Telecommunications and Networks for Business

Transcription

1 Telecommunications and Networks for Business Internetworking the Enterprise When computers are networked, two industries computing and communications converge, and the result is vastly more than the sum of the parts. Suddenly, computing applications become available for business-to-business coordination and commerce, and for small as well as large organizations. The global Internet creates a public place without geographic boundaries cyberspace where ordinary citizens can interact, publish their ideas, and engage in the purchase of goods and services. In short, the impact of both computing and communications on our society and organizational structures is greatly magnified. Thus, telecommunications and network technologies are internetworking and revolutionizing business and society. The Internet, the Web, and intranets and extranets are networking business processes and employees together, and connecting them to their customers, suppliers, and other business stakeholders. This topic presents the telecommunications and network foundations for these developments. Business Application Trends The changes in telecommunications industries and technologies are causing a significant change in the business use of telecommunications. The trend toward more vendors, services, Internet technologies, and open systems, and the rapid growth of the Internet, the World Wide Web, and corporate intranets and extranets, dramatically increases the number of feasible telecommunications applications. Thus, telecommunications networks are now playing vital and pervasive roles in electronic commerce, enterprise collaboration, and internal business applications that support the operations, management, and strategic objectives of both large and small companies. An organization's local and global computer networks can dramatically cut costs, shorten business lead times and response times, support electronic commerce, improve the collaboration of workgroups, develop online operational processes, share resources, lock in customers and suppliers, and develop new products and services. This makes telecommunications a more complex and important decision area for businesses that must increasingly find new ways to compete in both domestic and global markets. The Business Value of Telecommunications What business value is created by the trends in business applications of telecommunications we have identified? A good way to summarize the answer to this question is shown in Figure 1 below. Telecommunications and Networks 1

2 Examples of the business value of electronic commerce applications of telecommunications. Figure 1 Information technology, especially in telecommunications-based business applications, helps a company overcome geographic, time, cost, and structural barriers to business success. Figure 1 outlines examples of the business value of these four strategic capabilities of telecommunications and other information technologies. This figure emphasizes how several applications of electronic commerce can help a firm capture and provide information quickly to end users at remote geographic locations at reduced costs, as well as supporting its strategic organizational objectives. For example, traveling salespeople and those at regional sales offices can use the Internet, extranets, and other networks to transmit customer orders from their laptop or desktop PCs, thus breaking geographic barriers. Point-of-sale terminals and an online sales transaction processing network can break time barriers by supporting immediate credit authorization and sales processing. Teleconferencing can be used to cut costs by reducing the need for expensive business trips since it allows customers, suppliers, and employees to participate in meetings and collaborate on joint projects. Finally, business-to-business electronic commerce websites are used by the business to establish strategic relationships with their customers and suppliers by making business transactions fast, convenient, and tailored to the needs of the business partners involved. The Internet Revolution Suddenly it seems that the Internet is everywhere. After two decades of relative obscurity as a government and research network, the Internet burst upon the 1990s to penetrate the public consciousness, capturing headlines and attracting millions of users around the world. Every indication points to even faster growth in the 21st century. The explosive growth of the Internet is a revolutionary phenomenon in computing and Telecommunications and Networks 2

3 telecommunications. The Internet has become the largest and most important network of networks today, and has evolved into a global information superhighway. The Internet is constantly expanding, as more and more businesses and other organizations and their users, computers, and networks join its global web. Thousands of business, educational, and research networks now connect millions of computer systems and users in more than 200 countries to each other. The Internet has also become a key platform for a rapidly expanding list of information and entertainment services and business applications, including enterprise collaboration and electronic commerce systems. The Internet evolved from a research and development network (ARPANET) established in 1969 by the U.S. Defense Department to enable corporate, academic, and government researchers to communicate with and share data and computing resources. The Net doesn't have a central computer system or telecommunications center. Instead, each message sent has a unique address code so any Internet server in the network can forward it to its destination. Also, the Internet does not have a headquarters or governing body. The Internet Society in Reston, Virginia, is one of several volunteer groups of individual and corporate members who promote use of the Internet and the development of new communications standards. These common standards are the key to the free flow of messages among the widely different computers and networks of the many organizations and Internet service providers (ISPs) in the system. The Internet is growing rapidly. For example, the Internet grew from over 40 million to over 70 million host computers from early 1999 to early Figure 6.5 gives you a good idea of the historical growth of the Internet. Telecommunications and Networks 3

