Part 3 Networks

7.1 Overview of Telecommunications Systems

No matter what the business is, effective communication is critical to organizational success. Efficient communications is one of the most valuable organizational resources, because it enables a company to keep in touch with its operating divisions, customers, suppliers, and stockholders. For example, Ford Motor Company developed an integrated telecommunications system to reduce costs related to transporting auto parts and components to Ford's 20 North American plants and to streamline car production schedules. The system integrated the plant's individual parts-ordering systems and connected them to suppliers for real-time information about component and part inventories, as well as real-time tracking of deliveries.

When computer first were produced, they were stand-alone devices. As computers became widely used, hardware and software were designed so computer could exchange data, information, and instructions with other computers -- a process called communications. All types of Communications form a major part of any business system. Therefore, you must gain an appreciation of communication concepts, media, devices, and technologies -- as well as an understanding of how these factors may best be employed to develop effective and efficient business systems.

Telecommunications can be defined as communication of data and information by electronic means such as telephone, radio, television, and computer, usually over some distance. When referring to computers, telecommunications (also called computer communications) describe a process in which one computer transfers data, information and instructions to another computer. Today, even the smallest computers and devices communicate directly with one another, with hundreds of computers on a company network, or with millions of other computers around the globe -- often via the Internet. Telecommunications may change not only the way businesses operate, but also may alter the nature of commerce itself. As networks are connected with one another and information is transmitted more freely, a competitive marketplace will make excellent quality and service imperative for success. Computer communications is accomplished through the use of telecommunications technology.

Figure 7-1 An example of telecommunication System

A telecommunications system is a collection of compatible hardware and software arranged to communicate data, information and instructions from one location to another (Figure 7-1). Telecommunications systems can transmit text, graphic images, voice, and video information through communications channel. Figure 7-2 illustrates a general model of a typical telecommunications system. The telecommunications model consists of the following:

• A sending device that initiates an instruction to transmit data, information, or instructions. It can be a computer system, a terminal, a cellular telephone, a WebTV™, a GPS receiver, an Internet-enabled PDA, or another device that originates the message.

• A communications device that connects the communications channel to a sending device.

• A communications channel or transmission media on which the data, instructions, or information travel.

• A communications device that connects the communications channel to a receiving device.

• A receiving device that accepts the transmission of data, information, or instructions.

• A communications software that controls and manages the activities and functions of the communications network.

Figure 7-2 General model of telecommunications

As shown in Figure 7-1, all types of computers and mobile devices serve as sending and receiving devices in a telecommunication system. To send and receive data and information over some distance, a telecommunications system must perform a number of functions. A telecommunications system transmits data and information; establishes the interface between the sender and the receiver; routes messages along the most efficient communications paths; performs elementary processing of the information to ensure that the right message gets to the right receiver; converts messages from one speed into the speed of a communications line or from one format to another; control the flow of messages; and performs some editorial tasks on the data.

This chapter presents various uses of telecommunications, discusses different types of networks, and examines several types of communications devices, channels, and services.

7.2 Telecommunications System Components

7.2.1   Communications Devices

Telecommunications system components include communications devices and communications software. A communications device is one of various hardware that allows electronic communications to occur. Almost every telecommunication system uses one or more of these devices to transmit or convert signals. The type of communications device used in a telecommunications system depends on the type of sending and/or receiving devices, as well as the type of transmission media.

At the sending end, a communications device sends the data, instructions, or information from the sending device to a communications channel. At the receiving end, a communications device receives the signals from the communications channel. The most common type of sending and receiving device in telecommunications is a computer. Other devices, such as fax machine, digital cameras, cellular telephone, smart phone, PDA, Internet appliances, and Web-enabled devices, also can function as sending and/or receiving devices.

Today, thousands of computer networks exist, ranging from small networks operated  by home users to global networks operated by numerous telecommunications firms. Interconnecting these many types of network requires various types of communications devices. Many network devices and software allow these networks to communicate with other networks that employ different transmission media and/or protocols. Devices that handle the movement of data in a computer network include modem, router, hub, network interface cards, multiplexer, front-end processor, and the host computer (Figure 7-3).

Figure 7-3 Hardware components in computer communications

Modems

One type of communications devices that connects a communications channel to a sending or receiving device such as a computer is a modem. Computers process data as digital signals. Data, instructions, and information travel along a communication channel in either analog or digital form, depending on the communications channel. An analog signal consists of a continuous electrical wave. A digital signal consists of individual electrical pulses that represent bits grouped together into bytes. For communications channels that use digital signals, the modem transfers the digital signals between the computer and the communications channel. If a communications channel uses analog signals, however, the modem first converts between analog and digital signals.

Dial-up modem is the communications device that performs the function of modulation/demodulation. Converting signal from digital to analog is called modulation, and converting signal from analog to digital is called demodulation. The word, modem, is derived from the combination of the words MOdulation and DEMdulation. Dial-up modem can automatically dial telephone numbers, originate message sending, and answer incoming calls and messages.  Both the sending and receiving ends of a standard telephone line must have a dial-up modem for data transmission to occur.

	Figure 7-4 A dial-up modem for a desktop computer usually is in the form of an adapter card installed in the system unit.

A dial-up modem usually is in the form of an adapter card that you insert in an expansion slot on a computer's motherboard (Figure 7-4). One end of a standard telephone cord attaches to a port on the modem card and the other end plugs into a telephone outlet. Devices other than computers also use modems. A stand-alone fax machine, for example, has a modem that converts a scanned digitized image into an analog signal that is sent to a recipient's fax machine. If a notebook or other mobile computer does not have built-in modem capabilities, mobile users can insert a PC Card modem in a PC Card slot on the computer. The PC Card modem attaches to a telephone outlet with a standard telephone cord. Mobile users without access to a telephone outlet also can use a special cable to attach the PC Card modem to a cellular telephone, thus enabling them to transmit data over a cellular telephone.

Figure 7-5 A DSL modem

If you access the Internet using ISDN or DSL, you need a communications device to send and receive the digital ISDN or DSL signals. A digital modem is a modem that sends and receives data and information to and from a digital telephone line such as ISDN or DSL (Figure 7-5). According to the definition of a modem (to convert from analog to digital signals and vice versa), digital modems are not really modems. Although the original term for these devices was terminal adapter, the industry refers to ISDN, DSL, and cable modems as digital modems. A DSL or ISDN modem sends digital data and information from a computer to a DSL or ISDN line and receives digital data and information from a DSL or ISDN line. ISDN and DSL modems usually are external devices, in which one end connects to the telephone line and the other end connects to a port on the system unit. Most include built-in connectivity.

	Figure 7-6 A typical cable modem installation

A cable modem is a digital modem that sends and receives digital data over the cable television (CATV) network (Figure 7-6). With more than 110 million homes wired for cable television, cable modems provide a faster Internet access alternative to dial-up for the home user and have speeds similar to DSL. Cable modems currently can transmit data at speeds that are much faster than either a dial-up modem or ISDN. Cable modems typically include built-in Wi-Fi connectivity. As shown in Figure 7-6, CATV service enters your home through a single line. To access the Internet using the CATV service, the CATV company installs a splitter inside your house. From the splitter, one part of the cable runs to your televisions and the other part connects to the cable modem. A cable modem usually is an external device, in which one end of a cable connects to a CATV wall outlet and the other end plugs in a port in the system unit.

Figure 7-7 Wireless modems allow users to access the Internet wirelessly using their mobile computers and devices.

Some mobile users have a wireless modem that allows access to the Internet wirelessly from a notebook computer, a PDA, a smart phone, or other mobile device (Figure 7-7). Wireless modems, which have an external or built-in antenna, typically use the same waves used by cellular telephones. These modems are available as PC Cards, ExpressCard modules, and flash cards. A wireless access point is a central communications device that allows computers and devices to transfer data  wirelessly to a wired network. Wireless access points have high-quality antennas for optimal signals. For the best signal, some manufacturers suggest positioning the wireless access point at the highest possible location.

Devices Connecting Networks

Today, thousands of computer networks exist, ranging from small networks operated by home users to global networks operated by numerous telecommunications firms. Inter-connecting these many types of networks requires various types of communications devices. Most networks use bridges, gateways, and routers. Many times these terms (bridge, gateway, router) are used interchangeably -- although they do have slightly different meanings.

	Figure 7-8 Through a router, a network can share access to a high-speed Internet connection.

