Definition and Objectives

Computer-based communication refers to the movement of data and information generated by computers from one location to another via various types of communication lines. Computer-based communication systems often are called communication networks. The networks allow users to collect data and information from and transmit these to powerful central computers or terminals.

The objectives of communication networks are:

• To offer more timely interchange of information and data among users.

• To reduce the effort and cost required to collect and transmit business data and information.

• To support better performance of tasks and improved management control over an organization especially with several remote locations.

Information society dominated by computers and communications is in front of us. In this society, the strategic resource is knowledge and information, and the value of that resource will be increased by the movement through the information pathway. A communication network is the pathway that allows information, data, and knowledge to be spread. Thus, the mainstream of the information age is communication.

Communications involve many technologies. In addition, many new communication technologies are being incorporated into the fabric of the information society as fast as people can learn how to maintain and use the technologies. This may be the most important reason for users to study communications.

Computer users who work in this society need to know technology incorporated into the communication networks. Although they use only applications involved in the communication networks, they still need to know how the communication networks work to improve their performance by using networks more efficiently and effectively.

Now let's see what are generic forms of networks that can exist in computer communications. A network can be defined simply as an interconnection of multiple teleprocessing devices. Three basic approaches to network processing exist in modern communications systems.

Timesharing

The earliest approach is known as timesharing. Timesharing occurs in networks in which multiple users gain access to a single host computer from their individual terminals. An example of timesharing is the use of a central computer at a nationwide company's headquarters by a variety of sales professionals dispersed around the nation.

The next approach gaining popularity in computer networking systems was the distributed processing method. This type of system involves the interconnection of minicomputers and microcomputers throughout a business organization. Various types of resources are shared by the interconnected terminals, while the individual computers retain control over their own processing.

The third approach to computer network processing, client/server computing, is currently attracting the most interest among management information systems professionals. The popularity of this approach is due to its ability to allow companies to selectively centralize and decentralize certain key operations. Thus, users can view client/server computing as a cross between the timesharing method (which emphasizes centralized computing) and distributed processing (which emphasizes decentralized computing). In client/server computing, the various applications available in the network are shared by several clients and one or more host computers (or servers). Clients obtain access to the network by means of desktop computers. The server, which can be a microcomputer, minicomputer, or mainframe, provides control for the entire network.

The hardware and software configuration used in client/server computing is the most complex because the network must enable any one of its subsystems to enter any other available subsystem. Thus, microcomputers in the network can take advantage of the unused computing capacity of other computers in the system. Also, the host computer can download data and information to a client. The role of the server is to provide control for the vital functions of the network, such as hardware and software sharing, database management, batch processing, and network management.

Economic, technological, and organizational factors have led to the current popularity of client/server systems. Client/server systems provide an economic benefit by allowing firms to maximize the efficiency of their information resources. Recent technological advances such as improvements in the power of microcomputers, graphical user interfaces, communications channels, and database management systems have increased the feasibility of client/server systems for all business organizations. Modern business organizations have decentralized many of their functions. This phenomenon has also sparked increased interest in client/server systems.

There are many components for a communication network. The basic three components are a host computer, a client, and a circuit. Besides these components, others include front-end processors, teleprocessing software, input terminals, and specialists.

Host Computer (Server)

The host computer or server (a mainframe, minicomputer, or microcomputer) is the main unit in the communication network. It stores data and programs that can be accessed by the clients. The host computer is responsible for performing the data processing tasks of the network. After processing has occurred, the data or information is routed back to a front-end processor. In distributed processing or client-server computing, several host computers may be tied together by the data communication network.

The client is a hardware device (a terminal or microcomputer) at the opposite end of a host computer of a communication circuit. It usually provides end users with access to the data and software on the server. The client is an input/output device.
The most widely used computer input/output devices for a communication network are the terminals. There are several types of terminals in common use in business today. The most familiar type of input/output terminal for most end users is the keyboard terminal. This terminal usually has a keyboard as an input device and a monitor and/or a printer as an output device. Push- button telephones can also be used as input/output terminals in computer-controlled communications networks. This type of terminal represents the most readily accessible and least expensive terminal in common use today. There are many special purpose terminals. One is a point of sale (POS) terminal. These terminals enable retailing institutions to instantaneously record sales data and update inventory records at the time of sale. Another is a data collection terminal. The term data collection terminal refers to a type of terminal which can collect information on the ongoing performance of factory employees. A remote job entry (RJE) terminal is another type. It is used in situations in which large amounts of information must be printed out at a remote location. A cash register used in many fast-food restaurants is another special type of terminal. It can update sales and inventory records.

The circuit is a communication channel that allows the data and information to move through the pathway. It includes several types of lines, such as twisted wires, coaxial cables, and fiber optic cables. It also contains wireless transmission circuits such as microwaves and satellites, and it is becoming more popular.

The front-end processor handles the incoming and outgoing communications exchanges between the host computer and the peripheral terminals and other host computers both inside and outside the network. Thus, the front-end processor serves as both an input unit and output unit for the host computer. Usually, the front-end processor is a minicomputer, while a mainframe serves as the host.

Certain front-end processors can send messages from one terminal to another without involving the host computer. This ability is known as message switching. Front-end processors that perform message switching usually have a store and forward capability. When a terminal is unable to receive a transmission at a particular time, the front-end processor will hold the message in its secondary storage and resend it later.

Besides the hardware mentioned previously in this chapter, the host and the front-end processor must contain software in order for the network system to run effectively. Software may also be located in the terminals and the cluster control units. The types of software located in the host and the front-end processor will be discussed here.

• TCM: The teleprocessing software located in the host computer is known as the TeleCommunications Monitor (TCM). The typical functions of the TCM include: Setting priorities of incoming messages, providing network security, settling an interface between the teleprocessing system and the DBMS (database management system), and saving the contents of primary storage to secondary storage.

• NCP: The Network Control Program (NCP) is the name for the software located in the front- end processor. Its main functions are: determining if a terminal desires to obtain access to the network, organizing data and information within the network by stamping date and time, translating the codes received from other types of computer system, inspecting format errors in incoming delivery, saving recent incoming messages in secondary storage, and maintaining statistical records on network activity.

Obviously, the growing importance of networks in business organizations will lead to an increase in the number of specialists who can manage these data communications systems. This section discusses the role of a network manager and his or her staff in a corporate environment. The network manager is responsible for performing all of the major management functions such as network planning, implementing, operating and controlling that relate to the communications system. The network planning and network controlling are the most important among them.