4 Internet Applications The most popular Internet applications are , browsing the sites on the World Wide Web, and participating in newsgroups and chat rooms. Internet messages usually arrive in seconds or a few minutes anywhere in the world, and can take the form of data, text, fax, and video files. Internet browser software like Netscape Navigator and Microsoft Explorer enables millions of users to surf the World Wide Web by clicking their way to the multimedia information resources stored on the hyperlinked pages of businesses, government, and other websites. Websites offer information and entertainment, and are the launch sites for electronic commerce transactions between businesses and their suppliers and customers. E-commerce websites offer all manner of products and services via online retailers, wholesalers, service providers, and online auctions. The Internet provides electronic discussion forums and bulletin board systems formed and managed by thousands of special-interest newsgroups. You can participate in discussions or post messages on thousands of topics for other users with the same interests to read and respond to. Other popular applications include downloading software and information files and accessing databases provided by thousands of business, government, and other organizations. You can make online searches for information at websites in a variety of ways, using search sites and search engines such as Yahoo!, Google, and Fast Search. Logging on to other computers on the Internet and holding real-time conversations with other Internet users in chat rooms are also popular uses of the Internet. Popular Uses of the Internet Surf. Point and click your way to thousands of hyperlinked websites and resources for multimedia information, entertainment, or electronic commerce. . Exchange electronic mail with millions of Internet users. Discuss. Participate in discussion forums or post messages on bulletin board systems formed by thousands of special-interest newsgroups. Chat. Hold real-time text conversations in website chat rooms with Internet users around the world. Buy and Sell. You can buy and sell practically anything via E-commerce retailers, wholesalers, service providers, and online auctions. Download. Transfer data files, software, reports, articles, pictures, music, videos, and other types of files to your computer system. Compute. Log on to and use thousands of Internet computer systems around the world. Other Uses: Make long-distance phone calls, hold desktop videoconferences, listen to radio programs, watch television, play video games, explore virtual worlds, etc. A Telecommunications Network Model Generally, a communications network is any arrangement where a sender transmits a message to a receiver over a channel consisting of some type of medium. Figure 2 illustrates a simple conceptual model of a telecommunications network, which shows that it consists of five basic categories of components: Terminals, such as networked personal computers, network computers, or information appliances. Any input/output device that uses telecommunications net works to transmit or receive data is a terminal, including telephones and various computer terminals. Telecommunications processors, which support data transmission and reception between Telecommunications and Networks 4

5 terminals and computers. These devices, such as modems, switches, and routers, perform a variety of control and support functions in a telecommunications network. For example, they convert data from digital to analog and back, code and decode data, and control the speed, accuracy, and efficiency of the communications flow between computers and terminals in a network. The five basic components in a telecommunications network: (1) terminals, (2) telecommunications processors, (3) telecommunications channels, (4) computers, and (5) telecommunications software. Figure 2 Telecommunications channels over which data are transmitted and received. Telecommunications channels may use combinations of media, such as copper wires, coaxial cables, or fiber optic cables, or use wireless systems like microwave, communications satellite, radio, and cellular systems to interconnect the other components of a telecommunications network. Computers of all sizes and types are interconnected by telecommunications net works so that they can carry out their information processing assignments. For example, a mainframe computer may serve as a host computer for a large network, assisted by a midrange computer serving as a front-end processor, while a microcomputer may act as a network server in a small network. Telecommunications control software consists of programs that control telecommunications activities and manage the functions of telecommunications networks. Examples include network management programs of all kinds, such as telecommunications monitors for mainframe host computers, network operating systems for network servers, and web browsers for microcomputers. No matter how large and complex real world telecommunications networks may appear to be, these five basic categories of network components must be at work to support an organization's telecommunications activities. This is the conceptual framework you can use to help you understand the various types of telecommunications networks in use today. Types of Telecommunications Networks There are many different types of telecommunications networks. However, from an end user's point of view, there are only a few basic types, such as wide area and local area networks and Telecommunications and Networks 5