A bridge connects two pieces of land together offering a path from one to another. Networks also can have a bridge -- a device that connects two networks making each accessible to the other. Usually the two networks use the same protocol, such as Ethernet. Bridges can be used to connect two different types of networks but are usually used to separate one large network into two smaller networks for performance purposes. A bridge knows all of the addresses on each side of the bridge and can send information accordingly. To use a bridge, the transmission media used in the networks does not have to be the same. One network could use coaxial cable, while the other might use twisted-pair cable. A gateway is a communications processor that connects networks that use different protocols by providing the translation from one set of protocols to another. Gateways also are used between e-mail systems so that users on different e-mail systems can exchange messages.

A router is an intelligent bridge for large networks. A router can listen to the traffic on the entire network and determine the least congested route to its destination. Routers connect multiple networks and routs communications traffic to the appropriate network using the fastest available path. Routers direct most of the traffic on the Internet, thus ensuring that data arrive at the correct destination. A router can be used on any size of network. On the largest scale, routers along the Internet backbone forward data packets to their destination using the fastest available path. For smaller business and home networks, a router allows multiple computers to share a single high-speed Internet connection such as through a cable modem or DSL modem (Figure 7-8). These routers connect from 2 to 250 computers.

To prevent unauthorized users from accessing files and computers, many routers are protected by a built-in firewall, called a hardware firewall. Some also have built-in antivirus protection. Routers also support wireless communications, eliminating the need for a separate wireless access point in a wireless network. If the network has a separate wireless access point, it connects to the router via a cable. Some routers also include additional functionality such as including a built-in print server. Today's routers or combination wireless access point/routers are easy to configure and secure against unauthorized access. 

Multiplexing is a technique that allows several telecommunications signals to be transmitted over a single communications medium at the same time. A multiplexer is a device that combines two or more input signals from various devices into a single stream of data and then transmits it over a single transmission medium. By combining the separate data streams into one, a multiplexer increases the efficiency of communications and reduces the need for using multiple separate transmission media. As with modems, both the sending and the receiving devices need a multiplexer for data transmission to occur. The multiplexer at the sending end codes each character with an identifier before combining the data streams. It then compresses the data and sends it over the communications channel. The multiplexer at the receiving end takes the transmitted signal, use the character identifiers to separate the combined data stream into its original parts, and send the data to the appropriate device.

A front-end processor is a small computer dedicated to communications management and is attached to the main, or host, computer in a large computer system. The front-end processor perform special processing related to communications such as error control, formatting, editing, controlling, routing, and speed and signal conversion. The front-end processor is largely responsible for collecting and processing input and output to and from terminals and grouping characters into complete messages for submission to the CPU of the host computer. The front-end processor relieves the host computer of a variety of communications-related processing duties. The host can instead concentrate on overall system control and the execution of applications software.

	Figure 7-9 Examples of network cards

A network card, also called network interface card (NIC), is an adapter card, PC Card, ExpressCard module, USB network adapter, or flash card that enables the computer or device that does not have built-in networking capability to access a network. The network card coordinates the transmission and receipt of data, instructions, and information to and from the computer or device containing the network card. Network cards are available in a variety of style (Figure 7-9). A network card for a desktop computer is an adapter card that has a port to which a cable connects. A network card for mobile computers and devices is in the form of a PC Card, ExpressCard module, USB network adapter, or a flash card. Many of these network cards have more than one type of port, which enable different types of cables to attach to the card. Network cards that provide wireless data transmission also are available. This type of card, called a wireless network card, often has an antenna. Some network cards include support for both wired and wireless networks. A network card follows the guidelines of a particular network communications standard, such as Ethernet or token ring. An Ethernet card is the most common type of network card. Some network cards also are a combination Ethernet and dial-up modem card.

A repeater is a device that accepts a signal from a transmission medium, amplifies it, and retransmits it over the medium. As a signal travels over a long distance, the signal undergoes a reduction in strength, an occurrence called attenuation. Repeaters regenerate analog or digital signals that can be distorted by attenuation.

A hub or switch is a device that provides a central point for cables in a network (Figure 7-10). Some hubs include routers. That is, the hub receives data from many directions and then forwards ti to one or more destinations.

Figure 7-10 A hub or switch is a central point that connects several devices in a network together

7.2.2   Communications Channels

An important aspect of communications is the communications channel, which is the communications path between two devices. Communications channel is composed of one or more transmission media, which consists of materials or techniques capable of carrying a signal. Figure 7-11 illustrates a typical communications channel and shows the variety of transmission media used to complete the connection. Although many media and devices are involved, the entire communications process could take less than one second.

Figure 7-11 An example of sending a request over the Internet using a communications channel

Various types of transmission media are available. Each type exhibits its own characteristics, including transmission capacity and speed. In developing a telecommunications system, the selection of media depends on the purpose of the overall information and organizational systems, the purpose of the telecommunications subsystems, and characteristics of the media. The proper media will help a company link its subsystems to maximize effectiveness and efficiency.

The amount of signals that can travel over a communications channel sometimes is called the Bandwidth. The higher the bandwidth, the more data and information the channel can transmit. Baseband transmission media can transmit only one signal at a time. By contrast, broadband media can transmit multiple signals simultaneously. Media that use broadband transmit signals at a much faster speed than those that use baseband. In many cases, download transfer rates of broadband are faster than its upload transfer rates. For transmission of text only, a lower bandwidth is acceptable. For transmission of multi-media such as photographs, music, videos, virtual reality images, or 3-D games, however, you need a higher bandwidth. When the bandwidth is too low for the application, you will notice a considerable slowdown is system performance. Home and business users today opt for  broadband Internet access because of the much faster transfer rates.

	Figure 7-12 Physical transmission media

Latency is the time it takes a signal to travel from one location to another on a network. Several factors that can negatively affect latency include the distance between the two points, the type of transmission media, and the number of nodes through which the data must travel over the communications channel. For best performance, bandwidth should be high and latency low.

Transmission media are one of two types: physical or wireless. Physical transmission media use wire, cable, and other tangible materials to send communications signals. Wireless transmission media send communications signals through the air or space using radio, microwave, and infrared signals. The following sections discuss these types of media.

Physical Transmission Media

Physical transmission media used in communications include twisted-pair cable, coaxial cable, and fiber-optic cable (Figure 7-12). These cables typically are used within building or underground between buildings. Ethernet and token ring LANs often use physical transmission media. Figure 7-13 lists the transfer rates of LANs using various physical transmission media.

A twisted-pair wire cable consists of one or more twisted-pair wires bundled together. Each twisted-pair wire consists of two separate insulated copper wires that are twisted together. The wires are twisted together to reduce noise, which is an electrical disturbance that can degrade communications. One type of twisted-pair cable, called shielded twisted-pair (STP) cable, has a metal wrapper around each twisted-pair wire, which further reduces noise. Cables that do not have this shielding are called unshielded twisted-pair (UTP). Inexpensive and easy-to-install, UTP cables commonly are used in telephone networks. Because STP cables more insulated than UTP cables, STP cables are used in environments susceptible to noise, such as in a local area network.

A coaxial cable consists of a single copper wire surrounded by at least three layers: (1) an insulating material, (2) a woven or braided metal, and (3) a plastic outer coating. Cable television wiring often uses coaxial cable because it can be cabled over longer distances than twisted-pair cable. Coaxial cable also is insulated more heavily than twisted-pair cable, and thus is not as susceptible to noise. Most of today's computer networks, however, do not use coaxial cable because other transmission media such as fiber-optic cable transmit signals at faster rates.

	Figure 7-13 The speeds of various physical communications media when they are used in LANs

The core of a fiber-optic cable consists of dozens or hundreds of thin strands of glass or plastic that use light to transmit signals. Each strand, called an optical fiber, is as thin as a human hair. These high-intensity light beams are generated by lasers and are conducted along the transparent fibers. Inside the fiber-optic cable, an insulating glass cladding and a protective coating surround each optical fiber. Because it transmits via light rather than electricity, fiber-optic cable has several advantages over cables that use wire. These advantage include (1) capability of carrying significantly more signals; (2) faster data transmission; (3) less susceptible to noise from other devices; (4) better security for signals during transmission; and (5) smaller size. Disadvantages of fiber-optic cable are higher cost and difficulty to install and modify. Many telephone companies and cable TV operators are replacing existing telephone and coaxial cables with fiber-optic cables, enabling them to offer fiber Internet access to home and business users. Many companies also are using fiber-optic cables in high-traffic networks or as the backbone in a network. 

Wireless Transmission Media

Many users opt for wireless transmission media because it is more convenient than installing cables. In addition to convenience, businesses use wireless transmission media in locations where it is impossible to install cables. Wireless telecommunications technologies transport digital communications without wires between communications devices. Wireless transmission media are used when it is inconvenient, impractical, or impossible to install cables. Wireless transmission media used in communications include broadcast radio, cellular radio, microwaves, communications satellites, and infrared. Each technology utilizes specific ranges within the electromagnetic spectrum (in megahertz) of electromagnetic frequencies that are specified by national regulatory agencies to minimize interference and encourage telecommunications.  Figure 7-14 lists transfer rates of various wireless transmission media.