Network Planning

It involves anticipating the future computer-based communications needs of the organization. Network planning consists of three parts:

• Capacity planning involves estimating the quantity of data communications that will be required

• Staff planning consists of determining the number of employees that will be necessary to efficiently maintain the network system

• Performance monitoring consists of anticipating the effects of changes in the network's design.

Network Control

It involves activities in which the performance of the network is compared to predetermined standards to determine when corrective action or improvements are needed.

More Details on Input/Output Devices

This section discusses modems that users use to receive and send their data and information through communications channels. All communications must flow through some types of channels. Most of the computer communications are transmitted via ordinary telephone lines. Therefore, many require devices to convert the digital signals (represented by the presence or absence of an electronic pulse) into analog signals. This converter is called a modem.

The modem is an acronym for "MOdulator- DEModulator." A modem converts the digital signals into continuous analog signals (Modulation), and converts from analog to digital (Demodulation). The modem enables digital microcomputers to communicate across analog telephone lines. Both voice communications and data communications can be carried over the same telephone line.

Microcomputers require modems to send and receive messages over telephone lines. However, not all computer communications should have converters such as modems. Computer systems connected by coaxial or fiber- optic cables can send and receive digital signals directly through these circuits. Most modems today are both data and fax modems in that they can transfer computer data and also acting as fax machines.

Communications Speeds

The modem handles the dialing and answering of the call and controls transmission speed. Communications speeds are expressed in bits per second (bps). A baud is commonly and erroneously used to specify bits per second for modem speed. This is not always true. Modems are classified by the speed at which they operate. The modems used with microcomputers usually operate at 2400, 9600, 14400, and 28800 bits per second.

The higher the speed, the faster users can transmit a document and therefore the cheaper your line costs. For example, transmitting a 100-page single-spaced report takes 25 minutes at 2400 bps. It takes 6 1/4 minutes at 9600 bps, about 4 1/6 minutes at 14400 bps, and 2 1/12 minutes at 28800 bps.

External vs. Internal Modems

• External Modem: This is a modem separated from the system unit in the computer case. It is connected to the serial port of the computer by means of a cable. It is connected to the telephone wall jack by another cable.

• Internal Modem: An internal modem is a circuit board (a modem card) that can be added to the system unit of the computer. It takes one of the expansion slots.

Standard vs. Intelligent Modems

• Standard Modems: Most modems used today are called standard modems. These modems are usually operated by commands entered from a microcomputer keyboard. Users control the functions (dialing, etc.) of a modem through the keyboard. Modems may use different command languages to control their functions, but most modems (called "Hayes compatible") use a popular language developed by a company "Hayes Microcomputer Products, Inc." The command ATDSX4, for example, indicates that pay attention (AT), dial stored telephone number (DS), and then display the progress of the call (X4).

• Intelligent Modems: Intelligent modems are also called advanced modems. These modems can accept new instructions and then respond to the commands while transmitting data and information. These can be done by microprocessor chips and internal read only memory (ROM) contained in the modem. Of course, these modems are more expensive.

Short-Haul and Wireless Modems

• Short-Haul Modems: Short- haul modems are devices that transmit signals down the cable through any COM1 port. They sometimes are called modem eliminators, because they do not require an external power source. They are also called line drivers that can send data for a distance of more than one mile. This type of modem can be used within or across several buildings in a company or a university campus.

• Wireless Modems: Wireless modems transmit the data signals through the air instead of by using a cable. They sometimes are called a radiofrequency modem. This type of modem is designed to work with cellular technology, and wireless local area networks. Wireless modems are not yet perfected, but the technology is rapidly improving.

Many different types of modems exist in market today. There are several standards for modems, and modems that conform to a standard can communicate with other modems having the same standard. Many new modems currently developed support several standards.

V.22/V.22bis Modems

V.22 is the Consultative Committee on International Telegraph and Telephone (CCITT) standard for 600 bps or 1,200 bps full-duplex modems for many years, but this standard is becoming obsolete. The modem uses frequency modulation (FSK). V.22bis is the CCITT standard for 2400 bps full-duplex modems. It uses QAM modulation.

V.32/V.32bis Modems (High Speed)

V.32 is the CCITT standard for 4,800 bps and 9,600 bps full-duplex modems over dial-up telephone circuits. It uses QAM modulation. V32bis is the CCITT standard that extends V.32 to data rate of 14,400 bits per second.

V.34/V.34bis Modems (Error Checking)

V.34 is the CCITT standard for the digital (not analog) transmission telephone network beyond a local loop. It supports the data rates up to 28,800 bps. V.34bis is an extension of the V.34 standard. This modem provides a better error checking and voice transmissions. It supports the higher data transmission rates of 33,600 bps.

V.42/V.42bis Modems (Data Compression)

V.42 is the CCITT standard for error correction that uses two protocols. The main protocol is LAPM and secondary protocol is MNP Classes 2 through 4. V.42bis is the CCITT standard for error correction and data compression. It provides a compression technique to increase transmission speed around 4 times the bps rating by using a dictionary of 4- byte character combinations.

Baud

This is a switching speed or signaling rate of a line. It refers to the number of transitions (frequency changes) made per second. Baud equals to bits per second at only low speeds. (e.g., 300 baud is the same as 300 bps. However, the V.22 modem can generate 1,200 bps at 600 baud.)

COM1 Port

Logical name assigned to serial port #1 in DOS and OS/2. COM ports are usually connected to a modem or mouse. DOS version 3.3 or higher supports up to COM4, and OS/2 supports eight COM ports. Also, called RS232 port. See LPT1 port.

LPT1 Port

Logical name assigned to parallel port #1 in DOS and OS/2. This port is usually connected to a printer. A second parallel device is assigned LPT2.

RS232

A 25-wire electrical interface between a computer and a peripheral device, such as a modem, a mouse, etc. It is an EIA (Electronic Industries Association) standard for serial transmission that uses a 25-pin DB-25 or a 9-pin DB-9 connector.

Frequency Modulation

A communications transmission technique that modulates a data signal into a fixed carrier frequency by modifying the carrier frequency.

FSK (Frequency Shift Keying)

A simple communications modulation technique that merges binary data into a carrier frequency. It usually creates only two changes in the frequency, one for the 0 bit and another for the 1 bit.