6 interconnected networks like the Internet, intranets, and extranets, as well as client/server and interorganizational networks. Wide Area Networks Telecommunications networks covering a large geographic area are called wide area networks (WANs). Networks that cover a large city or metropolitan area (metropoli tan area networks} can also be included in this category. Such large networks have be come a necessity for carrying out the day-to-day activities of many business and government organizations and their end users. For example, WANs are used by many multinational companies to transmit and receive information among their employees, customers, suppliers, and other organizations across cities, regions, countries, and the world. Local Area Networks Local area networks (LANs) connect computers and other information processing devices within a limited physical area, such as an office, a classroom, a building, man ufacturing plant, or other work site. LANs have become commonplace in many orga nizations for providing telecommunications network capabilities that link end users in offices, departments, and other workgroups. LANs use a variety of telecommunications media, such as ordinary telephone wiring, coaxial cable, or even wireless radio and infrared systems, to interconnect microcomputer workstations and computer peripherals. To communicate over the net work, each PC usually has a circuit board called a network interface card. Most LANs use a more powerful microcomputer having a large hard disk capacity, called a file server or network server, that contains a network operating system program that controls telecommunications and the use and sharing of network resources. For example, it distributes copies of common data files and software packages to the other microcomputers in the network and controls access to shared laser printers and other network peripherals. Intranets and Extranets Intranets are designed to be open, but secure, internal networks whose web browsing software provides easy point-and-click access by end users to multimedia information on internal websites. Intranet websites may be established on internal web servers by a company, its business units, departments, and workgroups. For example, a human re sources department may establish an intranet website so employees can easily access up-to-the-minute information on the status of their benefits accounts, as well as the latest information on company benefits options. One of the attractions of corporate intranets is that their Internet-like technology makes them more adaptable, as well as easier and cheaper to develop and use than either traditional client/server or main frame-based legacy systems. Extranets are networks that link some of the intranet resources of a company with other organizations and individuals. For example, extranets enable customers, suppliers, subcontractors, consultants, and others to access selected intranet websites and other company databases. Organizations can establish private extranets among themselves, or use the Internet as part of the network connections between them. Telecommunications and Networks 6

7 A local area network (LAN). Note how the LAN allows users to share hardware, software, and data resources. Figure 3 Local Area Network Many organizations use virtual private networks (VPNs) to establish secure intranets and extranets. A virtual private network is a secure network that uses the Internet as its main backbone network, but relies on the fire walls and other security features of its Internet and intranet connections and those of participating organizations. Thus, for example, VPNs would enable a company to use the Internet to establish secure in tranets between its distant branch offices and manufacturing plants, and secure ex tranets between itself and its customers and suppliers. Figure 4 Intranets and Extranets Telecommunications and Networks 7

8 Cient/Server Networks Client/server networks have become the predominate information architecture of enterprisewide computing. In a client/server network, end user PC or NC workstations are the clients. They are interconnected by local area networks and share application processing with network servers, which also manage the networks. (This arrangement of clients and servers is sometimes called a two-tier client/server architecture.) Local area networks are also interconnected to other LANs and wide area networks of client workstations and servers. A continuing trend is the downsizing of larger computer systems by replacing them with client/server networks. For example, a client/server network of several interconnected local area networks may replace a large mainframe-based network with many end user terminals. This typically involves a complex and costly effort to install new application software that replaces the software of older, traditional mainframebased business information systems, now called legacy systems. Client/server networks are seen as more economical and flexible than legacy systems in meeting end user, workgroup, and business unit needs, and more adaptable in adjusting to a diverse range of computing workloads. Network Computing The growing reliance on the computer hardware, software, and data resources of the Internet, intranets, extranets, and other networks has emphasized that for many users, "the network is the computer." This network computing or network-centric concept views networks as the central computing resource of any computing environment. Client System Types: PCs, Network Computers, Workstations, Macintoshes. Functions: Provide user interface, perform some/most processing on an application. Servers Types: Servers, Workstations, or Midrange Systems Functions: Shared computation, application control, distributed databases. Host Systems/ Superservers Types: Mainframes and Midrange Systems, Functions: Central database control, security, directory management, heavy-duty processing. In network computing, network computers and other thin clients provide a browser-based user interface for processing small application programs called applets. Thin clients include network computers, Net PCs, and other low-cost network devices or information appliances. Application and database servers provide the operating system, application software, applets, databases, and database management software needed by the end users in the network. Network computing is sometimes called a three-tier client/server model, since it consists of thin clients, application servers, and database servers. Interorganizational Networks Many business applications of telecommunications involve the use of the Internet, extranets, and other networks to form interorganizational networks. Such networks link a company's headquarters and other locations to the networks of its customers, suppliers, and other organizations. For example, you can think of a customer account inquiry system that provides intranet access by employees and extranet access by customers as an example of an interorganizational network. So is the use of electronic data interchange (EDI) systems that link the computers of a company with those of its suppliers and Telecommunications and Networks 8