	Figure 7-14 Transfer rates for various types of wireless transmission media

Broadcast radio is a wireless transmission medium that distributes radio signals through the air over long distances such as between cities, regions, and countries and short distances such as within an office or home. For radio transmissions, you need a transmitter to send the broadcast radio signal and receiver to accept it. To receive the broadcast radio signal, the receiver has an antenna that is located in the range of the signal. Some networks use a transceiver, which both sends and receives signals from wireless devices. Broadcast radio is slower and more susceptible to noise than physical transmission media but it provides flexibility and portability. Bluetooth, Wi-Fi, UWB, and WiMAX communications technologies use broadcast radio signals. Bluetooth and UWB is an alternative to infrared communications, with the latter designed for high bandwidth transmissions. Hot spots use Wi-Fi, WiMAX, and Bluetooth networks.

Cellular Radio is a form of broadcast radio that is used widely for mobile communications, specifically wireless modems and cell phones. A cell phone is a telephone device that uses high-frequency radio wave to transmit voice and digital data messages. Because only a limited number of radio frequencies exist, cellular network providers reuse frequencies so they can accommodate the large number of users. Cellular transmission uses radio waves, therefore, it is possible for people with special receivers to listen to cell phone conversations.

With cellular transmission, a local area, such as a city, is divided into cells. As a vehicle with a cellular device moves from one cell to another, the cellular system passes the phone connection from one cell to another (Figure 7-15). The base stations communicate with a mobile telephone switching office, which sends and receives voice and data traffic to and from the public switched telephone network. The signals from one cell are transmitted to a receiver and integrated into the regular phone system. Cell phone users can thus connect to anyone that has access to regular phone service or other cell phone users. They can also connect their notebook computer or other mobile computer to a cell phone to access the Web, send and receive e-mail, enter a chat room, or connect to an office or school network while away from a standard telephone line.

Figure 7-15 As a person with a cell phone drives from one cell to another, the radio signals transfer from the base station (microwave station in one cell to a bas station in another cell.

Several categories of cellular transmissions exist, defining the development of cellular networks:

• IG (first generation) transmitted analog data

• 2G (second generation) transmit digital data at speeds from 9.6 kbps to 19.2 kbps

• 3G (third generation) transmit digital data at speeds from 114 kbps to 2.4 Mbps

• 4G (fourth generation) transmit digital data at speeds up to 15 Mbps

3G technology allows users quickly to display multimedia and graphics, browse the Web, watch television or a video, have a video conference, and transfer data on a cellular device. The most recent cellular network category, the 4G network, uses the mobile wireless WiMAX communication standard. Several major communications companies have worked together to develop a nationwide 4G network that reaches more than 100 million people.

	Figure 7-16 Microwave stations

Personal Communications Services (PCS) is the term used by the US Federal Communications Commission (FCC) to identify all wireless digital communications. Devices that use PCS include cell phones, PDAs, pagers, and fax machines. These devices have voice mail, call forwarding, fax capability, caller ID, and wireless modems for Internet and email access.

Microwaves are high-frequency radio waves that are sent through the atmosphere and space. Microwaves provide a high-speed signal transmission, and can transmit data at rates up to 4,500 times faster than a dial-up modem. Microwave transmission involves sending signals from one microwave station to another, thus called fixed wireless (Figure 7-16). A microwave station is an earth-based reflective dish that contains the antenna, transceivers, and other equipment necessary for microwave communications.

Microwaves are limited to line-of-sight transmission, which means that microwaves must be transmitted in a straight line with no obstructions between microwave antennas. To avoid possible obstructions, such as buildings or mountains, microwave stations often sit on the tops of buildings, towers, or mountains.  Typically, microwave stations are placed in a series -- one station will receive a signal, amplify it, and retransmit it to the next microwave transmission tower. Microwave signals can carry thousands of channels at the same time.

	Figure 7-17 Communications satellites are placed about 22,300 miles above the Earth's equator.

A communications satellite is basically a microwave station placed in outer space (Figure 7-17). The satellite receives the signal from the earth, amplifies the relatively weak signal, and then rebroadcasts it at a different frequency to any number of earth-based stations. These earth-based stations often are microwave station. Other devices, such as smart phones and GPS receivers, also can function as earth-based stations. Transmission from an earth-based station to a satellite is an uplink. Transmission from a satellite to an earth-based station is a downlink. The advantage of satellite communications is the ability to receive and broadcast over larger geographic regions. Applications such as air navigation, television and radio broadcasts, weather forecasting, videoconferencing, paging, global positioning systems, and Internet connections use communications satellites. Most of today's communications satellites are owned by companies that rent or lease satellite communications capacity to other companies. However, several large companies are now using their own satellites for internal telecommunications. With the proper satellite dish and a satellite modem card, consumers can access the Internet using satellite technology. With satellite Internet connections, however, uplink transmissions usually are slower than downlink transmissions. This difference in speeds usually is acceptable to most Internet satellite users because they download much more data than they upload. Although a satellite Internet connection is more expensive than cable Internet of DSL connections, sometimes it is the only high-speed Internet option in remote areas.

	Figure 7-18 Many computers and device have IrDA ports that enable the transfer of data from one device to another using infrared light.

Infrared is a wireless transmission media that sends signals using infrared light waves. Infrared transmission requires a line-of-sight transmission and short distances -- under a few hundred yards. Infrared transmission can be used to connect various devices and computers. Many computers and devices, such as a mouse, printer, smart phone, and digital camera, have an IrDA port that enables the transfer of data from one device to another using infrared light waves. For example, infrared transmission has been used to allow handheld computers to transmit data and information to larger computers within the same room. This means of transmission can be used to establish a wireless network with the advantage that devices can be moved, removed, and installed without expensive wiring and network connections (Figure 7-18).

7.2.3  Communications Software, standards, and Protocol

Communications software consists of programs that (1) help users establish a connection to another computer or network; (2) manage the transmission of data, instructions, and information; and (3) provide an interface for users to communicate with one another. The first two are system software and the third is application software. The principal functions of communications software are network control, access control, transmission control, error detection/correction, and network security. A variety of examples of application software for communications include e-mail, FTP, Web browser, newsgroup/message boards, chat rooms, instant messaging, video conferencing, and VoIP.

Sometimes, communication devices are pre-programmed to accomplish communications tasks. Other communications devices require separate communications software to ensure proper transmission of data. Communication software works with the network standards and protocols to ensure data moves through the network or the Internet correctly. Communication software usually is bundled with the operating system or purchased network devices.

Often, a computer has various types of communications software, each serving a different purpose. One type of communications software, for example, helps users establish an Internet connection using wizards, dialog boxes, and other on-screen messages. Another allows home and small office users to configure wired and wireless networks and connect devices to an existing network.

Software tools and utilities are available for managing networks. Network control programs route messages, poll network terminals, determine transmission priorities, maintain a log of network activities, and check for errors. Access control programs establish connection between terminals and computers in the network, establishing transmission speed, mode, and direction. Transmission control programs enable computers and terminals to send and receive data, programs, commands, and messages. Error-control programs detect and correct errors, and then retransmit the corrected data. Security-control programs monitor utilization, log-ons, passwords, and various authorization procedures to prevent unauthorized access to a network.  

A telecommunications network typically contains diverse hardware and software components that need to work together to transmit data and information across many types of networks, such as wide area, local area, and wireless. For the different devices on various types of networks to be able to communicate, the network must use similar techniques of moving data through the network from one application to another. For example, an IBM mainframe computer cannot communicate directly with an Apple Macintosh network--some form of translation must occur for devices on these two types of networks to communicate.

To alleviate the problems of incompatibility and ensure that hardware and software components can be integrated into any network, various organization such as ANSI and IEEE propose, develop, and approve network standards. A network standard defines guidelines that specify the way computers access the medium to which they are attached, the types of medium used, the speeds used on different types of networks, and the types of physical cable and/or the wireless technology used. Different components in a network can communicate by adhering to a common set of rules that enable them to communicate each other. This set of rules and procedures governing transmission between components in a network is called a protocol.

