Data Communication and Networks

Introduction:
Information is carried out in data communication systems as signals between two or more points. To understand transmission one needs to study electrical form that the messages take while they are in transit and of media and transmission technologies.

Signals:
Signals are electric or electromagnetic encoding of data, and signaling is propagation of signal along suitable communication medium.

Transmission:
Transmission is communication of data achieved by the propagation and processing of signals.

Parallel and Serial Transmission:
In parallel data transmission, there are multiple parallel lines connecting the transmitting and receiving units. Each wire carries a bit of information.

In series data transmission, each bit is sent sequentially one after another and it requires one pair of wires for connecting the receiving and transmitting units.

Series transmission is slower than parallel transmission and is used primarily for transferring data between devices at the same site. Communication between computers is almost always serial.

Synchronous and Asynchronous Transmission:
The mode of transmission is the way in which coded characters are assembled for the purpose of transmission and permits receiving devices to identify where the coding for each character begins and ends within the torrent of bits. When two computers communicate, they must have a way to synchronize the flow of data so that the receiving computer can read at the same speed at which the sending computer transmits.

In synchronous transmission characters are transmitted as groups, with control characters in the beginning and at the end of the bit train. The transmission and receiving intervals between each bit are precisely timed permitting the grouping of bits into identifiable characters. In synchronous mode, intervals between characters are uniform within space between consecutive bytes.

In asynchronous transmission, each character is transmitted separately, that is, one character at a time. Each character begins with a start bit and ends with a stop bit.

The start and stop bits and the interval of time between consecutive characters allow the receiving and sending computers to synchronize the transmission.

Simplex, Half-duplex, Full-duplex Communication:
The direction in which information can flow over a transmission path is determined by the properties of both the transmitting and the receiving devices.

There are three basic options:
In Simplex mode, the communication channel is used in one direction. The receiver receives the signals from the transmitting device. The simplex mode is rarely used for data communication.

In Half-duplex mode, the communication channel is used in both directions, but in one direction at a time. This requires the receiving and transmitting devices to switch between send and receive modes after each transmission.

In Full-duplex mode, the communication channel is used in both directions at the same time. Typical example of this mode of transmission is the telephone in which both parties talk to each other at the same time.

Local Area Network (LAN):
A LAN is a data communication network, which connects many computers of workstations (computer terminal, printer etc.) and permits exchange of data and information among themselves, within a localized area, typically confined to a building, or a cluster of buildings.

LAN topology:
A network topology refers to the physical layout of the network in which all the devices are connected. This includes all the hardware that makes up the network. The points of connection to the network by the stations are called Nodes or link stations. There are several types of typographical design and strategies used to implement LAN. The majority of these are based on three types of topologies:
  • Star
  • Bus
  • Ring

Star Topology:
In this topology, a number of stations are connected directly to a central station or controller. Communications on the connecting links between the stations and the central station of a star topography can be bi-directional and are point-to-point.

A station on this type of network passes information to the central controller, which then forwards the information to the destination station. The central controller manages and controls all communications between stations on the network.

Bus Topology:
All stations are connected to a single communication line. This single communication line is referred to as a bus. Information frames originating at a station are propagated away from the station in both directions on the bus. Each station on the bus interrogates the information frame destination address field for its own address. If the destination address field matches the station address, only then it accepts the information frame and processes the frame.

Ring Topology:
Local area networks that have each station attached to an adjacent station using point-to-point links form a physical ring. Each station attached and active to the ring regenerates the information frame, and then re-transmits the information frame on the ring. The ring itself is logically circular and the information travels in one direction.

Failure of a station in a ring topology disrupts the ring because the information frame is not regenerated. Additions or deletions of stations to the ring can be disruptive, if the changes are not managed properly.

Lan Access Method:
Access methods are the means or way by which stations actually gain the use of the common channel to transmit messages. The right to transmit is an issue only in broadcast where work stations share a single channel.
Many techniques have been proposed, but two of these are commonly used.
  • Carrier-Sense Multiple Access with Collision Detection (CSMA/CD)
  • Token passing.

CSMA/CD:
CSMA/CD access method is used with bus networks. The bus operates in a Multiple Access (MA) mode. A node is allowed to transmit on the bus, if it senses that the medium is free (carrier sense). Occasionally, two or more nodes may simultaneously sense that the medium is free and begin to transmit. This creates a collision as the contents of transmitted information frames will collide resulting in corruption of the information frame. This collision is detected (collision detection) by the transmitting node. The two (or more) nodes involved then wait for a further short random time interval before trying to retransmit a frame.

Token Passing:
Another way of controlling access to a share medium is by the use of a control (permission) token. The control token is passed from one node to another according to a defined set of rules understood and adhered to by all nodes.
In token passing, a logical ring of all nodes connected to the physical medium is first established and a single token is generated; the control token passes from one node to another traversing the logical ring. The token keeps on circulating the logical ring until it is received by a node waiting to send an information frame. After receiving the token, waiting station transmits its waiting frames on the physical medium after which it passes the control token to the next node in the logical ring.

Wide Area Network:
A Wide Area Network, or WAN, is a network that links separate geographical locations. A WAN can be a public system such as the Public Switched Telephone Network (the PSTN) or one of the various packet switched services provided by the public telecommunication authorities. WANs can also use most other types of circuit including satellite networks, ISDN, Value Added Networks (VANs/ VADs).
The main distinguishing feature between a WAN and LAN is that the LAN is under the complete control of the organization which owns it, whereas the WAN needs the involvement of another authority like the telephone company.