QAM

?Refers to Quadrature Amplitude Modulation. A modulation technique that generates 4 bits per baud.

LAPM

Refers Link Access Procedure Modem. The protocol defined in CCITT V.42. It uses LAPD methods (LAP-D channel) used for the data channel of an ISDN transmission.

MNP

Refers to Microcom Networking Protocol. This is a data compression protocol that has been a standard in the United States. It was developed from microcom, Inc.

Data communications can be carried from one location to another through a variety of communications channels. These communications media include telephone lines (twisted pairs), coaxial cable, fiber-optic cable, microwave, and satellite. In general, there can be two ways of connecting microcomputers with each other and with other equipment: guided and radiated media.

• Guided Media: Refer to channels that allow the transmission of data and information through a physical media such as a twisted pair wire, coaxial cable, or fiber optic cable.

• Radiated media: Refer to those that transmit data and information through the air such as microwave, or satellite.

Twisted Pair

The telephone lines used to carry most of the voice and data communications consist of a pair of thin-diameter insulated copper wires (called twisted pairs). The wires are twisted around each other to minimize interference from other twisted pairs in the cable. Twisted pairs have fewer bandwidth than coaxial cable or optical fiber. They have been the standard communication channels for voice, data and information, but are now diminishing because of more reliable media such as coaxial cable, optical fibers, microwave, or satellite.

Coaxial cable can be used for telephone lines for transmission at a high frequency. Coaxial cable consists of a single core of solid copper. A coaxial cable can handle 80 times as many telephone transmissions as twisted pair media. Many computers in local area networks are linked by coaxial cables. Because of its sturdiness, coaxial cable is often used for telephone lines that must be carried under bodies of water. Because coaxial cables have very little distortion and are less prone to interference, they have low error rates.

A fiber-optic cable consists of tubes of glass through which data are transmitted as pulses of light. Although a fiber- optic cable is diametrically smaller than a human hair, it has 26,000 times the transmission capacity of twisted pair media. A major advantage of fiber-optic media is its high level of security. These communications channels are not susceptible to electronic interference. Therefore, they are a more reliable form of data transmission. Fiber-optic cables are also significantly less expensive than coaxial cable. A disadvantage of fiber-optic channels is that they cannot carry information over great distances.

Microwave data transmission differs from the previously mentioned communications channels in that data is transmitted through the air instead of through cables or wires. Microwaves are high-frequency radio waves that can only be directed in straight lines. Consequently, microwave transmission is usually limited to communications occurring within the limits of a particular city or community. For microwave transmissions to be able to occur over larger distances, data messages must be relayed from one location to another using antennas placed at high altitudes usually twenty to thirty miles apart.

Instead of antennas, satellites can also be used to transfer microwave messages from one location to another. Satellites rotate approximately 23,300 miles above the earth in precise locations. Satellite transmission stations that can both send and receive messages are known as earth stations. A major advantage of satellite transmission is that large volumes of data can be communicated at once. A particular drawback of several disadvantages is bad weather can severely affect the quality of satellite transmissions. Another one is that it has a serious security problem, because it is easy to intercept the transmission as it travels through the air.

Bandwidth

Refers to transmission capacity of a communications channel. It is the difference between the lowest and highest frequencies transmitted. It is expressed in cycles per second (Hertz).

Satellite

Many of these are offered by Intelsat, the International Telecommunications Satellite Consortium, which is owned by 114 governments and forms a worldwide communications system.

Tips

In late 1994, RCA introduced a direct broadcast satellite (DBS) system that enables homeowners or businesses to install 18-inch KU-band VSATs to receive satellite broadcasts for about $700. DBS lets owners receive more than 150 TV channels of higher quality video and audio than traditional cable TV. The real potential for DBS lies not only in the replacement of cable TV, but with the coming integration of video, voice, and data, as another high speed circuit into the home and office.

Ku-band

Ku-band satellites use very short waves that can be caught and concentrated in much smaller dish antennas, called VSAT (Very Small Aperture Terminal), which can be installed on virtually any buildings (or vehicles).

Coding

In all digital communications channels, computers transmit data and information in forms of binary codes. Both sender and receiver of the data and information should have a standard for both to understand them.

A coding scheme for communications is a binary system, as in the computer systems. The system consists of groups of bits (0 or 1) that represent characters. In computer systems, a byte is a group of bits and represents a character. In data communications, a byte is the same, but some codes use different number of bits such as 5, 7, 8 or 9.

Two predominant coding schemes ASCII and EBCDIC. ASCII refers America Standard Code for Information Interchange. It is the most popular code for data communications and is the standard code on most communications terminals. Among two types of ASCII, a 7-bit code can make 128 character combinations, and an 8-bit can do 256 combinations. EBCDIC refers Extended Binary Coded Decimal Interchange Code. It is IBM's standard information code, and has 8 bits for a character.

Each types of communications media has different transmission speed. The bandwidth is a measure of the transmission rate of communications channels.

• Baseband: Digital signals are commonly called baseband signals. Baseband is a communications technique in which digital signals are placed onto the transmission line without change in modulation. It transmits up to a couple of miles, and does not require the complex modems. Typical Token Ring and Ethernet use baseband signals.

• Broadband: Broadband is a technique for transmitting large amounts of data, voice and video over long distances simultaneously by modulating each signal onto a different frequency. Using the FDM (Frequency division multiplexing) technique, several streams of data can be transmitted simultaneously.
  Broadband is the bandwidth used for direct communication between very high-speed computers (e.g., large mainframe computers). This bandwidth includes microwave, satellite, coaxial cable, and fiber-optic media.

Two forms of data movement exist: parallel data transmission and serial data transmission.

• Parallel Transmission: Parallel data transmission involves the concurrent flow of bits of data through separate communications lines. This pattern resembles the flow of automobile traffic on a multilane highway. Internal transfer of binary data in a computer uses a parallel mode. If the computer uses a 32-bit internal structure, all the 32 bits of data are transferred simultaneously on 32 lane connections. \ Parallel data transmission is commonly used for interactions between a computer and its printing unit. The printer usually located close to the computer, because parallel cables need many wires and may not work stably in long distance.

• Serial Data Transmission: Most data transmitted over telephone lines use a serial pattern. That is, each individual bit of information travels along its own communications path; the bits flow in a continuous stream along the communications channel. This pattern is analogous to the flow of traffic down a one-lane residential street.
  Serial transmission is typically slower than parallel transmission, because data are sent sequentially in a bit-by-bit fashion.