A protocol is based on agreed-upon and established standard, and in this way all manufacturers of hardware and software that are using the protocol do so in a similar fashion to allow for interoperability. Interoperability is the capability of two or more computer systems to share data and resources, even though they are made by different manufacturers. Hardware and software manufacturers design their products to meet the guidelines specified in a particular standard, so that their devices can communicate with the network. The principal functions of protocol in a telecommunications network include:

• identifying each device in the communication path;

• securing the attention of the other device;

• verifying correct receipt of the transmitted message;

• determining that a message requires retransmission if it is incomplete or has errors;

• performing recovery when errors occur.

The goal of network architectures is to promote an open, simple, flexible, and efficient communications environment. This is accomplished by the use of standard protocols, standard communications hardware and software interfaces, and the design of a standard multilevel interface between end users and computer systems. Just as standards are important in building computer and database systems, established protocols help ensure communications among computers of different types and from different manufacturers. Networks use a variety of communications protocols and operating systems. Depending on the task, a single computer can use multiple protocols. For example, a computer might use one protocol to communicate with another computer on the LAN and a different protocol to communicate with a computer at an Internet service provider. A number of communications protocols are used by companies and organizations of all sizes. The following subsections discuss some of the more widely used network communications standards and protocols for both wired and wireless networks. As data moves through the network from ore program to another, it may use one or more of these standards.

Ethernet

Ethernet  is a network standard that specifies no central computer or device on the network (nodes) should control when data can be transmitted; that is, each node attempts to transmit data when it determines the network is available to receive communications. If two computers on an Ethernet network attempt to send data at the same time, a collision will occur, and the computers must attempt to send their messages again.

Ethernet is based on a bus topology, but Ethernet networks can be wired in a star pattern. The Ethernet standard defines guidelines for the physical configuration of a network, e.g., cabling, network cards, and nodes. Today, Ethernet is the popular communications protocol often used with local area networks (LAN) because it is relatively inexpensive and easy to install and maintain. Ethernet networks often use cables to transmit data. The original Ethernet standard is not very fast by today's standards. A more recent Ethernet standard, called Fast Ethernet, has a data transfer rate of 100 Mbps (million bits per second), ten times faster than the original standard. Gigabit Ethernet provides an even higher speed of transmission, with transfer rates of 1 Gbps (1 billion bits per second). The 10-Gigabit Ethernet standard supports transfer rates up to 10 Gbps.

Token Ring

Token Ring is another popular network standard for LANs.  This standard specifies that computers and devices on the network share or pass a special signal, called a token, in a unidirectional manner and in a preset order. A token is a special series of bits that unction like a ticket. The device with the token can transmit data over the network. Only one token exists per network. This ensures that only one computer transmits data at a time. Token ring is based on ring topology (although it can use a star topology). The token ring standard defines guidelines for the physical configuration of a network. Some token ring networks connect up to 72 devices. Others use a special type of wiring that allows up to 260 connections. The data transfer rate on a token ring network can range from 4 Mbps to 1 Gbps.

TCP/IP

Transmission Control Protocol/Internet Protocol (TCP/IP) is a network standard that defines how messages (data) are routed from one end of a network to the other, ensuring the data arrives correctly. TCP/IP describes rules for dividing messages into small pieces, called packets; providing addresses for each packet; checking for and detecting errors; sequencing packets; and regulating the flow of messages among the network.

TCP/IP has been adopted as a network standard for Internet communications. Thus, all hosts on the Internet follow the rules defined in this standard. As shown in Figure 7-19, Internet communications also use other standards, such as the Ethernet standard, as data is routed to its destination. When a computer sends data over the Internet, the data is divided into packets. Each packet contains the data, as well as the recipient (destination), the origin (sender), and the sequence information used to reassemble the data at the destination. Each packet travels along the fastest individual available path to the recipient's computer via communications devices called routers. This technique of breaking a message into individual packets, sending the packets along the best route available, and then reassembling the data is called packet switching. 

Figure 7-19 Example of how communications standards work together

Although more than 100 protocols make up the entire TCP/IP protocol suite, the two most important of these are TCP and IP. TCP provides transport functions, ensuring, among other things, that the amount of data received is the same as the amount transmitted. IP provides the addressing and routing mechanism that acts as postmaster. Figure 7-20 display TCP/IP’s four-layer reference model:

	Figure 7-20 TCP/IP four-layer reference model

• Application layer—serves as the window for users and application processes to access network services

• Transport layer—handles end-to-end packet transportation.

• Internet layer—formats the data into packets, adds a header containing the packet sequence and the address of the receiving device, and specifies the services required from the network.

• Network interface layer—places data packets on the network for transmission.

The TCP/IP suite of applications includes five protocols:

• File Transfer Protocol (FTP): It allows files containing text, programs, graphics, numerical data, and so on to be downloaded off or uploaded onto a network.

• Simple Mail Transfer Protocol (SMTP): It is TCP/IP’s own messaging system for e-mail.

• Telnet Protocol: It provides terminal emulation that allows a personal computer or workstation to act as a terminal, or access device, for a server.

• Hypertext Transfer Protocol (HTTP): It allows Web browsers and servers to send and receive Web pages.

• Simple Network Management Protocol (SNMP): It allows the management of networked nodes to be managed from a single point.

802.11 (Wi-Fi)

802.11, developed by IEEE, is a series of network standards that specifies how two wireless devices communicate over the air with each other. Using the 802.11 standard, computers or de ices that have the appropriate wireless capability communicate via radio waves with other computers or devices. Figure 7-21 outlines various 802.11 standards and their data transfer rates. A designation of 802.11 a/b/g on a computer or device indicates it supports all three standards. The newest standard, 802.11n, uses multiple transmitters and receivers, known as MIMO (multiple-input multiple-output), to reach speeds from 2 to 10 times faster than 802.11g.

	Figure 7-21 Series of 802.11 standards

The 802.11 standard often is called the wireless Ethernet standard because it uses techniques similar to the Ethernet standard to specify how physically to configure a wireless network. Thus, 802.11 networks easily can be integrated with wired Ethernet networks. When an 802.11 network accesses the Internet, it works in conjunction with the TCP/IP network standard.  

The term Wi-Fi (wireless fidelity) identifies any network based on the 802.11 series of standards. Wi-Fi Certified products are guaranteed to be able to communicate with each other. Windows Vista and Windows Mobile include support for Wi-Fi. Most of today's computers and many personal mobile devices are Wi-Fi enabled.

One popular use of the Wi-Fi network standard is in hot spots that offer mobile users the ability to connect to the Internet with their wireless computers and devices. Many home and small businesses also use Wi-Fi to network computers and devices together wirelessly. In open or outdoor areas free from interference, the computers or devices should be within 300 feet of each other. In closed areas, the wireless network range is about 100 feet. To obtain communications at the maximum distances, you may need to install extra hardware.

Some entire cities are set up as a Wi-Fi mesh network in which each mesh node routes its data to the next available node until the data reaches its destination--usually an Internet connection. A Wi-Fi mesh network is more flexible than a hot spot because each node in a mesh network does not have to be directly connected to the Internet.

Bluetooth

Bluetooth is a network standard that defines how two Bluetooth devices use short-range radio waves to transmit data. The data transfers  between devices at a rate of up to 3 Mbps. A Bluetooth computers and device contains a small chip that allows it to communicate with other Bluetooth devices. For computers and devices not Bluetooth-enabled, you can purchase a Bluetooth wireless port adapter that will convert an existing USB port or serial port into a Bluetooth port. To communicate with each other, Bluetooth devices often must be within about 10 meters but can be extended to 100 meters with additional equipment. Windows Vista has built-in Bluetooth support.

UWB

UWB, which stands for ultra-wideband, is a network standard that specifies how two UWB devices use short-range radio waves to communicate at high speeds with each other. At distances of 10 meters (about 33 feet), the data transfer rate is 110 Mbps. At closer distances, such as 2 meters (about 6.5 feet), the transfer rate is at least 480 Mbps. UWB can transmit signals through doors and other obstacles. Because of its high transfer rates, UWB is best suited for transmission of large files such as video, graphics, and audio. 

IrDA

IrDA is a standard for transmitting data wirelessly to each other via infrared light waves. The devices transfer data at rates from 115 Kbps to 4 Mbps between their IrDA ports. Infrared requires a line-of-sight transmission, which means that the sending device and the receiving device must be in line with each other so that nothing obstructs the path of the infrared light wave.  Because Bluetooth and UWB do not require line-of-sight transmission, some industry experts predict that these technologies will replace infrared.

RFID

RFID (radio frequency identification) is a standard that defines how a network uses radio signals to communicate with a tag placed in or attached to an object, an animal, or a person. The tag, called a transponder, consists of an antenna and a memory chip that contains the information to be transmitted via radio waves. Through an antenna, an RFID reader, also called a transceiver, reads the radio signals and transfers the information to a computer or computing device.