Communication Switching Techniques:
In a WAN, two computing devices are not connected directly. Networks of switching nodes provide a transfer path between the two devices.
The process of transferring data blocks from one node to another is called data switching.

There are three switching techniques commonly employed and these are:

Circuit Switching:
In Circuit switching there is a dedicated communication path between the sending and receiving devices. The dedicated path is a connected sequence of links between switching nodes. A conventional telephone network, where a dedicated path is set between the caller and the called party for the duration of a telephone call, is an example of circuit switching.

Circuit switching is mainly used for voice telephone network, but is not all that effective for data communication networks, as channel capacities are not fully utilized.

Message Switching:
Message Switching is an alternative switching technique, where it is not necessary to establish a dedicated path between the sending and receiving devices. In Message Switching, the sending device appends the destination address to the message and passes it to the network; the message is then passed through the network from one node to another till it reaches the intended destination. Each switching node receives a message, stores it briefly and then transmits it to the next node. Examples of a message are electronic mails, computer files, telegrams and transaction queries and responses.

Packet Switching:
Packet Switching combines the advantages of message and circuit switching. Packet switching is functionally similar to message switching, in which data is transmitted in block, stored by the first switching node it meets in the network and is forwarded to the next and subsequent downstream nodes until it reaches the destination.

Network/ File server system:
For sharing data in a LAN, users stores files on a file server. A file server is a central node (Computer in the network) that stores data files where all users can access them. Typically, the file server in LAN acts as a central hub for sharing peripherals like printers, print queues and modems. In a LAN's file server, in many cases entire files are pumped across the network on behalf of the operations taking place on LAN computers. A file server does not involve in processing of an application. It simply stores files for applications that run on LAN computer.

Client/ Server system:
Any local area network could be considered as client/ server system, since the workstation (clients) requests services such as data, programme files, or printing from server. A client/ server (C/S) has three distinct components, each focusing on a specific job: a database server, a client application and a network.

A server gives stress on efficiently managing a resource such as a database of information. Its main job is to manage its resources optimally among various clients that concurrently request the server for the same resource.

Computer Output Devices

The Visual Display Unit:
The Visual Display Unit (VDU) is a device used for interactive processing, i.e. data that is being keyed in, is displayed on the screen or monitor. Messages and processed information are also displayed on the screen. The combination of keyboard and the VDU is usually referred to as Video Display Terminal (VDT), which is an input/ output (I/O) device.

The formation of images is controlled by the video controller. The video controller along with the memory is termed as the display adaptor.
The number of dots (pixels) on the screen is the measure of resolution of monitor.

Display adaptors are of various types and are normally classified on the basis of resolution, colour and display mode:

MGA: Monochrome Graphics Adaptor (MGA). The term graphics is a misnomer since the device supports text only. Although the option is monochrome or single colour, it offers various attributes e.g. the brightness and intensity of characters which can be changed.

CGA: Colour Graphics Adaptor supports both text and graphics mode.
It functions in colour and monochrome modes in various resolutions. The CGA works with different types of monitors. However, it gives poor display quality in the text mode. The typical resolution is 640 * 200 (i.e. 640 rows * 200 columns) in the graphics mode.

HGA: Hercules Graphics Adaptor is a monochrome adaptor with an additional graphics display mode which provides high resolution monochrome graphics. In text mode it functions like the MGA but in the graphics mode it offers a resolution of 750 * 350 dots, even better than the CGA. In the graphics mode the HGA has no colour although it offers brightness and intensity variations.

EGA: Enhanced Graphics Adaptor was developed in an effort to unify the variety of display adaptors. The EGA combines all the features of other adaptors and provides higher resolutions using higher quality colour monitors. The EGA supports 16 colours at a time.

VGA: Video Graphics Array supports 256 colors at a time with a high resolution.

SVGA: Super Video Graphics Adaptor supports 24-bit true colour up to 1024* 768 dots.

Printers:
The results of processing could be written by the computer onto a tape or disk, to be used at a later time or to be given to another computer as input. However, the most common form of computer output is printed output - also called hard copy output. Printers are classified by how they print and how fast they operate.

Character Printers:
Character Printers print one character at a time and are used for low-volume printing jobs.

Dot Matrix Printer:
The dot matrix printer is a versatile low-cost device capable of printing in various languages, printing letters of various types, in bold, italics or underlined. It can also be used to print graphics.

Ink Jet Printers:
Droplets of ink are electrically charged after leaving a nozzle. The droplets are guided to the proper positions on the paper by electrically charged deflection plates. Print quality is good because each character is formed by dozens of ink dots. Text and graphics produced are of better quality and the printer is comparatively fast. They also have the ability to use multiple-nozzle print heads, thus enabling it to print in several colors.

Laser Printers:
Laser printers are page printers. A page of text or pictures is composed at a time. A laser printer utilizes a laser beam that sensitized selected areas on a print page. The laser - exposed areas attract a toner (an ink powder) that attaches itself to the laser generated charges on the drum. The toners is then permanently fused on the paper with heat or pressure. The resolution of print image is up to 1200 dots per inch.

The printer, besides being fast, also produces a high quality print. It can handle large volumes of printed output.