9 customers for the electronic exchange of business documents. Of course, electronic commerce applications for buying and selling products and services on the World Wide Web depend on Internet, intranet, extranet, and other interorganizational networks established among banks, businesses, customers, and suppliers. Thus, the business use of telecommunications networks has moved beyond the boundaries of the enterprise. Now many business firms are using the Internet and other networks to extend their information systems to their customers and suppliers, both domestically and internationally. Such interorganizational systems build strategic business relationships and alliances with those stakeholders in an attempt to increase and lock in their business, while locking out competitors. Also, transaction processing costs are frequently reduced, and the quality of service to customers and suppliers improves significantly. Network computers and other clients provide a browser-based user interface for applet processing. Application servers for multi-user operating systems, web server software, and application software applets. Database servers for Internet/intranet web databases, operational databases, and database management software Technical Telecommunications Alternatives Telecommunications is a highly technical, rapidly changing field of information systems technology. Most business professionals do not need a detailed knowledge of its technical characteristics. However, it is necessary that you understand some of the important characteristics of the basic components of telecommunications networks. This understanding will help you participate effectively in decision making regarding telecommunications alternatives. Key telecommunications network components and alternatives. Media Processors Software Channels Topology/architecture Twisted-pair wire, coaxial cable, fiber optics, microwave radio, communications satellites, cellular and PCS systems, wireless mobile and LAN systems Modems, multiplexers, switches, routers, hubs, gateways, front-end processors, private branch exchanges Network operating systems, telecommunications monitors, web browsers, middleware Analog/digital, switched/nonswitched, circuit/message/packet/cell switching, bandwidth alternatives Star, ring and bus topologies, OSI, and TCP/IP architectures Above are outlined key telecommunications components and alternatives. Remember, a basic understanding and appreciation, not a detailed knowledge, are sufficient for most business end users. Telecommunications Media Telecommunications channels make use of a variety of telecommunications media. These include twisted-pair wire, coaxial cables, and fiber optic cables, all of which physically link the devices in a network. Also included are terrestrial microwave, communications satellites, cellular phone systems, and packet and LAN radio, all of which use microwave and other radio waves. In addition, there are infrared systems, which use infrared light to transmit and receive data. Telecommunications and Networks 9

10 Twisted-Pair Wire Ordinary telephone wire, consisting of copper wire twisted into pairs (twisted-pair wire), is the most widely used medium for telecommunications. These lines are used in established communications networks throughout the world for both voice and data transmission. Thus, twisted-pair wiring is used extensively in home and office telephone systems and many local area networks and wide area networks. Figure 5 Telecommunications wire & cables Coaxial cable Coaxial cable consists of a sturdy copper or aluminum wire wrapped with spacers to insulate and protect it. The cable's cover and insulation minimize interference and distortion of the signals the cable carries. Groups of coaxial cables may be bundled together in a big cable for ease of installation. These highquality lines can be placed underground and laid on the floors of lakes and oceans. They allow highspeed data transmission and are used instead of twisted-pair wire lines in high-service metropolitan areas, for cable TV systems, and for short-distance connection of computers and peripheral devices. Coaxial cables are also used in many office buildings and other work sites for local area networks. Fiber optics Fiber optics uses cables consisting of one or more hair-thin filaments of glass fiber wrapped in a protective jacket. They can conduct pulses of visible light elements (photons) generated by lasers at transmission rates as high as 320 billion bits per second. This is about 640 times greater than coaxial cable and 32,000 times better than twisted-pair wire lines. Fiber optic cables provide substantial size and weight reduc tions as well as increased speed and greater carrying capacity. A half-inch-diameter fiber optic cable can carry over 500,000 channels, compared to about 5,500 channels for a standard coaxial cable. Fiber optic cables are not affected by and do not generate electromagnetic radia tion; therefore, multiple fibers can be placed in the same cable. Fiber optic cables have less need for repeaters for signal retransmissions than copper wire media. Fiber optics also has a much lower data error rate than other media and is harder to tap than electrical wire and cable. Fiber optic cables have already been installed in many parts of the world, and they are expected to replace other communications media in many applications. New optical technologies such as dense wave division multiplexing (DWDM) can split a strand of glass fiber into 40 channels, which enables each strand to carry 5 million calls. In the future, DWDM technology is expected to split each fiber into 1,000 channels, enabling each strand to carry up to 122 million calls. In addition, newly developed optical routers will be able to send optical signals up to 2,500 miles without needing regeneration, thus eliminating the need for repeaters every 370 miles to Telecommunications and Networks 10