RFID tags are passive or active. An active RFID tag contains a battery that runs the chip's circuitry and broadcasts a signal to the RFID reader. A passive RFID tag does not contain a battery and thus cannot send a signal until the reader activates the tag's antenna by sending out electromagnetic waves. Because passive RFID tags contain no battery, these can be small enough to be embedded in skin. To date, more than 100,000 people have been implanted with RFID tags. The most common application of human-implanted RFID tags is to control access to buildings and other secure areas. Depending on the type of RFID reader, the distance between the tag and the reader ranges from 5 inches to 15 feet. Readers can be handheld or embedded in an object such as the tollbooth shown in Figure 7-22.

Figure 7-22 How electronic RFID toll collection works

WiMAX

WiMAX (Worldwide Interoperability for Microwave Access), also known as 802.16, is a newer network standard developed by IEEE that specifies how wireless devices communicate over the air in a wide area. Using the WiMAX standard, computers or devices with the appropriate WiMAX wireless capability communicate via radio waves with other computers or devices via a WiMAX tower.

The WiMAX Forum is a wireless industry association of more than 480 suppliers and Internet access providers dedicated to developing specifications and testing equipment. Two types of WiMAX specifications defined by the WiMAX forum are fixed wireless and mobile wireless. With fixed wireless WiMAX, a customer accesses the Internet from a desktop computer at home or other permanent location. Mobile wireless WiMAX, by contrast, enables users to access the WiMAX network with mible computers and mobile devices. Fixed wireless WiMAX has data transfer rates up to 40 Mbps, while mobile wireless WiMAX has data transfer rates up to 15 Mbps.

The WiMAX standard provides wireless broadband Internet access at a reasonable cost over long distances to business and home users. Expert predict that WiMAX service eventually could surpass other broadband Internet access services such ad DSL and cable because it can reach rural and remote areas easily and inexpensively. The WiMAX standard, similar to the Wi-Fi standard, connects mobile users to the Internet via hot spot. Many computers and mobile devices have built-in WiMAX capability.

WAP

The Wireless Application Protocol (WAP) is standard that specifies how some mobile devices can display the content of Internet services such as the Web, e-mail and chat rooms. For example, users can check weather, sports scores, and headline news from their WAP-enabled smart phones.

WAP uses a client/server network. The wireless device contains the client software, which connects to the Internet access provider's server. Devices that support WAP, called WAP-enabled devices, include Web-enabled telephones, pagers, and handheld computers. For example, to display a Web page on a smart phone, the phone would be WAP enabled and contain a microbrowser. On WAP-enabled devices, data transfer rates range from 9.6 to 153 Kbps depending on the type of service. WAP works in conjunction with the TCP/IP network standard.

OSI Reference Model

	Figure 7-23 The seven layers of the OSI model

The Open Systems Interconnection (OSI) reference model, developed by the International Organization for standardization (ISO), serves as a standard model for network architectures. The OSI model divides data communications functions into seven distinct layers to promote the development of modular networks that simplify the development, operation, and maintenance of complex telecommunications networks, as shown in Figure 7-23. A simple way to understand the OSI model is to think of it as an elevator. On the sending end, data enters at the top floor and travels to the bottom floor. Each layer communicates with the layers immediately above and below it. When a layer receives data, it performs specific functions, adds control information to the data, and passes it to the next layer. The top layer, the application layer, serves as the interface between the user and the network. The presentation layer translates the converted message data into a language the receiving computer can process and also may compress or encrypt the data. The session layer establishes and maintains communications sessions. A session is the period between establishment of a connection, transmission of the data, and termination of the connection. The transport layer ensures that data arrives correctly and in proper sequence. The network layer routes the message from sender to receiver. The data link layer supervises the transmission of the message to the next network node by specifying the network technology and grouping data accordingly. Finally, the physical layer encodes the packets into a signal recognized by the medium that will carry them and sends the packets along that medium to the receiving computer.

The lower layers (1 to 3) represent local communications, while the upper layers (4 to 7) represent end-to-end communications. Each layer contributes protocol functions that are necessary to establish and maintain the error-free exchange of information between network users. For many years, users thought the OSI model would replace TCP/IP as the preferred technique for connecting multi-vendor networks. But the slow pace of OSI standards as well as the expense of implementing complex OSI software and having products certified for interoperability will preclude this from happening.

7.3 Data Transmission

Telecommunications involves the transmission of data, information, and instructions among computers. Any transmissions sent during these communications can be categorized by a number of characteristics including the signal type, transmission mode, transmission direction, and transmission rate.

Signal Type: Analog or Digital

Recall that computers produce digital signals, which are simply the presence or absence of an electric pulse. The state of being on or off represents the binary number of 1 or 0, respectively. Some communications channels accept digital transmission directly, and the trend in the communications industry is toward digital transmission. However, telephone equipment originally was designed to carry only voice transmission in the form of an analog signal, which represented by a continuous waveform that passes through a communications medium. For traditional telephone lines to carry digital signals, all digital signals must be translated into analog signals before they can be transmitted. The device that performs this translation is called a modem. A modem converts the digital signals of a computer into analog form for transmission over ordinary telephone lines, or converts analog signals back into digital form for reception by a computer (Figure 7-24).

Figure 7-24 Digital signal and analog signal in telecommunication

Transmission Modes: Asynchronous and Synchronous

When two devices exchange data, the data flows between the devices as a continuous stream of bits. There are two basic transmission techniques for separating the groups of bits: asynchronous transmission and synchronous transmission (Figure 7-25). These methods are necessary for devices to know when a byte begins or ends.

Figure 7-25 Asynchronous and synchronous transmission

Asynchronous transmission transmits one byte at a time over a line at random intervals, each byte framed by controls -- a start bit for marking the beginning of the byte, a stop bit for marking the end of the byte, and a parity bit for error checking. Asynchronous transmission is relatively slow and used for low-speed transmission.

Synchronous transmission transmits groups of bytes simultaneously at regular intervals, with the beginning and ending of a block of bytes determined by the timing circuitry of the sending device and receiving devices. Although synchronous transmission requires more sophisticated and expensive communications devices, it provides much higher speeds and greater accuracy than asynchronous transmission. Some people use the term synchronous to refer to real-time live communications and the term asynchronous to refer to communications that are not real time.

Transmission Direction

	Figure 7-26 Data flow can be simple, half-duplex, full-duplex and multiplex

The direction in which data flows along transmission media is characterized as simplex, half-duplex, full-duplex or multiplex (Figure 7-26).

Simplex transmission sends data in one direction only. Simplex transmission is used only when the sending device does not require a response from the receiving device. One example of simplex transmission is television broadcasting. Half-duplex transmission allows data transmission in either direction, but only one way at a time. Many fax machines, credit card verification systems and automatic teller machines use half-duplex transmission. In full-duplex transmission, data can flow in both directions at the same time. A regular telephone line, for example, supports full-duplex transmission, allowing both parties to talk at same time. In multiplex transmission, several different types of signals can be carried at once through the same line. 

7.4 Networks

7.4.1 Network Topologies

A network is a collection of computers and devices connected by telecommunications channels that allows users to facilitate communications, and to share data, information, software, and hardware with other users. In a networked environment, any authorized user can use a computer on a network to access data and information stored on other computers in the network, to access hardware that shared in the network, to use software stored on a server's hard disk, to transfer funds, and to communicate efficiently and easily via e-mail, chat, and videoconferencing. A network can be internal to an organization or span the world by connecting itself to the Internet. Networks facilitate communications among users and allow users to share resources with other users.

There are a number of different ways available to organize telecommunications components to form a network. Thus networks can be classified in multiple ways. Network can be classified by their configuration or physical layout called topology, by their geographic scope, and by the type of services provided. A network topology is a description of the possible physical connections within a network. In a network topology, a component is called a node, which refers to any device connected to a network, including the server, computers, telephones, and other devices. Three commonly used network topologies are bus, ring, and star. However, a pure form of any of these three basic topologies is seldom found in practice. Most computer networks are hybrids—combinations of these topologies.

	Figure 7-27 Devices in a bus network share a single data path

A bus network consists of a single central cable, to which all the network nodes are attached (Figure 7-27). The bus is the physical cable that connects the computers and other devices. The bus in a bus network transmits data, instructions, and information in both directions. When a sending device transmits data, address of the receiving device is included with the transmission so that the data is routed to the appropriate receiving device. All the signals are broadcast in both directions to the entire network, with special software to identify which nodes receive each message. However, only one node can transfer items at one time. There is no central host computer to control the network. Nodes can be attached to or detached from the network without affecting the network. If one node in the network fails, none of the other nodes in the network is affected. However, the greatest risk to a bus network is that the bus itself might become inoperable. This topology is commonly used for local area networks (LAN).