Printing takes time, and often the CPU is the idle while the printing process takes place. Techniques of spooling or printer buffer are used to optimize CPU resources.

Spool: Simultaneous Peripheral Operations On-line.
Using the spooling technique, output is not sent directly to the printer. The output is first transferred to an intermediary storage medium, such as a disk file. Outputs can be stored in separate files and printed out at a later stage as time and resources permit.

Printer buffer:
The CPU transfers print output to temporary memory called a "buffer".
The printer then accesses this information and prepares one or more lines of printed output. During this time the CPU performs other tasks. When all the information is printed, the CPU refills the buffer with more data. Printers contain built-in buffers.

Besides, there are many other types like Daisy Wheel Printer/ Band Printer etc., also.

Data Storage Devices:
Data processing system often requires access to very large quantities of data. The computer memory or Immediate Access Storage (IAS) is unsuitable for this task. The amount of data needed for any commercial system is too large to be held in the processor, and secondly, normal processor memory is volatile, i.e. it does not retain its contents once power is switched off or even if there is a temporary break in power. Some form of backing storage is therefore required.

Main memory:
The memory unit of the CPU is a place where programmes or instructions and data are stored while processing. The device consists of a number of storage locations. Each storage location may be identified by a unique number which is called its address. During processing, data may be stored in any location which is identified by the address of the location.

The storage area may be designed to store a fixed number, of characters which are treated as a single entity or word.

RAM:
Typically, semiconductor elements are used in primary storage sections or main memory. Semiconductor storage elements are small integrated circuits. The storage cell circuits and the support circuitry needed for reading and writing data are packaged on chips of silicon. A number of semiconductor storage technologies are in use. However, chips that use Metal-Oxide Semiconductor (MOS) technology are usually used in the primary storage section. The components are called Random Access Memory (RAM) chips. It is possible to read from and write to any location within RAM by specifying its location or address. New data can be written onto any location. However, while doing so the previous existing data is erased.

RAM chips may be classified as i) Dynamic RAM chips and ii) Static RAM chips.

Dynamic RAM chips: The storage cell circuits contain a transistor (functions like a mechanical on-off light switch) and a capacitor used to store an electric charge.

Depending on the switching of the transistor, the capacitor may have no charge (0 bit) or hold a charge (1 bit). The charge on the capacitor must be periodically refreshed or recharged.

In the event of power loss, dynamic RAM loses its contents. It is thus called volatile storage.

Static RAM chips: They are also volatile storage devices. However, as long as they are supplied with power, they do not require special regenerator circuits to retain the stored data. More transistors and other devices are needed to store a bit in static RAM. These chips are more complicated than dynamic RAMs. Static RAMs are used in specialized applications. Dynamic RAMs are typically used on the primary storage section.

ROM:
There are certain essential functions that the computer must perform when it is switched on (e.g.: establishing connection within the various components of the computer and its peripherals). These low-level or machine-level functions are carried out through a series of programmes or microprogrammes. These microprogrammes are stored on chips. These Read-Only Memory chips (ROM) contain data which can be read randomly when required but cannot be written onto.

Data is hardwired onto these chips at the time of manufacture. They cannot be changed by the user.

While both RAM and ROM are storage devices and can be accessed randomly, they differ in that data can be written onto RAM while ROM does not permit the user to write onto it.

ROM retains the data in it even in the absence of power and is thus nonvolatile storage.

PROM:
There are some types of read only memory called Programmable Read-Only Memory (PROM). Critical or lengthy operations that are slowly carried out by software can be converted into micro programs and fused into a programmable read-only memory chip. Once they are in hardware form, these tasks take a fraction of the time. Each bit can be individually programmed to a ''1" or "0" by burning out a fusible link within the selected cells. A fused link cannot be restored. Operations once written cannot be erased. PROM can be programmed only once.

EPROM:
The Erasable Programmable Read-Only Memory (EPROM) makes it possible for the user to repeatedly erase and reprogram the ROM. Erasing is done by exposing the EPROM to ultra-violet rays of a specific frequency.

EEPROM:
An Electrically Erasable Programmable Read-Only Memory (EEPROM) –
-can be programmed through the use of special electrical pulses. It is possible to integrate the circuitry into the computer, so that the EPROM does not have to be removed from its socket for programming.

CD-ROM:
CD-ROM (Compact Disk Read-Only Memory) is a non-erasable disk used for storing computer data.

The disk is formed from a resin, such as polycarbonate and coated with a highly reflective surface, usually aluminum. Information is imprinted as a series of microscopic pits on the reflective surface. A master disk is created using a finely-focused, high-intensity laser. The master is used to make copies. Atop coat of clear lacquer protects the pitted surface from dust and scratches. A laser shines through the clear protective coating while a motor spins the disk past it. When it encounters a pit, the intensity of the reflected light of the laser changes. The change is detected by photo sensors and converted into a digital signal.
By the use of CD writer it has become possible to use CDs as erasable storage device. Data can be written onto the CD or erased like in a floppy disc.

Virtual Memory:
The size of the memory unit in the CPU is often inadequate, when large programmes need to be stored or large amounts of data need to be processed. To overcome this drawback, the technique of virtual memory is used. In this technique, which is implemented by hardware or software or both, the memory of the computer appears to have been increased. The programme is divided by the software into pages or segments. Only that portion of the programme and data that is being processed is kept in primary storage. Part of backing storage is treated as an extension of the main memory and information that is not required is swapped in and out of main store by the operating system. Thus the size of memory becomes equal to the size of primary storage plus the size of secondary storage being used. Processing time is thus automatically increased.

