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
[email protected]
For
discussion on this column, join [email protected]
Copyright
© 1994 - 1998 by VACETS and Thao M. Le
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