A ring network links all nodes together in a circular chain (Figure 7-28). Data messages travel in only one direction from device to device around the entire ring. The node examines any data that passes by to see if it is the addressee; if not, the data is passed on to the next node in the ring. If a node on a ring network fails, all nodes before the failed node are unaffected, but those after the failed nodes cannot function. A ring network can span a larger distance than a bus network, but it is more difficult to install. The ring topology primarily is used for LANs, but also is used in wide area networks (WAN).

Figure 7-28 On a ring network, all connected devices form a continuous loop

The token ring network is a variant of the ring network. In the token ring network all the devices on the network communicate using a special signal called token. The token is a predefined packet of data, which includes data indicating the sender, receiver, and whether the packet is in use. The token may contain a message or be empty. A token moves from node to node in the network, and each node examines the token as it passes. If the token contains data and is meant for that node, the node accepts the data and marks the packet as empty. If a node wants to send a message, it finds an available token, loads the message onto the token, and marks it as used. If no message is pending, the token passes unchanged. The ring topology primarily is used for local area networks, but also to connect a mainframe to a wide area network.

On a star network, all of the computers and devices (nodes) on the network connect to a central device, thus forming a star (Figure 7-29). Two types of devices that provide a common central connection point for nodes on the network are a hub and a switch.  The hub/switch is responsible for managing the network. All data that transfers from one node to another node passes through the hub/switch. Star networks are fairly easy to install and maintain. Nodes can be added to and removed from the network with little or no disruption to the network. On a star network, if one node fails, only that node is affected. Any connection failure between a node and the hub will not affect the overall system. However, if the hub computer fails, the entire network fails. Most large star networks keep backup hubs/switches available in case the primary one fails.

	Figure 7-29 A star network contains a single, centralized hub or switch through which all the devices in the network communicate

7.4.2 LAN, MAN, and WAN

Many different types of networks serve as the telecommunications infrastructure for the Internet and the intranets and extranets of inter-networked enterprises. However, from an end user's point of view, computer networks can also be classified by the proximity of their nodes. Usually, networks are classified ad a local area network, metropolitan area network, or wide area network. The main differentiation among these classifications is their area of coverage.

A local area network (LAN) is a network that connects computers in a limited geographical area, such as a school computer laboratory, department, or closely positioned group of buildings (Figure 7-30). A LAN, the most common network, consists of a communications channel, networked computers and devices, a network interface card, and a network operating system. The most common use of LANs is for linking personal computers within a building or office to share information and expensive peripheral device. Another popular application of LANs is in factories, in which they link computers and computer-controlled machines.

Figure 7-30 An example of a LAN

Local area networks can be built at various levels of sophistication. A local area network can be a ring, bus, or star network. It can be built around powerful personal computers, minicomputers, or mainframe computers. Each computer in the LAN usually requires a network interface card, which is a board that is placed in a computer's expansion slot to allow it to communicate with the network. A LAN uses the network gateway to connect to public networks or other corporate networks so that the LAN can exchange data with networks external to it. Recall that a gateway is generally a communications processor that can connect dissimilar networks by translating from one set of protocols to another.

A wireless LAN (WLAN) is a LAN that uses no physical wires. Computers and devices that access a wireless LAN must have built-in wireless capability or the appropriate wireless network card, PC Card, ExpressCard module, USB network adapter, or flash card. Very often, a WLAN communicates with a wired LAN for access to its resources, such as software, hardware, and the Internet.

	Figure 7-31 An example of a WAN

If a network is designed for a city, that is, it connects local area networks in a metropolitan area and handles the bulk of communications activity, or traffic, across that region, this network is called metropolitan area network (MAN). A MAN typically includes one or more LANs but covers a smaller geographic area than a WAN. A MAN usually is managed by a consortium of users or by a single network provider who sells the service to the users. Local and state governments, for example, regulate some MANs. Telephone companies, cable television operators, and other organizations provide users with connections to the MAN.

A wide area network (WAN) is a network that covers a large geographical area using a communications channel that usually combines many different types of transmission media (Figure 7-31). A WAN can be one large network or can consist of two or more LANs connected together. Common communications carriers such as AT&T typically determine transmission rates or interconnections between lines, but customers are responsible for their telecommunications contents and management. It is up to the individual organization to establish the most efficient routing of messages, and to handle editing, protocols, error checking, and communications management. Individual business firms may maintain their own wide area networks. However, private wide area networks are expensive to maintain, and firms may not have the resources to manage their own wide area networks. In such instances, companies may choose to use commercial network services to communicate over vast distances. Today, a WAN typically consists of two or more LANs connected by router that ensure that data are delivered to the correct destination. The Internet is the world's largest WAN.

Many home users are connecting multiple computers and devices together in a home network. Home networking saves the home user money and provides many conveniences. Each networked computer in the house has the following capabilities:

• Connect to the Internet at the same time

• Share a single high-speed Internet connection

• Access files and programs on the other computers in the house

• Share peripherals such as a printer, scanner, external hard disk, or DVD drive

• Play multiplayer games with players on other computers in the house.

• Connect game consoles to the Internet

• Subscribe to and use VoIP

Many vendors offer home networking packages that include all the necessary hardware and software to network your home using wired or wireless techniques. Some of these packages also offer intelligent networking capabilities. An intelligent home network extends the basic home network to include features such as lighting control, thermostat adjustment, and a security system.

As with other networks, a home network can use wires, be wireless, or use a combination of wired and wireless. To network computers and devices that span multiple rooms or floors in a home, it may be more convenient to use a wireless strategy.  One advantage of wireless networks is that you can take a mobile computer outside, for example in the backyard, and connect to the Internet through the home network, as long as you are in the network's range. Most home networks use a Wi-Fi network because Wi-Fi networks are fairly easy to configure. Figure 7-32 shows the steps to set up hardware for a Wi-Fi home network. Wireless networks do have the disadvantage of interference. Walls, ceilings, and electrical devices such as cordless telephones and microware ovens can disrupt wireless communications.

Figure 7-32 How to set up hardware for a Wi-Fi home network

	Figure 7-33 An example of a client/server network

7.4.3 Network Architectures

The design of computers, devices, and media in a network, sometimes called the network architecture, is categorized as either client/server or peer-to-peer. The major difference between these two types of LANs lies in how the data and information is stored. The following paragraphs discuss these network architectures.

A client/server network is a network in which one or more computers are designated as a server(s) and other computers on the network, called clients, can request services from the server, such as providing database access or queuing print jobs (Figure 7-33). A server, also called host computer, controls access to the hardware and software on the network and provides a centralized storage area for programs, data, and information. The other computers (clients) on the network rely on the servers for these resources, such as files, devices, processing power, and storage. The major difference between the server and the client computers is that the server ordinarily is faster and has more storage capacity. Thus, the server generally performs most of the processing tasks. Sometimes the server and the client computers share processing. Some servers are dedicated to performing a specific task. For example, a file server stores and manages files; a print server manages printers and print jobs; and a database server stores and provides access to a database. A network server manages network traffic and activity). A client/server network typically provides an efficient means to connect many computers together. Most client/server networks have a network administrator, who is the operations person in charge of the network.

Figure 7-34 Each computer on a peer-to-peer network shares its hardware and software with other computers on the network.

A peer-to-peer network is a simple, inexpensive network. All computers in a peer-to-peer network have equal status; no one computer is in control. Each computer, called a peer, in the network can share the hardware, data, or programs located on any other computer in the network. Each computer stores files on its own storage devices. Each computer in the network also must install a network operating system and application software (Figure 7-34). However, only one computer on the network needs to connect to peripherals; the other computers in the network share these hardware resources. Peer-to-peer networks are typically used in very small business and organizations. Some operating systems, such as Windows, include a peer-to-peer networking utility that allows users to set up a peer-to-peer network.

Internet peer-to-peer (P2P) describes an Internet network on which users access each other's hard disks and exchanges files directly over the Internet (figure 7-35). This type of peer-to-peer network is also called a file sharing network because users with compatible software and an Internet connection copy files from someone else's hard disk to their hard disks. As more users connect to the network, each user has access to shared files on other users' hard disks. When users log off the network, others no longer have access to their hard disks. To maintain an acceptable speed for communications, some implementations of P2P limit the number of users. Example of networking software that support P2P are BitTorrent, Gnutella, Kazaa, and LimeWire, which allow users to swap music and other files via the Web. For example, when one user requests a song, the program searches through lists of shared files--which are stored on one or more connected computers, called supernodes. If a match is found, the music file is copied from the computer on which it resides to the requesting computer. These programs initially stirred much controversy with respect to copyright infringement of music because they allowed users easily to copy music and movie files free from one computer to another. To help reduce copyright infringement, today's music and movie sharing services typically are fee based, and music and movie files often are encrypted as they travel across the Internet.