Cache Memory:
In an instruction cycle, the CPU accesses main memory to fetch the instruction. It also accesses memory one or more times to fetch operands or store results. The rate at which the CPU can execute an instruction is, therefore, limited by the speed of main memory. To build main memory with the same technology required for CPU registers, so that memory cycle times are comparable to processor cycle times, is very expensive. As a solution, a small, fast memory is provided between the CPU and main memory. This is called cache memory.
A copy of portions of main memory is maintained in cache. When the CPU reads a word of memory, it first reads cache. If found, the word is delivered to the CPU.

Gallium Arsenide Chips:
Chips made of gallium arsenide are expected to be five times faster, consume less power and operate at higher temperatures than silicon. These chips are going to replace the presently used integrated chips.

Magnetic Tape:
Magnetic tapes are convenient, inexpensive devices which can be used to store large volumes of data. The magnetic tape is similar to the commonly used audio-tape recorders. The access time in the case of magnetic tapes is quite high.

Uses:
Large volumes of data can be stored.
Data can be transported easily.
Cost effective.
Re-usable medium.
For data that has to be loaded sequentially.

Cartridge Tapes:
Used mainly with micro computers, they are also called "floppy tapes" since it is easy to "flop" the cartridge tape in and out of the recording device. Unlike commercial tape, it does not have to be manually threaded while inserting it onto the tape drive.
They are used with low-speed micro computers and for small-scale applications.

Streamer Tapes Magnetic Disks:
A magnetic disk is a circular platter that may be made of smooth metal or Mylar plastic. This is coated with magnet sable material. Data is stored or retrieved from the disk using a conducting coil called the head. During a read/ write, the head is stationary while the platter rotates beneath it.

Floppy Disks:
Floppies are made of Mylar plastic coated with magnetic oxide. The flexible material is cut into circular pieces 5 1/4 " in diameter. There are mini floppy disks 3 1/2" in diameter.

Since they are made of flexible tape unlike the hard disk, they are called "floppy disks."

The circular pieces are packaged in 5 1/4 " square plastic covers. The 3 1/2 " floppy is covered by a rigid plastic case.

A long slit is provided for the read/ write head to access the disk. A hub in the centre is used for mounting the disk drive. A hole is used to sense index marking.

Optical Disk:
Most storage devices are based on the principle of magnetism. Some storage devices, however, are based on the use of light. Optical disks use this technology. Streams of digital data, in the form of tiny pits, are burned onto a thin coating of metal or other material deposited on a disk. A beam of laser light is used to read these pit patterns. These pits once burned onto the disk cannot be erased. The disk cannot be used for re-recording.

Erasable optical disk:
The erasable optical disk is a recent development. Data can be repeatedly written and overwritten as done with a magnetic disk. The disk is coated with a magnetic material. A laser beam heats a specific spot on the medium and a magnetic field changes the orientation of that spot while its temperature is elevated.
For reading, the direction of magnetism can be detected by polarized laser light.

WORM:
The Write Once Read Many (WORM) disk is prepared in such a way that it can be written once with a laser beam of modest intensity. It is possible to read the disk a number of times.

A high powered laser is used to produce a series of blisters of the disk. When the pre-formatted medium is placed in a WORM drive, a low-powered laser can produce just enough heat to burst the prerecorded blisters. During a disk read operation, a laser in WORM drive illuminates the disks surface. Since the burst blisters provide higher contrast than the surrounding area, these are easily recognised by simple electronics.

Virtual Disk:
RAM disk storage of "Virtual disk" storage is a facility offered by some versions of operating software. A section of the main memory is treated as a disk, with data being organized in files and the same commands being used to access or control it as for a backing storage device requiring Input/ Output. Instead of writing to backing storage, however the data is written onto RAM chips in memory.

Number systems:
All digital computers store numbers, letters, and other characters in coded form. The code used to represent characters is the binary code, i.e., a code made up of binary digits or bits. Every character is represented by a string of "0s" and "1s" - the only digits found in the binary numbering system.

A sequence of 8 bits is called a byte. 1024 bytes make a kilobyte. 1024 kilobytes make a megabyte (MB). 1024 megabytes make a gigabyte. A word consists of several bytes. Most computers have words that consist of 8 or 16 bits. However in larger computers the number of bits could be 16, 32, 36 or 40 bits.

When data is typed into a computer, the keyboard converts each keystroke into a binary character code. This code is then transmitted to the computer. When the computer transmits the data to the printer, or to the screen or to the disk, each individual character is communicated in binary code. It is then converted back to the specific character while displaying or printing the data.

Unit of Information:
Most computers however do not represent characters as pure binary numbers.
They use a coded version of true binary to represent letters and special symbols as well as decimal numbers.

In the English language there are 26 characters. If we also take into consideration the uppercase, special symbols like *, %, +, - etc, the ten decimal digits and non-printable control characters like the carriage, return etc. We have 128 characters. We would. Require 7 digits to uniquely represent all the 128 characters uniquely. Coding of characters has been standardized to enable transfer of data between computers.

ASCII:
The most popular and common standard is the American Standard Code for Information Interchange (ASCII). ASCII uses 7 bits per character. With 7 bits it is possible to provide 128 (27) different arrangements.