Besides the previously mentioned ways in which data may travel, there are three directional modes of travel in data transmission.

• Simplex Communication: Simplex communication is a mode in which data only flows in one direction. Because most modern communications require a two-way interchange of data and information, this mode of transmission is not as popular as it once was. However, one current usage of simplex communications in business involves certain point-of-sale terminals in which sales data is entered without a corresponding reply.

• Half-duplex Communication: Half-duplex communication adds an ability for a two-way flow of data between computer terminals. In this directional mode, data travels in two directions, but not simultaneously. Data can only move in one direction when data is not being received from the other direction. This mode is commonly used for linking computers together over telephone lines.

• Full-duplex Communication: The fastest directional mode of communication is full-duplex communication. Here, data is transmitted in both directions simultaneously on the same channel. Thus, this type of communication can be thought of as similar to automobile traffic on a two-lane road. Full-duplex communication is made possible by devices called multiplexers. Full-duplex communication is primarily limited to mainframe computers because of the expensive hardware required to support this directional mode.

Another way of classifying data communications flow is as synchronous or asynchronous.

• Synchronous Transmission: Large volumes of information can be transmitted at a single time with synchronous transmission. This type of transmission involves the simultaneous flow of several bytes of data. Because a large block of data being sent synchronously cannot be interrupted, a synchronized clock is necessary to carefully schedule the transmission of information. This special communications equipment is expensive; but this cost can be made up in part by faster, less expensive transmission of information.

• Asynchronous Transmission: Conversely, asynchronous transmission involves the sending and receiving of one byte of data at a time. This type of transmission is most often used by microcomputers and other systems characterized by slow speeds.

To avoid chaos in computer communications, rules must be established for the exchange of data from one site to another. These rules are known as line protocol. Communications software packages control the speed and mode of communications between computer systems.

Many different standard network protocols exist to perform addressing, routing, and packetizing. All provide formal definitions for how addressing and routing is to be executed, and specify packet structures to transfer this information between computers. OSI, TCP/IP, IPX/SPX, and X.25 are commonly used routing protocols.

Open Systems Interconnection (OSI)

A major problem of early networked computer systems was that a lack of consistency existed among the protocols of different types of computers. Consequently, various efforts have resulted in the establishment of standards for data transmission protocols. For example, the International Standards Organization (ISO) developed a set of standard protocols called the Open Systems Interconnection (OSI). The OSI model separates each network's functions into seven layers of protocols, or communication rules. This model identifies functions that should be offered by any network system.

It is important to note that the physical layer, data link layer, and network layer appear in the user and host computers as well as units such as the front-end processor and the cluster control unit. The remaining layers appear only in the user and host computers.

• Physical Layer: The physical layer sends data from the user computer to a host computer (and vice versa). This layer is concerned essentially with computer hardware, whereas the upper layers are interested in communications software. This layer, for example, manages voltage of electricity, timing factors, or connector standards.

• Data Link Layer: The data link layer formats the received data into a record called a frame. This layer also is in charge of error detection.

• Network Layer: The network layer provides the physical layer with the ability to transfer records from one computer to another. This layer provides for the functions of internal network operations such as addressing and routing.

• Transport Layer: The transport layer allows communication to take place between the user and host computers. That is, this layer takes care of end to end validity and integrity of the transmission. OSI transport services takes place through layer 1 to layer 4, which are collectibly responsible for acquiring data and information from the sender to the receiver.

• Session Layer: The session layer is in charge of starting, maintaining, and ending each logical session between the interacting end user computers.

• Presentation Layer: The presentation layer formats incoming data so that it can be presented on the receiving terminal for end users. In other words, it is in charge of displaying, formatting, and editing user inputs and outputs.

• Application Layer: The application layer controls the input from the user computer and allows the application program to be run on the host computer. This layer is the end user's access to the network.

TCP/IP (Transmission Control Protocol/Internet Protocol) is a set of communications protocols developed for internetworking dissimilar systems. This is supported by many hardware vendors from microcomputers to mainframes. It is used by most universities, federal governments, and many corporations. TCP/IP has two parts. TCP protocol controls data transfer that is the function of the transport layer in the OSI model. IP protocol provides the routing and addressing mechanism that are the roles of the network layer in the OSI model.

The TCP/IP may be the oldest networking standard, and is also the most popular network protocol, used by almost 50 percent of all installed backbone, MAN (metropolitan area network), and WAN (wide area networks). TCP/IP is widely compatible with many other protocols. Although TCP/IP supports many protocols, it is usually associated with Ethernet. TCP/IP is also the network protocol used on the Internet.

IPX/SPX (lnternetwork Packet Exchange/Sequenced Packet Exchange) is a Novell NetWare communications protocol used to route messages from one end to another. It is the major network protocol used by Novell NetWare, and about 40 percent of all installed LAN (local area networks) use this protocol.

IPX/SPX has two parts, and is similar to TCP/IP. SPX controls the transport layer in the OSI model. It guarantees that an entire message arrives intact. IPX manages the role of the network layer in the OSI model and is used as delivery mechanism for SPX. IPX/SPX can be linked with many other protocols.

X.25 is a CCITT standard developed by ITU- TSS for WAN (wide area networks). It defines the interface between an end user computer and packet switching network. This is an international standard used for many worldwide corporations. It also has two parts. Packet layer protocol (PLP) is the routing protocol that manages the network layer and X.3 controls the transport layer.

ITU-TSS

This is an abbreviation of International Telecommunications Union-Telecommunications Standardization Sector.

CCITT

It refers Consulative Committee for International Telephony and Telegraphy. CCITT is an international organization for communications standard.

Networks can be classified by their topology, which is the basic geometric arrangement of the network. Different types of network configurations exist for network designers to choose from. Communications channels can be connected in different arrangements using several different topologies. This arrangement allows users to exchange information and share resources (software and hardware).

Four basic types of network configurations are star, bus, ring, hierarchical and mesh. Ring, bus, and star topologies are commonly used in LANs and BNs. Star and mesh topologies are commonly used in MANs and WANS. The networks are usually built using a combination of several different topologies.

A star topology is one in which a central unit provides a link through which a group of smaller computers and devices is connected. The central computer is commonly called a host computer. A host computer is usually a large computer such as a minicomputer or a mainframe. A file server is a large storage device that provides volumes of data and programs to the other units in the network.

