSI standards were developed by the International Standards Organization (ISO) and ITU in response to the need for multi-vendor computer interoperability. The OSI is designed to foster communication among machines with diverse hardware architectures to use almost any packet switched network hardware. The TCP/IP provides the reliable services on which many application protocols depend and offers a well-documented and mature set of rules for networking.

During the past 12 years, there have been many innovations in the communications field, with fiber optic technology among them. Using light for communication, fiber optic transmission has become the medium of choice in the telephone networks. Fiber optics systems already have reshaped the face of telecommunications the world over. In the United States, the largest fiber optic user to date, the telephone industry has been the driving force in fiber optic technology. Two fiber optic technology applications, Fiber Distributed Data Interface (FDDI) and Synchronous Optical Network (SONET), have just become commercially available. These new technologies are capable of transporting data at a very high-speed in a local area network or wide area network via fiber optic cables.

The world of communications networks is continuing to be developed at an accelerating rate. Merging communications and computer technologies have sparked innovations that are transforming global and local activities of all sorts. One most promising development in the evolution of packet switching is the fast packet switching technology. This technology consists of the frame relay technology and cell relay technology that lead to the developments of Integrated Services Digital Network (ISDN), Broadband-ISDN (B-ISDN)/Asynchronous Transfer Mode (ATM), and Switched Multi-Megabit Data Service (SMDS).

As a result of end-user demand, more and more network applications require higher speeds to across a Local Area network (LAN)/ Wide Area Network (WAN). While X.25 packet switching and TDM circuit switching technologies have a limitation of low speed, high delay, and limited bandwidth, Frame Relay and ISDN enable WANs to solve many issues of this limitation, including LAN-LAN, LAN-WAN interconnections, transmission speeds, bandwidth, routing, flow control, network congestion, and recovery from a line failure.

Today, Asynchronous Transfer Mode (ATM) is considered as the most advanced technology that can help an organization dealing with various communications issues raised by current network technologies. For example, AT&T recently offered General DataComm's (GDC) Asynchronous Transfer Mode (ATM) systems as part of its broadband ATM solutions for public and private networks. ATM is also becoming an important technology for the provisioning of strategic as well as tactical military communications. The use of ATM, and dependency on, in the satellite communications is increasing in the military arena.

It is certain that the flexibility of ATM and its advanced features and potential low cost could lead to vast improvement and simplification of most networks architectures. In addition to providing the solutions as Frame Relay and ISDN do, ATM provides applications that can be used for productive gains for that competitive edge. Some of these include:

* video-on-demand; * distance-learning; * multi-party video-conferencing; * multi-player games; * personalized news on demand; * concurrent engineering using CAD/CAM; * optimised distributed database updates; * real-time transaction processing; * remote visualization of high resolution images as found in telemedicine and teleradiology or as in molecular modeling; * multimedia kiosks; and * multimedia patient record systems.

The communications technology plays an important role in our life today. It supports not only from internetworking to electronic publishing, from electronic fund transfer to financial industry reports, but also from videoconferencing to medical image, more affordable health care, to expanding telecommuting, work at home, and many, many more.


Thao Mong Le
tle@gatewayone.com

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Copyright © 1994 - 1998 by VACETS and Thao M. Le

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