Besides codes for character, codes are also defined to convey information such as end of file, end of page etc., to the computer. These codes are called non-printable control characters. The ASCII code is used to represent data internally in personal computers.

EBCDIC:
Another code also exists called External Binary Coded Decimal Interchange Code (EBCDIC). EBCDIC uses 8 bits per character. Thus 256 characters can be represented using EBCDIC. The EBCDIC code is used in IBM mainframe models and other similar machines.

Electronic circuits are available to transform characters from ASCII to EBCDIC and vice versa. We can also achieve the same results using a computer program.

Computer CACHE Controller

Pronounced ''Cash". A special area of memory, managed by a cache controller that improves performance by storing the contents of frequently accessed memory locations and their addresses. When the processor references a memory address, the cache checks to see if it holds that address. If it does, the information is passed directly to the processor; if not, a normal memory access takes place instead. A cache can speed up operations in a computer whose RAM access is slow compared with its processor speed, because the cache memory is always faster than normal RAM.

There are several types of cache architecture:

Direct-Mapped Cache:
A location in the cache corresponds to several specific locations in memory, so when the processor calls for certain data, the cache can locate it quickly. However, since a several blocks in RAM correspond to the same location in the cache, the cache may spend its time refreshing itself and calling main memory.

Fully Associative Cache:
Information from RAM may be placed in any free blocks in the cache, so that the most recently accessed data is usually present; however, the search to find that information may be slow because the cache has to index the data to find it.

Set-Associative Cache:
Information from RAM is kept in sets, and these sets may have multiple locations, each holding a block of data; each block may be in any of the sets, but it will only be in one location within that set. Search time is shortened, and it is less likely that frequently-used data will be overwritten. A set-associative cache may use two, four or eight sets.

Memory Cache:
An area of high-speed memory on the processor that stores commonly used code or data obtained from slower memory, replacing the need to access the system's main memory to fetch instructions.

Disk Cache:
An area of computer memory where data is temporarily stored on its way to or from a disk. A disk cache mediates between the application and the hard disk and improves hard disk performance.

Caps Lock Key:
A toggle key on the keyboard that shifts the alphabetical characters on the key board into uppercase when it is on. The Caps Lock key does not change the case of the numbers or punctuation keys on the keyboard; you must use the Shift key for this.

Card:
A printed circuit board or adapter that you plug into your computer to add support for a specific piece of hardware not normally presents on the computer.

Expansion Bus:
An extension of the main computer bus that includes expansion slots for use by compatible adapters, such as including memory boards, video adaptors, hard disk controllers, and SCSI interface cards.

PC Card:
A term describing add-on cards that conform to the PCMCIA (Personal Computer Memory Card International Association) standard.

PCMCIA:
Abbreviation for PC Memory Card International Association. The majority of PCMCIA devices are modems, Ethernet and Token Ring network adapters, dynamic RAM, flash memory cards, mini-hard disks, wireless LAN adapters, and SCSI adapters etc.

PDA:
Abbreviation for personal digital assistant. A tiny pen based palmtop computer that combines fax, e-mail, PCMCIA support, and simple word processing into an easy-to-use unit that fits into a pocket. PDAs are available form several manufacturers.

Card Services:
Part of the software support needed for PCMCIA hardware devices in a portable computer, controlling the use of system interrupts, memory, or power management.
When an application wants to access a PC Card, it always goes through the card services software and never communicates directly with the underlying hardware.

Cascade:
In a windowed environment, the arrangement of several overlapping windows so that their title bars are always visible. The windows appear to be stacked, one behind the other.

Cascading Menu:
In a graphical user interface, a menu selection that leads to one or more further menus; usually indicated by a right-pointing triangle.
If you choose one of these commands, the cascading menu opens to the right of the original menu, and you make your selection from this menu just as you would from any menu. Cascading menus are often used to reduce the number of entries in a main menu.

CD-R:
Abbreviation for CD Record able. A type of CD device that brings CD-ROM publishing into the realm of the small business or home office. From a functional point of view, a CD-R and a CD-ROM are identical; you can read CD-R discs using almost any CD-ROM drive, although the processes that create the discs are slightly different.

CD-ROM:
Acronym for Compact Disc-Read Only Memory, pronounced "see-dee-rom". A high-capacity, optical storage device that uses compact disc technology to store large amounts of information up to 650 MB.
CD-ROMs are important components of multimedia PCs, and are used to store encyclopedias, dictionaries and other large reference works, libraries of fonts and clip art for desktop publishing, and are increasingly used as the distribution mechanism for large software packages.
CD-ROMs are usually considered to be WORM (write once, read many) devices, but several vendors (including Kodak) are working toward a format that will allow home users to add information to an existing multi session compact disk.

Clip Art:
A set of non-copyrighted or public domain graphical images, photographs, maps, or line art, usually on disk, that you can import into a word processor, desktop publishing program, or presentation graphics program and incorporate into other documents.
You can then resize, rotate, or edit the image to your satisfaction.

Clipboard:
An area of memory reserved for temporary storage of text or graphics being transferred within the same file, between files in the same application program, or between applications. Material placed on the clipboard remains there until it is replaced by another selection, or the computer is turned off or restarted.

Clock:
An electronic circuit that generates regularly-spaced timing pulses at speeds of up to millions of cycles per second.
These pulses are used to synchronizes the flow of information through the computer's internal communications channels.