In the star network, all interactions between different computers in the network travel through the host computer. The central unit will poll each to decide whether a unit has a message to send. If so, the central computer will carry the message to the receiving computer.

Star networks represent a very popular form of configuration for time-sharing systems in which a central computer makes available resources and databases for several "client" computers to share. As such, the star network is appropriate for systems that demand centralized control. The disadvantage of the star network is that a processing problem in the central computer can be paralyzing to the entire system.

In a star network, the central unit may be a host computer or a file server. The host computer is a large centralized computer, usually a minicomputer or a mainframe. In contrast, the file server is a large-capacity hard-disk storage device. It stores data and programs files shared by the users on the network. Also, called a network server.

In a bus configuration, each computer in the network is responsible for carrying out its own communications without the aid of a central unit. A common communications cable (the bus) connects all of the computers in the network. As data travels along the path of the cable, each unit performs a query to determine if it is the intended recipient of the message. The bus network is less expensive than the star configuration and is thus widely in use for systems that connect only a few microcomputers and systems that do not emphasize the sharing of common resources.

The problem in a computer on a bus topology does not frustrate the operation of the network, but a crack in the central cable will stop the whole network. Bus topology is popular because many computers can be connected to a single central cable. In a bus topology, each end user computer in the network handles its own communications control. There is no host computer or file server. As the information passes along the bus, it is examined by each terminal to see if the data is for it.

A ring configuration features a network in which each computer is connected to the next two other computers in a closed loop. Like the bus network, no single central computer exists in the ring configuration. Messages are simply transferred from one computer to the next until they arrive at their intended destinations. Each computer on the ring topology has a particular address. As the messages pass around the ring, the computers validate the address. If the message is not addressed to it, the node transmits the message to the next computer on the ring.

This type of network is commonly used in systems that connect widely dispersed mainframe computers. A ring network allows organizations to engage in distributed data processing system in which computers can share certain resources with other units while maintaining control over their own processing functions. However, a failure in any of the linked computers can greatly affect the entire network.

The ring arrangement is the least frequently used with microcomputers. However, as stated above, it often is used to link mainframes over wide geographical areas to build distributed data processing system. The loss of a mainframe usually does not restrain the operation of the network, but a cable problem will stop the network altogether.

A hierarchical network (or a tree network) resembles a star network in that several computers are connected to a central host computer (usually a mainframe). However, these "client" computers also serve as host computers to next level units. Thus, the hierarchical network can theoretically be compared to a standard organizational chart or a large corporation. Typically, the host computer at the top of the hierarchy is a mainframe computer. Lower levels in the hierarchy could consist of minicomputers and microcomputers. It should be noted that a system can sometimes have characteristics of more than one of the above topologies.

This topology is effective in a centralized corporation. For example, different divisions within a corporation may have individual microcomputers connected to divisional minicomputers. The minicomputers in turn may be connected to the corporation's mainframe, which contains data and programs.

This is a net-like communications network in which there are at least two pathways to each node. In a mesh topology, computers are connected to each other by point-to-point circuits. In the topology, one or more computers usually become switching centers, interlinking computers with others.

Although a computer or cable is lost, if there are other possible routes through the network, the damage of one or several cables or computers may not have vital impact except the involved computers. However, if there are only few cables in the network, the loss of even one cable or device may damage the network seriously.

Communications networks differ in geographical size. Networks may be constructed within a building or across several buildings. Networks may also be citywide and even international, using both cable and air connections. There are three major network types: LAN (local area networks), MAN (metropolitan area networks), and WAN (wide area networks).

A computer communications network contained in a small area such as a commercial building is known as a local area network (LAN). A local area network's linkages usually are accomplished with either telephone, coaxial, or fiber-optic cables. Often, LANs use a bus configuration. The major benefit of a local area network is that it can help to reduce costs by allowing people and microcomputers to share expensive resources. Also, local area networks enable end users to participate in office automation technology. In addition, LANs may be connected to other networks by means of a network gateway. Network designers can choose the method in which messages are controlled in a local area network. Two basic methodologies are the token-passing approach and the contention-based approach. The token- passing approach allows the designers of a network to achieve a degree of centralized control. A group of data bits, or "token," is passed from one network node to another. A node can only send a message when it is in possession of the token.

In the contention-based approach, a node that wishes to send a message first listens to determine if another node is currently sending a message. If not, the node attempts to send its message. However, the lack of centralized control can result in a collision - two nodes attempting to send messages simultaneously. Consequently, contention-based approach is usually not suitable for networks with a large amount of communications activity.

Why Use an LAN?

There are two basic reasons for developing an LAN: information sharing and resource sharing.

• Information sharing: This refers to having users who access the same data files, exchange information via electronic mail, or search the Internet for information. The main benefit of information sharing is improved decision making, which makes it generally more important than resource sharing.

• Resource sharing: It refers to one computer sharing a hardware device (e.g., a printer) or a software package with other computers on the network. The main benefit of resource sharing is cost savings.

Types of LANs

By the categories, there are three common types of LANs. Dedicated server LANs account for more than 70 percent of all installed LANs.

• Dedicated Server Networks: A dedicated server LAN can connect with almost any other network, can handle very large databases, have a dedicated network server, and uses sophisticated LAN software. Moreover, high-end dedicated server LANs can be easily interconnected to form enterprise-wide networks or, Sometimes, replace the host mainframe central computer. Generally speaking, the dedicated server is a powerful microcomputer.
  Three software components must work together and with the network hardware to enable communications: the network operating system (NOS) in the dedicated server, the network communication software that interconnects the server to the user computers, and the application software that runs on the server and client computers. Four common types of dedicated server LANs are file servers, database servers, print servers, and communication servers.

• Peer-to-Peer Networks: This network is a local area network that allows all users access to data on all workstations. In this networks, any computer can perform as both a client and a server. Each computer on the network shares its resource such as hard disk and printer with any other computer on the same network.
  This network is usually slower, has less capability, supports a limited number of computers, provides less sophisticated software, and is more difficult to manage than dedicated server LANS. However, this LAN uses cheaper computers and programs. Examples of peer-to-peer LANs include Artisoft's LANtastic, Novell's NetWare Lite, and Windows for Workgroups.