11 regenerate signals. Wireless Technologies Wireless telecommunications technologies rely on radio wave, microwave, infrared, and visible light pulses to transport digital communications without wires between communications devices. Wireless technologies include terrestrial microwave, communications satellites, cellular and PCS telephone and pager systems, mobile data ra dio, wireless LANs, and various wireless Internet technologies. Each technology utilizes specific ranges (in megahertz) of electromagnetic frequencies that are specified by national regulatory agencies to minimize interference and encourage efficient telecommunications. Let's briefly review some of these major wireless communications technologies. Terrestrial Microwave Terrestrial microwave involves earthbound microwave systems that transmit high speed radio signals in a line-of-sight path between relay stations spaced approximately 30 miles apart. Microwave antennas are usually placed on top of buildings, towers, hills, and mountain peaks, and they are a familiar sight in many sections of the country. They are still a popular medium for both long-distance and metropolitan area networks. Communications satellites Communications satellites also use microwave radio as their telecommunications medium. Many communications satellites are placed in stationary geosynchronous orbits approximately 22,000 miles above the equator. Satellites are powered by solar panels and can transmit microwave signals at a rate of several hundred million bits per second. They serve as relay stations for communications signals transmitted from earth stations. Earth stations use dish antennas to beam microwave signals to the satellites that amplify and retransmit the signals to other earth stations thousands of miles away. While communications satellites were used initially for voice and video transmission, they are now also used for high-speed transmission of large volumes of data. Because of time delays caused by the great distances involved, they are not suitable for interactive, real-time processing. Communications satellite systems are operated by several firms, including Comsat, American Mobile Satellite, and Intellsat. A variety of other satellite technologies are being implemented to improve global business communications. For example, many companies use networks of small satellite dish antennas known as VSAT (very-small-aperture terminal) to connect their stores and distant work sites. Other satellite networks use many low-earth orbit (LEO) satellites orbiting at an altitude of only 500 miles above the earth. Companies like Globalstar and Indium offer cellular phone, paging, and messaging services to users anywhere on the globe. Cellular and PCS Systems Cellular and PCS telephone and pager systems use several radio communications technologies. However, all of them divide a geographic area into small areas, or cells, typically from one to several square miles in area. Each cell has its own low-power transmitter or radio relay antenna device to relay calls from one cell to another. Computers and other communications processors coordinate and control the transmissions to and from mobile users as they move from one area to another. Cellular phone systems have long used analog communications technologies operating at frequencies in Telecommunications and Networks 11

12 the 800 to 900 MHz cellular band. Newer cellular systems use digital technologies, which provide greater capacity and security, and additional services such as voice mail, paging, messaging, and caller ID. These capabilities are also available with the new PCS (Personal Communications Services) phone systems. PCS operates at 1,900 MHz frequencies using digital technologies that are related to digital cellular. However, PCS phone systems cost substantially less to operate and use than cellular systems and have lower power consumption requirements. The Wireless Web Wireless access to the Internet, intranets, and extranets is growing as more web-enabled information appliances proliferate. Smart telephones, pagers, PDAs, and other portable communications devices have become very thin clients in wireless networks. Agreement on a standard wireless application protocol (WAP) has encouraged the development of many wireless web applications and services. For example, the Palm VII PDA can send and receive and provides web access via a "web clipping" technology which generates custom-designed web pages from many popular financial, securities, travel, sport, entertainment, and E-commerce web-sites. Another example is the Sprint PCS Wireless Web phone which delivers similar web content and services via a web-enabled PCS phone. Figure 6 illustrates the wireless application protocol that is the foundation of wireless mobile Internet and web applications. The WAP standard specifies how web pages in HTML or XML are translated into a wireless markup language (WML) by filter software, and preprocessed by proxy software to prepare the web pages for wireless transmission from a web server to a web-enabled wireless device. Telecommunications Processors Modems Telecommunications processors such as modems, multiplexers, switches, and routers perform a variety of support functions between the computers and other devices in a telecommunications network. Let's take a look at some of these processors and their functions. Modems are the most common type of communications processor. They convert the digital signals from a computer or transmission terminal at one end of a communications link into analog frequencies that can be transmitted over ordinary telephone lines. A modem at the other end of the communications line converts the transmitted data back into digital form at a receiving terminal. This process is known as modulation and demodulation, and the word modem is a combined abbreviation of those two words. Modems come in several forms, including small stand-alone units, plug-in circuit boards, and removable modem cards for laptop PCs. Most modems also support a variety of telecommunications functions, such as transmission error control, automatic dialing and answering, and a faxing capability. The wireless application protocol (WAP) architecture for wireless Internet services to mobile information appliances. Telecommunications and Networks 12