Clock/Calendar board:
An internal time-of-day and month-year calendar that is kept up-to-date by a small battery-backup system. This allows your computer to update the time even when turned off. Appointment scheduling programs and programs that start at specific times use the output from the clock/calendar board.

Clock speed:
Also known as clock rate. The internal speed of a computer or processor, normally expressed in MHz.
The faster the clock speed, the faster the computer will perform a specific operation, assuming the other components in the system, such as disk drives, can keep up with the increased speed.

Cluster:
The smallest unit of hard disk space that DOS can allocate to a file, consisting of one or more contiguous sectors.
The number of sectors contained in a cluster depends on the hard disk type.

Crash:
An unexpected program halt, sometimes due to a hardware failure, but most often due to a software error, from which there is no recovery. You will probably have to reboot your computer to recover after a crash.

Compatibility:
The extent to which a given piece of hardware or software conforms to an accepted standard, regardless of the original manufacturer. In hardware, compatibility is often expressed in terms of certain other widely accepted models, such as a computer described as IBM-Compatible, or a modem as Hayes-compatible. This implies that the device will perform in every way just like the standard device.
In software, compatibility is usually described as the ability to read data file formats created by other vendors' software, or the ability to work together and share data.

Compiled basic:
A version of the BASIC programming language that allows you to compile the program, rather than run the BASIC interpreter.
Compiled programs generally tend to execute faster than interpreted programs, which translate and execute instructions line-by-line through the program.

Compressed file:
A file that has been processed by a special utility program so that it occupies as little hard-disk space as possible. When the file is needed, the same program decompresses the file back into its original form so that it can be read by the computer.

Computer-aided design:
Abbreviated CAD; the use of a computer and specialized CAD software in the design of a product. Specialized CAD systems are used to design buildings and landscapes, aircraft, mechanical parts, or printed circuit boards.
CAD reduces the time needed to create, edit, store, and transmit drawings by using high-performance computers and monitors, with input devices like scanners and graphics tablets

Computer-Aided Software Engineering:
Abbreviated CASE. Development software that is used to aid all aspects of the software life cycle, including the design, coding, testing, documenting, and maintenance of software.
CASE provides a set of programming and development tools that help programmers to automate the production of business, technical, and engineering software.

Computer-integrated manufacturing:
Abbreviated CIM. The integration of automated factory systems with office and accounting functions. Sales, billing, work order creation, machine tool scheduling inventory control, and purchasing all access a common database that is used throughout all aspects of the manufacturing process by all departments.
Some advanced CIM systems also include CAD functions and robotic assembly lines.

Control Menu:
In Microsoft Windows, the menu that appears when you click on the Control menu box at the top left corner of any application window, or when you click on any application icon.
The control menu lists options you can use to change the overall size and shape of the active window, as well as close the current window or switch to another application window.

Control Panel:
In the Macintosh and in Microsoft Windows, a selection that contains setting to control hardware options such as the mouse, display, and keyboard.
On the Macintosh, the Control Panel is a desk accessory, while in Windows, it is a program contained in the Main group window.

Copy:
To duplicate part of a document and reproduce it elsewhere. The material copied can range from a single character to pages of text and graphics. A copy operation leaves the original in place and unchanged.

Crop marks:
In desktop publishing, small intersecting lines at the corners of the page used if the final page size will be smaller than the paper size. The crop marks show where the paper should be cut down to the desired size.

Cursor:
A special character on a display screen to indicate where the next character will appear when it is typed. In text or character mode, the cursor is usually a blinking rectangle or underline. In a graphical user interface, the cursor can take many shapes, depending on the current operation, and may also change shape as it moves to different parts of the screen.

Cursor-movement keys:
The keys on the keyboard that move the cursor, including the four keys labeled with arrows, as well as the Home, PgUp, End, and PgDn keys.

Device:
ADOS and OS/2 command used in the start-up configuration file, CONFIG.SYS, to load a specific device driver into memory. For example, DOS does not contain software to manage a mouse, so before you can use one on your system, you must first load the appropriate device driver.

Device driver:
A small program that allows a computer to communicate with and control a device.
Each operating system contains a standard set of device drivers for the keyboard, the monitor, and so on, but if you add specialized peripherals such as a CD-ROM disk drive, or a network interface card, you will probably have to add the appropriate device driver so that the operating system knows how to manage the device.

Device name:
The name used by the operating system to identify a computer-system component.
For example, LPT 1 is the DOS device name for the first parallel part.

Diagnostic program:
A program that tests computer hardware and peripherals for correct operation. In the PC, some faults are easy to find, and these are known as "hard faults"; the diagnostic program will diagnose them correctly every time. Others, such as memory faults, can be difficult to find; these are called "soft faults" because they do not occur every time the memory location is tested, but only under very specific circumstances.

Dhrystone:
Pronounced "dry-stone". A standard general-purpose benchmark program used to quantify and compare the performance of different computers. The program reports system performance as the number of times that the program can operate per second. This benchmark program concentrates on string and general-purpose instructions.

Internet and Web Knowledge

Introduction:
The Internet is global collection of people's computers, which are linked together by cables and telephone lines making communication possible among them in a common language. However, the rigid technological definition of Internet is that it is a global collection of interconnected networks. By definition, a network allows computer users to share computer equipment and programs, messages, and the information available at one site.