• Zero-Slot LANs: This LAN operates like peer-to-peer LAN, but offers limited, simple abilities such as sharing files and printers one another, transfer files, and transmit e- mail. It is inexpensive. It does not require a network interface circuit card. Its adapter plug can be plugged into a serial or parallel port. This network usually can handle up to 30 computers.

LAN Components

There are five basic components to an LAN.

• Server: A server is a computer in a network that shared by multiple users. There are many kinds of servers in dedicated server networks. For example, file servers, printer servers, and communication servers are they.

• Client Computer: In a communications network, the client computer is the requesting machine from the supplying machine, server.

• NIC: The Network Interface Card (NIC) is a printed circuit board that plugs into a network server or client computer. It enables the computer to be physically connected to the network cable, which provides the physical layer connection among the computers in the network. The NIC performs the electronic functions of the access method, or data link protocol, such as Ethernet, Token Ring and LocalTalk.

• Network Cables/Hubs: Network cable physically connects each computer to the other computers in the network. The selection of an LAN topology can be influenced greatly by the type of cable.
  • Network Cable: Most LANs use a combination of unshielded twisted pair (UTP) wires, shielded twisted pair (STP), coaxial cable, and fiber optic cable. The advantage of the shielded and unshielded twisted pair wires is in the low cost. Coaxial cable is also commonly used and physically larger and heavier than twisted pairs. The fiber optic cable is thin and light. Its high capacity makes it perfect for backbone networks. It is beginning to use fiber optic cable for LANs.

  • Network Hubs: A network hub is called in many different names such as concentrator, multistation access unit, transceiver, or repeater. It serves two purposes. First, they provide an easy way to connect network cables. Second, hubs act as repeaters or amplifiers.

  • Wireless LANs: Wireless LAN is an alternative method of cabling a local area network. They use the same protocols as other LANS, but they deliver data and information through the air rather than through physical cable. Wireless LAN offers an alternative for an old building where wiring is difficult and expensive. It provides new capability for mobile computing for laptop computers. Its disadvantages are the noise and insecurity.

• NOS: The NOS (Network Operating System) is the control program that resides in a server or workstation within an LAN. This controls the network. The NOS handles the requests for data from all the users on the networks. It is responsible for data link layers and the network layers. It must interact with the application programs and the computer's own operating system.
  One of the well-known NOS for microcomputer is a NetWare. This is an NOS from Novell, Inc. This runs on 286 and higher personal computers and supports DOS, OS/2 and Macintosh operating system. It also supports many LAN access methods such as Ethernet, Token Ring, ARCNET and Starlan.

Two Most Common LANs

The two most commonly used LANs are Ethernet and Token Ring local area networks.

Ethernet

The Ethernet is a local area network developed by Xerox, Digital (DEC) and Intel and is the most popular LAN in the world, accounting for almost 50 percent of all LANs.

• Topology: Ethernet uses a bus topology. A major circuit running the length on the network connects all computers. When the Ethernet uses a central hub, the topology of it looks like a star (physically) from the outside, but it is really a bus (logically).

• Types of Ethernet:
  • 10Base5: The original Ethernet specification was a 10Mbps data rate using Baseband signaling on thick coaxial cable, called "Thick Ethernet (5)."

  • 10Base2: Today, thin coaxial cable is rapidly replacing the original thick coax because it is considerably cheaper and easier to work with. The 10Base2 standard is often called "Thin Ethernet (2)."

  • 10BaseT: This is the most commonly used type of Ethernet. The name means 10 million bits per second, Baseband, and the "T" means it uses twisted pair wiring. The extremely low cost of 10BaseT made Ethernet very inexpensive and the most popular type of LAN.

  • 10Broad36: It means 10 Mbps, broadband, with a maximum distance of 3600 meters. This standard is most commonly used in backbone networks.

Token Ring

This is a local area network developed by IBM that uses a special twisted wire and the token passing access method. Token Ring LANs are the second most popular type of LAN, with almost 40 percent of all LANs worldwide.

• Topology: A Token Ring network uses a ring topology. All messages pass to each computer in turn. Computers receive all messages, but only process those addressed to themselves. They deliver the rest of messages to the next computer in the network. When central hubs are used, it also looks like a star configuration (physically), but it really is a ring network (logically).

• Media Access Control: Token Ring network uses a token passing technology. A computer with a message to deliver waits until it receives free token. The computer then changes the free token into a busy token, attaches its message to it, and retransmits it on the circuit to the next computer.

• Types of Token Ring: There are two common types of Token Ring networks. The Token Ring developed first was Token Ring-4, transmitting data at 4Mbps over twisted wire. The newer Token Ring network is Token Ring- 16, delivering information at 16Mbps over high quality twisted wire.

MANs

The next larger network than LAN may be the Metropolitan Area Networks (MANs). MAN usually spans a geographical area that usually encompasses a city or county area. It interconnects various buildings or other facilities within this citywide area. For example, linkages can be established between two commercial buildings. A more recent use of MAN technology has been the rapid development of cellular phone systems.

A wide area network (WAN) is one that operates over a vast distance (e.g., nationwide). Its nodes may span cities, states, or national boundaries. This network interconnects computers, LANS, BNs, MANS, and other data transmission facilities. Typically, WAN will employ communications circuits such as long- distance telephone wires, microwaves and satellites. FOR example, nationwide automated teller machines used in banking represent a common application of a wide area network.

Backbone Networks

A BN is a large network to which many networks within an organization are connected. It usually is a network that interconnects all networks on a single site, but it can be larger if it connects all the organization's terminals, microcomputers, mainframes, local area networks, and other communication equipment.

NetBIOS

NETwork Basic Input/Output System is a commonly used transmission protocol for PC LANs. This is an extension to DOS, which examines all functions and direct to DOS. It provides a program called "redirector" that resides on top of DOS.

Most corporations do not have their own circuit or long distance communication route for use in MANs and WANs. They usually rent or lease them from common carriers. Common carriers provide a variety of telecommunications services for the corporations to use them for their LANs and MAN/WANs.

Many computer-based communications services are now being provided by common carriers. A common carrier is a private company (usually a telephone company) which offers computer networking services for a fee. These fees are regulated by agencies such as the Federal Communications Commission (FCC) and the Public Utility Commission (PUC). Two types of services are commonly provided by common carriers: public-measured service and private leased service.