13 Figure 6 Wireless Application Protocol Modems are used because ordinary telephone networks were first designed to handle continuous analog signals (electromagnetic frequencies), such as those generated by the human voice over the telephone. Since data from computers are in digital form (voltage pulses), devices are necessary to convert digital signals into appropriate analog transmission frequencies and vice versa. However, digital communications networks that use only digital signals and do not need analog/digital conversion are becoming commonplace. Since most modems also perform a variety of telecommunications support functions, devices called digital modems are still used in digital networks. Multiplexers A multiplexer is a communications processor that allows a single communications channel to carry simultaneous data transmissions from many terminals. Thus, a single communications line can be shared by several terminals. Typically, a multiplexer merges the transmissions of several terminals at one end of a communications channel, while a similar unit separates the individual transmissions at the receiving end. This is accomplished in two basic ways. In frequency division multiplexing (FDM), a multiplexer effectively divides a high-speed channel into multiple slow-speed channels. In time division multiplexing (TDM), the multiplexer divides the time each terminal can use the high-speed line into very short time slots, or time frames. The most advanced and popular type of multiplexer is the statistical time division multiplexer, most commonly referred to as a statistical multiplexer. Instead of giving all terminals equal time slots, it dynamically allocates time slots only to active terminals according to priorities assigned by a telecommunications manager. Internetwork Processors Telecommunications networks are interconnected by special-purpose communications processors called internetwork processors such as switches, routers, hubs, and gateways. A switch is a communications processor that makes connections between telecommunications circuits in a network so a telecommunications message can reach its intended destination. A router is a more intelligent communications processor that interconnects networks based on different rules or protocols, so a telecommunications message can be routed to its destination. A hub is a port switching communications processor. Advanced versions of hubs provide automatic switching among connections called ports for shared access to a network's resources. Workstations, servers, printers, and other network resources are connected to ports, as are switches and routers provided by the hub to other networks. Networks that use different communications architectures are interconnected by using a communications processor Telecommunications and Networks 13

14 called a gateway. All these devices are essential to providing connectivity and easy access between the multiple LANs and wide area networks that are part of the intranets and client/server networks in many organizations. The communications processors involved in a typical Internet connection. Figure 7 Communication Processors used in typical internet connections Telecommunications Software Software is a vital component of all telecommunications networks. Telecommunications and network management software, which may reside in PCs, servers, mainframes, and communications processors like multiplexers and routers. For example, mainframe-based wide area networks frequently use telecommunications monitors or teleprocessing (TP) monitors. CICS (Customer Identification Control System) for IBM mainframes is a typical example. Servers in local area networks frequently rely on Novell NetWare, Sun's Solaris, Unix, Linux, or Microsoft Windows 2000 Servers. Corporate intranets use network management software like the iplanet Portal Server, which is one of several programs for network management, electronic commerce, and application development in Sun Microsystems and Netscape's iplanet software servers for the Internet, intranets, and extranets. Many software vendors offer telecommunications software known as middle-ware, which can help diverse networks communicate with each other. A variety of communications software packages are available for microcomputers, especially Internet web browsers like Netscape Navigator and Microsoft Explorer. Telecommunications software packages provide a variety of communications support services. For example, they work with a communications processor (such as a modem) to connect and disconnect communications links and establish communications parameters such as transmission speed, mode, and direction. Network management packages such as LAN network operating systems and WAN telecommunications monitors determine transmission priorities, route (switch) messages, poll terminals in the network, and form waiting lines (queues) of transmission requests. They also detect and correct transmission errors, log statistics of network activity, and protect network resources from unauthorized access. Comparing modem and telecommunications technologies for Internet and other network access. Modem (56K bit/sec) Receives at 56K bit/sec. Telecommunications and Networks 14