How does Internet Work?
A computer network by definition allows sharing of resources. Since all software resources exist in computers in the form of files of data, one of the key aspect in network of many computers is to move data between two specific computers. For such a communication, we require:
1. The address of the destination.
2. A safe means of moving data in the form of electronic signals.

As far as safe movement of data is concerned, there exists a set of rules, which govern the sending and receiving of data on the Internet. These rules are implemented in two parts in the networks software and are called Transmission Control Protocol (TCP) and Internet Protocol (IP). These two are collectively called TCP/IP. For sending a large block of text/data to another machine, TCP divides the data into little data packets. It also adds special information, e.g. the packet position; error correction code etc., to make sure that packets at the destination can be reassembled correctly and without any damage to data. The role of IP here is to put destination-addressing information on such packets.
The following can be a typical person-understandable address on Internet: user name@host.domain

The user name in general is the name of the Internet account. This name is same as the one, which you may use when logging into the computer on which you have your Internet account (Logging in is the process of gaining access to your account on a computer, which is shared by several users). Your Internet account is created on it.

Hosts are in general, individual machines at a particular location. Resources of a host machine are normally shared and can be utilized by any user on Internet.
Domains are general category that a computer on the Internet belongs to. The most common high level domains are .com, .org etc.

Tools & Services on Internet:
To work with Internet and to utilize some of the points mentioned above we use certain tools. For example, Telnet is a tool, which is utilized for logging on remote computers on the Internet.

Electronic Mail:
One of the very useful things about the Internet is that it allows you almost instant exchange of electronic message (e-mail) across the world. E-mail is mainly used for sending electronic piece of text.

USENET and News Groups:
On Internet there exists another way to meet people and share information. One such way is through USENET newsgroups. These are special groups set up by people who want to share common interests ranging from current topics to cultural heritages.
The newsgroups are really meant for interaction of people who share your interests.
You can post your own questions as well as your answers to the questions of others, on the USENET.

Frequently Asked Questions (FAQ):
A great resource offered by USENET is the FAQs, that is the list of frequently asked questions and responses for them for particular newsgroup. FAQs are an excellent sharing place for learning about a topic.
These FAQs are generally text files or USENET articles.

Connecting to Remote Machine with Telnet:
Telnet is a program that allows an Internet host computer to become a terminal of another host on the Internet. Telnet allows becoming a user on a remote machine. You can run the Internet computer programs available on that machine.

Gopher:
Gopher displays a set of resources on the Internet in the form of menus or lists of items. You go around the Internet by selecting items from these menus. You need not know the addresses and commands. You just select an item of interest to see its content on the screen.

HTML:
The Hyper Text Markup Language (HTML) is a language used for creating documents for the World Wide Web. Although most browsers will display any document that is written in plain text, there are advantages to writing documents using HTML. When HTML documents are read by applications specifically designed for the WWW, they can include formatting, graphics and even links to other documents and websites on the World Wide Web.

SGML:
Standard Generalized Markup Language (SGML) is used for defining the structure and managing the contents of any digital document. HTML used in many World Wide Web document on the Internet is a part of SGML.

World Wide Web:
It is a huge collection of hyper text pages on the Internet. It is one of the most flexible and exciting tools in existence for surfing the Internet.
Hyper text link connects the pieces of information (text, graphics, audio or video) in separate HTML pages created at the same or at different Internet sites.

HTTP:
Hyper Text Transport Protocol (HTTP) is used to manage the links between one hyper text document and another.
HTTP is the mechanism that opens the related document when you click on a hyper text link, no matter where on the Internet that related document happens to be.

Home Page:
It is an initial starting page. A home page on the Internet may be related to a single person, a specific subject, or to a corporation, and is a convenient jumping-off point to other pages or resources.

URL:
Uniform Resource Locater (URL) is a method of accessing Internet resources. URLs contain information about both the access method to use and also about the resource itself, and are used by web browsers to connect you directly to a specific document or page on the World Wide Web, without you having to know where that resources is located physically.

Veronica:
A search service built into the Gopher Internet application. When you use Veronica to search a series of Gopher menus (files, directories, and other items), the results of the search are presented as another Gopher menu, which you can use to access the resources your search has located. Veronica supposedly stands for Very Easy Rodent oriented Net-wide Index to computer Archives.

WAIS:
Abbreviation for Wide Area Information Service, pronounced "ways". A service used to access text databases or libraries on the Internet.
WAIS uses simple natural-language queries, and takes advantage of index searches for fast retrieval. Unlike Gopher, which only searches through the names of Gopher resources, WAIS can search the content of all documents retrievable from WAIS databases. WAIS is particularly adept at searching through collections of articles, USENET newsgroups, electronic texts and newspaper archives.

Browser:
An application programme used to explore Internet resources.
A browser lets you wander from node to node without concern for the technical details of the links between the nodes or the specific methods used to access them, and presents the information- text, graphics, sound, or video - as a document on the screen.

Web browser:
A World Wide Web client application that lets you look at hyper text documents and follow links to other HTML documents on the Web. When you find something that interests you as you browse through a hyper text document, you can click your mouse on that object, and the browser automatically takes care of accessing the Internet host that holds the document you requested; you don't need to know the IP address, the name of the host system, or any other details.

Mosaic:
It is a World Wide Web client program. Mosaic uses a graphical user interface to give access to Internet resources, and allows users to navigate through hyper text documents quickly and easily using a mouse.