Two Types of Services

Public-Measured Service

In a public-measured service, various network systems share common communications channels with other networks. For example, an ordinary telephone line that users pass on your street every day may be carrying data communications generated by a variety of computers in your neighborhood.

Private Leased Service

In a private leased service arrangement, the common carrier provides its customer with an exclusive service. An illustration of this is when the common carrier allows a customer to have access to a dedicated line - one reserved for the exclusive use of the customer. Common carriers figure out their fees for communications services in much the same manner as phone companies decide your monthly phone bill. A dial-up arrangement represents the simplest billing system used by common carriers. Users can establish connection with other terminals by dialing the receiver's number. In this arrangement, customers pay a flat fee for the right to engage in an unlimited number of transactions. Long distance transmissions are charged on a per-transmission basis. Businesses that conduct many network activities with long distance transmissions may consider using a Wide Area Telecommunications Service (WATS). In this service, as long distance usage goes up, rates go down. Both WATS lines and dial-up arrangements are examples of voice grade circuits designed to transmit ordinary voice messages over telephone lines. These systems may be too noisy for computer networking systems that transmit vary large amounts of data in short periods. Therefore, many common carriers attempt to provide higher quality circuits by offering special private lines which filter out excess sound. These lines are known as above-voice grade circuits.

Communication services come in four basic groups. With a dedicated circuit, the organization pays a fixed monthly fee, while with the other three, the organization pays on a per-use basis.

Dialed Circuit Services

Dialed circuit services are regular dialed telephone calls from one point to another through the telephone networks. Dialed circuits are usually slow and noisy. The usual voice telephone network is used for data transmission with DDD (direct distance dialing). In this service, users dial the telephone number of the host computers through a modem to be connected.
This service contains an AT&T Megacom WATS (Wide Area Telephone Services), AT&T Megacom services, and DLAL-IT 900 services.

Dedicated Circuit Services

This service is point-to-point circuit service that a leasing corporation can use exclusively to connect two point. A common carrier lends this circuit for the exclusive use. The cable is faster and more noise-free than dialed circuit service.
This service includes voice grade channels, Wideband analog services, digital services, T carrier circuits, SONET, and satellite services.

Switched Circuit Services

A switched circuit service is the circuit in which the corporation installs network connection points at many locations and uses the common carrier's network to connect temporarily (switched connections) between locations when necessary. This service includes SMDS and ISDN. Let's see more details of ISDN.

ISDN

ISDN (Integrated Services Digital Network) is a special type of private leased service called the integrated services digital network (ISDN) is expected to dramatically affect the business environment in coming years. This type of network allows for simultaneous voice, data, text, and video digital communications. ISDN represents an effort to create a universal communications network nationwide. The ISDN consists of two types of channels.

The B-channel transmits user information with a transmission speed of 64 kilobits per second (Kbps). The D-channel provides a control function by initiating, terminating, and directing transmissions. In addition, there are two basic types of ISDN structures: basic access and primary access. Basic access networks have two B-channels and one D-channel with a transmission speed of 16 Kbps. These networks are primarily leased by residential users. Business end users are more likely to use the primary access system that uses 23 B-channels and one 64Kbps D-channel.

The ISDN system offers high-quality digital communications as well as many features that enhance the quality and flexibility of network use. For example, the B-channel enables users to simultaneously access multiple databases and computer systems. At the same time, users can take advantage of a variety of office automation technologies and multimedia systems. The D- channel can perform many useful functions such as transmission forwarding, automatic callback, and caller identification.

Packet Switched Networks

This works very much like a switched circuit, except that the user breaks data transmissions into pre-defined packets that conform to network protocols. This service includes packet switching, Frame relay, ATM, public data networks and software defined networks.

Common Carriers

A common carrier is a government-regulated private company that sells or leases communication services and facilities to the public. A tariff is the schedule of rates and description of services that are to be received when a particular type of communication service is purchased or leased. If the industry is deregulated, as it is in the United States, it may be more innovative, cost effective, and able to develop new services faster.

T Carrier Circuits

These services are leased digital circuits from common carriers with a wide range of transmission capacities.

SONET

SONET (Synchronous Optical Network) has recently been accepted by ANSI as a standard for optical transmission at gigabits per second speeds. It uses fiber optics.

ANSI

ANSI (American National Standards Institutes) is a nonprofit, privately-funded membership organization. It is the U.S. member body of ISO and IEC (International Electrotechnical Commission).

SMDS

SMDS (Switched Multimegabit Data Service) offered by most RBOCs (Regional Bell Operating Companies), is essentially a switched version of the T- carrier and SONET services.

ATM

Asynchronous Transfer Mode (ATM) is very similar to frame relay, except that it uses fixed-length packets. ATM is one of the fastest growing new technologies for packet switched networks. ATM is scaleable, and provides forward error correction on the header.

Computer-based communications systems allow end users to transmit data from one location to another via various types of communications lines. This type of communication has dramatically expanded the scope of business activities. The most vital component of any data communications system is the network. The network is any system that makes interaction between two or more computers possible. Today, computer users can tap into a vast number of resources to retrieve information on a variety of subjects and events. For example, a CEO of a nationwide corporation may wish to examine information about regional sales levels or expected economic conditions.

This section examines two types of network applications: Groupware and the Information Superhighway (Internet). These two applications are the future of information technology.

Groupware

This is a communications application that is growing in popularity (also called collaboration technology). Groupware is software that helps groups of people to work together more productively. Groupware allows people to exchange ideas, debate issues, make decisions, and write reports without actually having to meet face-to-face. The most important advantage of Groupware is its ability to help groups make decisions faster. This section focuses on four popular types of Groupware:

Electronic Mail

Electronic mail (E-Mail) is one of the earliest Groupware tools and is also the most heavily used tool today. E-mail is faster and cheaper than regular mail, and can substitute for telephone conversations in some cases. Several standards have been developed to ensure compatibility between different software packages. They include X.400, CMC (Common Messaging Calls), and MAPI (Massaging Application Program Interface).

Document-based Groupware (Notes)

Notes was the first document-based Groupware product. It is a document database designed to store and manage large collections of text and graphics to support ongoing discussions.

Group Support Systems (GSS)

Group Support System, one of the most popular uses of Groupware, is a software tool designed to improve group decision making in special purpose meeting rooms that provide networked computers and large screen video projection systems. These rooms are equipped with special-purpose GSS software that enables participants to communicate, propose ideas, analyze options, and evaluate alternatives. A group of employees could view a document displayed on a projector screen. Then, each employee could manipulate the data from his station while others observed the resulting changes.