	Figure 7-35 P2P describes an Internet network on which users connect to each other's hard disks and exchanges files directly.

Many businesses also see an advantage to using P2P. That is, companies and employees can exchange files using p2P, freeing the company from maintaining a network server for this purpose. Business-to-business e-commerce Web sites find that P2P easily allows buyers and sellers to share company information such as product databases.

7.4.4   Networks in Large Corporations

The network infrastructure for a large corporation consists of a large number of small local area networks linked to other local area networks and to firm-wide corporate networks. A number of powerful servers support a corporate Web site, a corporate intranet, and perhaps an extranet. Figure 7-36 provides an illustration of the complex, larger scale corporate-wide network. As you can see from this figure, a large corporate network infrastructure uses a wide variety of technologies—everything from ordinary telephone service and corporate data networks to Internet service, wireless Internet, and wireless call phone. One of the major problems facing corporations today is how to integrate all the different communication networks and channels into a coherent system that enables information to flow from one part of the corporation to another, from one system to another. As more and more communication networks become digital, and based on Internet technologies, it will become easier to integrate them.

Recognizing the efficiency and power of the Internet, many organizations apply Internet and Web technologies to their own internal networks. An intranet (intra means within) is an internal network that uses Internet technologies. Intranets generally make company information accessible to employees and facilitate working in groups. Simple intranet applications include electronic publishing of organizational materials such as telephone directories, event calendars, procedure manuals, employee benefits information and job postings. Additionally, an intranet typically includes a connection to the Internet. More sophisticated uses of intranets include groupware applications such as project management, chat rooms, newsgroups, group scheduling, and video conferencing.

An intranet essentially is a small version of the Internet that exists within an organization. It has a Web server, supports multimedia Web pages coded in HTML, and is accessible via a Web browser such as Internet Explorer, Firefox, Opera, and Safari. Users update information on the intranet by creating and posting a Web page, using a method similar to that used on the Internet.

Sometimes a company uses an extranet, which allows customers or suppliers to access part of its intranet. Package shipping companies, for example, allow customers to access their intranet to print air bills, schedule pickups, and even track shipped packages as the packages travel to their destinations.

Figure 7-36 Corporate network infrastructure

7.5 Telecommunication Applications

	Figure 7-37 Features of some communications

Telecommunication technologies have changed the way individuals interact, by allowing for instant and accurate information transfer, 24 hours a day. Today, uses of telecommunications technology are all around you. Many computer communications require that users subscribe to an Internet access provider. With other computer communications, an organization such as a business or school provides communications services to employees, students, or customers. In the course of a day, for example, you might use, or use information generated by, one or more of the following telecommunications technologies: e-mail, voice mail, fax, telecommuting, electronic conferencing, bulletin board system, digital information service, electronic data interchange, groupware, global positioning system, instant messaging, chat rooms, VoIP, FTP, Web, Web folders, and the Internet. The following pages discuss a variety of the computer communications. Figure 7-37 lists the features of some communications.

E-mail

Electronic mail (e-mail) is the exchange of text messages and computer files transmitted via a communications network such as a local area network or the Internet. Communications devices, such as modems, transfer the e-mail messages to and from computers or terminals on the same network or a separate network. To send and receive e-mail messages, you use e-mail software installed on your computer. When another user sends you an e-mail message, the message is placed in your mailbox, which is a storage location on the computer that connects you to the local area network or the Internet, such as the server operated by your Internet service provider. Using your e-mail software, you can retrieve, read, and reply to that message or delete it from you mailbox.

Voice Mail

Voice mail functions much like an answering machine, allows callers to leave a voice message for a called party. Unlike answering machines, however, a computer in the voice mail system converts an analog voice message into digital form. Once digitized, the message is stored in a voice mailbox, which is a storage location on a computer in the voice mail system. When the recipient is ready to listen, the messages are reconverted to audio form. A voice mail system usually provides individual voice mailboxes for many users. Some voice mail systems can send digital voice mail files to e-mail addresses. Others can convert a voice mail message to a text message for display on a computer or mobile device, which you can manage like any other text message. By accessing your voice mailbox, you can listen to messages; add comments to a message; and reply or forward a message to another voice mailbox in the voice mail system. Some voice mail systems allow you to send the same message to a specific group of individuals or everyone listed in the system's database.

Fax

A fax (facsimile) machine is a device that sends and receives documents via telephone lines. You can send or receive a fax using a stand-alone fax machine or a fax modem. A stand-alone fax machine scans and digitizes the document image. The digitized document is then transmitted over a network and reproduced in hard copy form by a receiving fax machine. A fax modem allows you to send and receive faxes using your computer. Because the fax modem must be connected to a computer, it can transmit only documents that already are digitized, such as a word processing letter or a digital photograph. Many computers include fax capability by using a fax modem. When a computer (instead of a fax machine) receives a fax, users can view the fax on the screen.

VoIP (Internet Telephony)

VoIP, also called Internet telephony, enables you to talk to other people over the Internet. Telephony uses the Internet to connect a calling party and one or more called parties. To place a telephony call, you need Internet telephone software. As you speak into a computer microphone, Internet telephone software and your computer’s sound card digitize and compress your conversation and then transmit the digitized audio over the Internet to the called parties. Software and equipment at the receiving end reverse the process so the receiving parties can hear what you have said, just as if you were speaking on a telephone.

Telecommuting

Telecommuting is a work arrangement in which employees work away from the company's standard workplace, but communicate with the office using some communications technology. A telecommuter often works at home and connects to the main office's network using a personal computer or mobile device equipped with communications software and a communications device. Once connected, the employee can read and answer e-mail, access databases, and send and receive project-related information.

Electronic Conferencing

People can meet electronically, even though they are geographically separated. Teleconferencing allows a group of people to confer simultaneously via telephone or via electronic mail group communication software. Dataconferencing includes the ability of two or more people at distant locations to work on the same document or data simultaneously. With dataconferencing, users at distant locations are able to edit and directly modify data files simultaneously. Videoconferencing also has the capability to let participants see each other face-to-face over video screens.

Digital Information Service

Digital information services enable networked computer and mobile device users to obtain information from outside instantaneously. Stock prices, historical references to periodicals, industrial supplies catalogs, legal research, news articles, reference works, weather forecasts, and travel information are just some examples of the electronic databases that can be accessed on-line. Many of these services have capabilities for e-mail, instant messaging, chat rooms, BBS, on-line discussion groups, shopping, and travel reservations.

Groupware

Groupware is a software application that provides functions and services to support the collaborative activities of work groups. Groupware is a component of a broad concept called workgroup computing, which includes hardware and programs for information sharing, project management, electronic meeting, scheduling, and group decision-making. Many software products allow you to conduct online meetings. In an online meeting, you can share documents with others in real time. All participants see the document at the same time. As someone changes the document, everyone can see the changes being made. To assist with these activities, most groupware provides personal information manager (PIM) functions.

Global Positioning System

A global positioning system (GPS) is a navigation system that consists of earth-based receivers that accept and analyze signals sent by satellites in order to determine the receiver's geographic location (Figure 7-38). A GPS receiver is a handheld, mountable, or embedded device that contains an antenna,  a radio receiver, and a processor. Many include a screen display that shows an individual's location on a map. Some also function as a portable media player allowing you to play music and view pictures on the device.

Many mobile devices such as smart phones have GPS capability built into the device or as an add-on feature. Some users carry a handheld GPS receiver, other mount a receiver to an object such as an automobile, boat, airplane, farm and construction equipment, or computer.

The first and most used application of GPS technology is to assist people with determining where they are located. The data obtained from a GPS, however, can be applied to a variety of other uses: creating a map, ascertaining the best route between two points, locating a lost person or stolen object, determining altitude, monitoring the movement of a person or object, and calculating speed. Many vehicles use GPSs to provide drivers with directions or other information, automatically call for help if the airbag is deployed, dispatch roadside assistance, unlock the driver's side door if keys are locked in the car, and track the vehicle if it is stolen. Hikers and remote campers may carry GPS receivers in case they need emergency help or directions.

Some GPS receivers work in conjunction with a cellular wireless network. When a user presses a button on the GPS receiver or at regularly scheduled times, the GPS receiver captures data from the satellite and sends location information to a cellular wireless network. Once the cellular wireless network receives the data, its computer calculates the exact location of a person or object.