FTP:
Abbreviation for the File Transfer Protocol. The protocol used to access a remote Internet host, and then transfer files between that host and your own computer. FTP is also the name of the program used to manage this protocol. FTP is based on client/ server architecture; you run an FTP client program on your system, and connect with an FTP server running on the Internet host computer.

Archie:
A system used on the Internet to locate files available by anonymous FTP. Once a week, special programs connect to all the known anonymous FTP sites on the Internet and collect a complete listing of all the publicly available files. This listing is kept in an Internet archive database, and when you ask Archie to look for a file, only this database is searched rather than the whole Internet; you can then use anonymous FTP to retrieve the file.

Modem:
Contraction of modulator/ demodulator; a device that allows a computer to transmit information over a telephone line. The modem translates between the digital signals that the computer uses, and analog signals suitable for transmission over telephone lines. When transmitting, the modem modulates the digital data onto a carrier signal on the telephone line. When receiving, the modem performs the reverse process, and demodulates the data from the carrier signal.

Internal modem:
A modem that plugs into the expansion bus of a personal computer.

External modem:
A stand-alone modem, separated from the computer and connected by a serial cable.
LEDs on the front of the chassis indicate the current modem status, and can be useful in troubleshooting communications problems.

X-modem:
In communication, a popular file transfer protocol available in many off the- shelf and shareware communications packages, as well as on many bulletin boards.
X-modem divides the data for the transmission to blocks; each block consists of the start-of-header character, a block number; 128 bytes is returned to the sender if the check sum calculation is identical to the sender's check sum; however, this requirement to acknowledge every block can cause poor performance.
An extension to X-modem, called X-modem-CRC, adds a more stringent error checking method by using a cyclical redundancy check to detect transmission errors rather than X-modem's simple additive check sum.

Y-modem:
Y-modem, a variation of the X-modem protocol, divides the data for the transmission into blocks; each block consists of the start-of-header character, a block number, 1K of data, and a check sum. Y-modem's larger data lock means less overhead for error control when compared with X-modem, but if the block has to be retransmitted because the protocol detects an error, there is more data to resend.
Y-modem also incorporates the ability to send multiple files in the same session, and to abort file transfer during the transmission.

Z-modem:
Z-modem is similar to X-modem and Y-modem but is designed to handle larger data transfer with fewer errors, Z-modem also includes a feature called checkpoint restart that allows an interrupted transmission to resume at the point of interruption, rather than starting again at the beginning of the transmission. If you have a choice between several protocols, choose Z-modem if you can; it is fast as well as convenient.

Newbie:
A newcomer coming to Internet world is known as a newbie.

Netiquette:
A contraction of network etiquette. The set of unwritten rules governing the use of e-mail, USENET news groups, and other online services.

USENET:
Contraction of User Network an international, non commercial network, linking many thousands of Unix sites.
Although there is a very close relationship between the Internet and USENET, they are not the same thing by any means.
USENET predates the Internet; in the early days, information was distributed by dial - up connections. Not every Internet computer is part of USENET, and not every USENET system can be reached from the Internet.
Like the Internet, USENET has no central governing body; USENET is run by the people who use it.

Moderated Newsgroup:
On the Internet, a USENET newsgroup of mailing list which is managed by one or more people in an attempt to maintain standards for the newsgroup. All posts to the newsgroup are reviewed by the moderator to make sure that they meet the standards the newsgroup has set for subject and commercial content before being passed on to the whole group. Moderation is not censorship, but an attempt to avoid some of the more extreme antics of those who enjoy flaming and flame wars.

Moderator:
A person or small committee of people who review the contents of all posts to a USENET newsgroup or mailing list in an attempt to ensure that the postings meet the standards set by the group.
Moderators are almost always volunteers.

Un-moderated newsgroup:
A USENET newsgroup or mailing list in which posts are not subject to review before distribution. You will find the discussions in un-moderated newsgroups to be wildly spontaneous, but they will also contain more than their fair share of flames and flame wars.

Flame:
A deliberately insulting e-mail message or post to a USENET newsgroup, usually containing a personal attack on the writer of an earlier post. Flames are often generated by established newsgroup members when a newbie posts a question which is answered in the newsgroup's FAQ.

Flame bait:
An insulting or outrageous posting to a USENET newsgroup specifically designed to provoke other subscribers into flaming the originator.

Flame war:
A prolonged series of flames in a USENET newsgroup which may have begun as a creative exchange of views but which quickly degenerates into personal attacks and crude name - calling.

Post:
An individual article or e-mail message sent to a USENET newsgroup or to a mailing list, rather than to a specific individual. Post can also refer to the process of sending the article to the newsgroup.

Anonymous posting:
In a USENET newsgroup, a public message posted via an anonymous server in order to conceal the identity of the original author. This server removes all the information from the message that could identify the sender, and forwards the message to its destination. If you ever use an anonymous server, don't forget to remove your signature from the bottom of your posting.

Mime:
Abbreviation for Multipurpose Internet Mail Extensions. A set of extensions that allows Internet e-mail users to add non - ASCII elements such as graphics, post script files, audio or video to their e-mail. Most of the common e-mail client programs include mime capabilities.

Anonymous FTP:
A method of accessing an internet computer with the FTP (file - transfer program) which does not require that you have an account on the target computer system. Just login to the Internet computer with the user name anonymous and use your e-mail.

EVERGREEN POSTS