Video Teleconferencing

This is a video conference among several users, which is provided by one or more video cameras and several display monitors set up in special purpose meeting rooms. It provides real-time transmission of video and audio signals to help people in different locations to have a meeting. The advantage of this technology is the time and cost savings. Another form of video teleconferencing, desktop video conferencing, is growing up fast.

The information superhighway or simply Internet is one of the most important developments in the history of information systems. It is growing fast. The Internet is not one network, but tens of thousands of networks linked together. In other words, it is a large network made up of thousands of smaller networks. Internet does not have overall central administration, because it is a collection of thousands of smaller networks. Internet provides four basic functions to its users:

• E-mail
  
• Telnet (Remote Login)
  
• Discussion Groups
  
• Information Resources
  

E-mail on the Internet

Electronic mail (E-mail) is one of the most rapidly growing developments in networked communications. Users of E-mail have their own file stored on a computer system. This file can be called a "mailbox." Access to a person's mailbox is protected by means of a password. Once logged on to an E-mail account, an end user may send messages and files to other mailboxes. An individual electronic mail transmission may be sent to one or many recipient accounts. This person may also read messages that have "arrived" in her or his mailbox from other E-mail accounts.

One of the main advantages of using E-mail is confidentiality. No one can access an individual's mailbox without knowledge of the password. E-mail also allows recipients to know the exact transmission times for each incoming message. Popular uses of E-mail have been to set up meetings within business organizations and to distribute memoranda throughout an organization. E-mail is increasing in popularity for communication between businesses. The main reasons for this are speed and cost; there is no reason to spend postage on a letter that will take three to seven days to arrive when electronic mail could be used for virtually nothing and arrive instantaneously.

Anyone with access to the Internet can send E-mail to anyone else on the Internet. Internet E-mail addresses have two parts, the individual user's account address and the address of the computer. The computer's address in turn has two parts, the computer name and its domain. The general format is therefore: user@computer.domain. Note that the "at" symbol (@) separates the user's account from the computer address, and that a period separates the name of the computer from its domain. Some computer names also have several parts separated by periods, so some addresses may have the format: user@computer.computer.computer.domain (e.g., user@sunset.backbone.olemiss.edu)

Telnet (Remote Login)

Internet offers a large number of services. Among them, the service that allows users to connect to a remote Internet host is called Telnet. Users on one computer in the Internet can login into other computers on the Internet by a special program, called Telnet, on your computer. This program uses the Internet to connect to the computer users specify. The users should know the account name and password of the remote computer. In Telnet, a user's computer is called the local computer. The other computer that the Telnet program connects is called the remote computer. An example of using Telnet is that users can read and send E-mail while traveling.

Discussion Groups

Discussion groups are lnternet users who have joined together to discuss some topic. There are many discussion groups on every topic imaginable, from cooking to biological science. Two groups are commonly used for business.

• Usenet Newsgroups: Usenet is a large collection of discussion groups involving millions of people from all over the world. This is the most formally organized among the discussion groups. To read Usenet articles, users use a program called a newsreader. There are a number of different newsreaders. In UNIX, nn, rn, tin, and trn are the most popular newsreaders.

• Listserv: A listserv is simply a mailing list developed on the large Bitnet network (not on the Internet). The listserv processor processes listserv commands such as requests to subscribe and unsubscribe, while the listserv mailer mails any message it receives to everyone on the mailing list.

Information Resources

The major use of the Internet is to find information. There are six major ways to find and achieve information:

• FTP: File Transfer Protocol (FTP) is the underlying set of specifications that support Internet file transfer. In other words, FTP is a service that allows us to copy a file from any Internet host to any other Internet host. A ftp program acts as a client and connects to the FTP server on a remote host. Internet users can download (copy files from the remote computer into users' computer) and upload (copy files from users' computer to a remote computer) files. An anonymous FTP site permits any Internet user to login using anonymous as the account name and a user's mailing address as the password.

• Archie: Throughout the Internet, there are a number of computers, called Archie servers, which provide a service to help users find the name of Anonymous FTP hosts that carry a particular file. Archie is a tool that allows users to search most of the publicly available anonymous FTP sites worldwide for specific files of interest.

• Gopher: The Gopher is a powerful system that allows users to access many resources of the Internet in a simple, consistent manner. To use the Gopher, all users need to do are making selections from a menu. In other words, Gopher is a menu-based tool that enables users to search for publicly available information posted on the Internet. The power of the Gopher lies in the fact that the resources listed in a menu may be anywhere on the Internet.

• Veronica: Veronica is a Gopher-based resource that users can use to search gopherspace for all the menu items that contain specified words. Veronica is to Gopher what Archie is to FTP. It enables users to search all publicly available Gopher sites by specifying key words.

• World Wide Web: WWW (World Wide Web) or simply Web is one type of information resource that is growing even faster than Internet itself is the World Wide Web. WWW is an attempt to organize all the information on the Internet as a set of hypermedia documents. Besides that, the WWW allows users to access all kinds of Internet resources, just by using a browser to read the appropriate document. The Web provides a graphical user interface and enables the display of rich graphical images, pictures, full motion video, and sound clips.

  The Web is the most common way for businesses to establish a presence on the Internet. This Web has two major components, a Web browser and a Web server. A browser is a software package for accessing a Web server that stores files using HTML. There are many Web browsers available, including Mosaic, Netscape, Cello, and WinWeb. Most browsers show these links by highlighting text in a different color, usually blue. To use a link to go to a new page, users simply click on the text, and the browser takes users to the new page.

  A Web server stores information in a series of text files called pages. These text files or pages use a structured language called HTML (Hypertext Markup Language) to store their information. HTML enables the author of a page to define different typestyles and sizes for the text, titles, and headings, and a variety of other formatting information. HTML also allows the author to define links to other pages that may be stored on the same Web server, or on any Web server anywhere on the Internet.

• WAIS: The WAIS stands for Wide Area Information Service. The original idea behind WAIS was to develop a generalized system of information retrieval that could access collections of data all around the world.

Bitnet

Bitnet is a worldwide network, separate from the Internet, which connects well over a thousand academic and research institutions in more than 40 countries. Many Bitnet sites are IBM mainframe computers running the VM operating system. Each country has different name for Bitnet.