Figure 7-38 How a GPS works

Wireless Messaging Services

	Figure 7-39 Users can use their smart phones to send and receive text messages, wireless instant messages, and picture/video messages.

Users can send and receive wireless messages to and from smart phones, cell phones, PDAs or other personal mobile devices using three techniques: text messaging, wireless instant messaging, and picture/video messaging (Figure 7-39). The type of messaging you use depends primarily on the services offered by the wireless Internet service provider that works with the cell phone or other personal mobile device you select.  A mobile device with text messaging capability allows users to send and receive short text messages on a phone or other mobile device. Most text messaging services limit messages to a specific number of characters and store incoming text messages for limited time.

Wireless instant messaging is a real-time Internet communications service that allows wireless mobile devices to exchange messages with one or more mobile devices or online users. Users can send graphics, pictures, video clips, and sound files, as well as short text message with picture/video messaging to another smart phone, PDA, or computer. If you send a picture message to a cell phone that does not have picture messaging capability, the cell phone usually displays a text message directing the user to a Web page that contains the picture message.

Public Internet Access Points

In many public locations, people connect wirelessly to the Internet through a public Internet access point using mobile computers or other devices. Three types of public Internet access point are hot spots, mobile wireless networks, and cybercafes.

A hot spot is a wireless network that provides Internet connections to mobile computers and other devices. Three hot spot technologies are Wi-Fi, WiMAX and Bluetooth. Mobile users can access any service on the Internet as long as their computers or devices have built-in wireless capability or the appropriate wireless network card or PC card. Hot spots are appearing in airports, hotels, schools, shopping malls, bookstores, restaurants, libraries, train stations, and coffee shops. Most hot spots span from 100 to 300 feet; some can extend to 15 miles and cover entire cities.

Instead of hot spots, some users access the Internet through mobile wireless networks that provide users with high-speed wireless Internet connections, as long as they are in the network's range. A mobile wireless network usually covers most major cities and airports. Two types of mobile wireless networks are 3G and 4G.

When mobile users travel without their notebook computer or Internet-enabled mobile device, they can visit a cybercafe to access the Internet service. A cybercafe, or Internet cafe, is a coffee house, restaurant, or other location that provides personal computers with Internet access to its customers. Cybercafes exist in cities around the world.

Collaboration

	Figure 7-40 Through an online meeting, all participants see a document at the same time

Many programs provide a means to collaborate, or work online, with other users connected to a server. Collaborative software includes tools that enable users to share documents via online meetings and communicate with other connected users. An online meeting allows users to share documents with others in real time (Figure 7-40). When the online meeting takes place on the Web, it is called a Web conference. In an online meeting, all participants see a document at the same time. As someone changes the document, everyone in the meeting sees the changes being made. During the online meeting, participants have the ability to open a chat window and type messages to one another. Collaborative software often has whiteboard and video/audio conferencing capabilities. Examples of collaborative software include Microsoft Office Live Meeting, Acrobat Connect, GoToMeeting and WebEx.

Instead of interacting in a live meeting, users often collaborate via e-mail. For example, if users want others to review a document, they can e-mail the document for review. When the recipients receive the document, they may add comments to the document. As the reviewers make changes to the document, both the original text and the changes are displayed. When the originator receives the document back from all reviewers, he can merge all comments and changes into a single document.

Some companies use document management systems to make collaboration possible among employees. A document management system provides for storage and management of a company's documents. Users then access these documents, depending on their needs. A document management system can track all changes made to a document. It also can store additional information such as the document's creation date, the user who created the document, a summary of the document, and any keywords associated with the document.

Web Services

Web services describe standardized software that enables programmers to create applications that communicate with other remote computers over the Internet or on an internal business network (Figure 7-41). Businesses are the primary users of Web services because this technology provides a means for departments to communicate with each other, suppliers, vendors, and with clients. For example, third-party vendors can use Web services to communicate with their online retailer's Web site to manage their inventory levels.

Web services do not require a specific programming language, operating system, or Web browser. Different applications from different platforms can communicate with each other by sending properly formatted XML. Web services do not have a user interface because the application's user interface interacts with the Web service.

Figure 7-41 An example of Web services

The Internet

The Internet is a worldwide collection of networks that links together millions of businesses, government agencies, educational institutions, and individuals via a variety of communications devices and media. The Internet itself is based on telecommunication technology. It offers a variety of services that also are based on this technology. Chapter 8 discusses the Internet in detail.

7.6 Telecommunication Carriers and Services

Telecommunications carriers provide communications facilities and technologies such as telephone lines, satellites, modems, or other communications technologies used to transmit data from one location to another. They also provide many types of computing services.

The public switched telephone network (PSTN) is the worldwide telephone system that handles regular voice telephone calls (Figure 7-42). While it initially was built to handle voice communications, the telephone network also is an integral part of computer communications. Nearly the entire telephone network today uses digital technology, with the exception of the final link from the local telephone company to a customer, which usually is analog. The PSTN uses a variety of transmission media and switching offices that route the analog and digital signals to their destinations. Data, information and instructions can be sent over the PSTN using various types of connections.

Figure 7-42 A sample telephone network configuration

Dial-Up Lines

A dial-up line is a temporary connection that uses one or more analog telephone lines for communications. Using a dial-up line to transmit data is similar to using a telephone to make a call. A modem at the sending end dials the telephone number of the modem at the receiving end. When the modem at the receiving end answers the call, a connection is established and the data is transmitted. When the transmission is completed, the modem at either end terminates the call by hanging up, and the communications connection ends.

One advantage of a dial-up line is that it costs no more than making a regular telephone call. Computers at any two locations establish an Internet or network connection using modems and the telephone network. Mobile users, for example, can use dial-up lines in hotels to connect to their main office network to read e-mail messages, access the Internet, and upload files. One disadvantage is that you cannot control the quality of the connection because the telephone company's switching office randomly selects the line.

Dedicated Lines

A dedicated line is a type of always-on connection that always is established between two communications devices. Businesses often use dedicated lines to connect geographically distant offices. Organizations can buy and maintain their own dedicated lines, or lease a dedicated line from a telephone or communications service company. Because dedicated lines provide a constant connection, the quality and consistency of the connection is better than a dial-up line. Five popular types of digital dedicated lines are ISDN lines, DSL, FTTP, T-carrier lines, and ATM. Although cable television lines and fixed wireless are not a type of standard telephone line, they are very popular ways for the home user to connect to the Internet. 

For the small business and home user, an ISDN line provides faster transmission rates than dial-up telephone lines. ISDN (Integrated Services Digital Network) is a set of standards for digital transmission of data over standard copper telephone lines. With ISDN, the same telephone line that could carry only one computer signal now can carry three or more signals at once, using multiplexing technology. ISDN requires that both ends of the connection have an ISDN adapter installed instead of a regular modem used in dial-up connections. The ISDN modem at your location must be within about 3.5 miles of the telephone company's ISDN modem. Thus, ISDN may not be an option for rural residents. ISDN lines also require a special ISDN telephone for voice communications. Home and business users who choose ISDN adapters benefit from faster Web page downloads and clearer videoconferencing. ISDN is not as widely used today as in the past.

DSL (Digital Subscriber Line) is a popular digital line alternative for the small business or home users. A digital subscriber line uses sophisticated techniques to transmit a greater number of bytes on a standard twisted-pair cable. Digital subscriber lines provide slightly higher transfer rates than ISDN lines. Some installations include a dial tone, providing users with both voice and data communications. These DSL installations often require that filters be installed to reduce noise interference when voice communications share the same line. To connect to DSL, a customer must have a special network card and a DSL modem. A DSL modem is different from the modem used for dial-up connections. Not all areas offer DSL service because the local telephone company or the lines in the area may not be capable of supporting DSL technology. As with ISDN, DSL may not be an option for rural residents because the user's location and the telephone company's DSL modem must be located within about 3.5 miles of each other. ADSL (asymmetric digital subscriber line) is a type of DSL that supports faster transfer rates when receiving data (the downstream rate) than when sending data (the upstream rate). ADSL is ideal for Internet access because most users download more information from the Internet than they upload.

FTTP (Fiber to the Premises) uses fiber-optic cable to provide extremely high-speed Internet access to a user's physical permanent location. Two specific types of FTTP are FTTH and FTTB. FTTH (Fiber to the Home) provides home users with Internet access via fiber-optic cable. Similarly, FTTB (Fiber to the Building) refers to small businesses that use fiber-optic cables to access the Internet. With FTTP service, an optical terminal at your location receives the signals and transfers them to a router connected to your computer. As the cost of installing fiber decrease