15 Sends at 28.8K bit/sec. Slowest technology DSL ( Digital Subscriber Line) Modem Receives at up to 256K bit/sec. Sends at 64K bit/sec. Users must be near switching centers ISDN (Integrated Services Digital Network) Sends and receives at 128K bit/sec. Users need extra lines Becoming obsolete Cable Modem Receives at 1.5 to 3M bit/sec. Sends at 128K bit/sec, Cable systems need to be upgraded Home Satellite Receives at 400K bit/sec. Sends via phone modem Slow sending, higher cost Local Microwave Sends and receives at 512K to 1.4M bit/sec. Higher cost alternative May require line of sight to base antenna Telecommunications and Networks 15

16 NETWORK TOPOLOGIES There are several basic types of network topologies, or structures, in telecommunications networks. Figure 8 illustrates three basic topologies used in wide area and local area telecommunications networks. A star network ties end user computers to a central computer. A ring network ties local computer processors together in a ring on a more equal basis. A bus network is a network in which local processors share the same bus, or communications channel. A variation of the ring network is the mesh network. It uses direct communications lines to connect some or all of the computers in the ring to each other. Another variation is the tree network, which joins several bus networks together. Client/server networks may use a combination of star, ring, and bus approaches. Obviously, the star network is more centralized, while ring and bus networks have a more decentralized approach. However, this is not always the case. For example, the central computer in a star configuration may be acting only as a switch, or message-switching computer, that handles the data communications between autonomous local computers. Star, ring, and bus networks differ in their performances, reliabilities, and costs. A pure star network is considered less reliable than a ring network, since the other computers in the star are heavily dependent on the central host computer. If it fails, there is no backup processing and communications capability, and the local computers are cut off from each other. Therefore, it is essential that the host computer be highly reliable. Having some type of multiprocessor architecture to provide a fault tolerant capability is a common solution. Ring and bus networks are most common in local area networks. Ring networks are considered more reliable and less costly for the type of communications in such networks. If one computer in the ring goes down, the other computers can continue to process their own work as well as to communicate with each other. Figure 8 Network Topologies NETWORK ARCHITECTURES AND PROTOCOLS Until quite recently, there was a lack of sufficient standards for the interfaces between the hardware, software, and communications channels of data communications net works. For this reason, it is quite common to find a lack of compatibility between the data communications hardware and Telecommunications and Networks 16

17 software of different manufacturers. This situation hampered the use of data communications, increased its costs, and reduced its efficiency and effectiveness. In response, computer manufacturers and national and international organizations have developed standards called protocols and master plans called network architectures to support the development of advanced data communications networks. Protocols. A protocol is a standard set of rules and procedures for the control of communications in a network. However, these standards may be limited to just one manufacturer's equipment, or to just one type of data communications. Part of the goal of communications network architectures is to create more standardization and compatibility among communications protocols. One example of a protocol is a standard for the physical characteristics of the cables and connectors between terminals, computers, modems, and communications lines. Other examples are the protocols that establish the communications control information needed for handshaking, which is the process of exchanging predetermined signals and characters to establish a telecommunications session between terminals and computers. Other protocols deal with control of data transmission reception in a network, switching techniques, internetwork connections, and so on. Network Architectures. The goal of network architectures is to promote an open, simple, flexible, and efficient telecommunications 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. The OSI Model The International Standards Organization (ISO) has developed a seven-layer Open Systems Interconnection (OSI) model to serve as a standard model for network architectures. Dividing data communications functions into seven distinct layers promotes the development of modular network architectures, which assists the development, operation, and maintenance of complex telecommunications networks. The Internet's TCP/IP The Internet uses a system of telecommunications protocols that has become so widely used that it is equivalent to a network architecture. The Internet's protocol suite is called Transmission Control Protocol/Internet Protocol and is known as TCP/IP. As Figure 6.23 shows, TCP/IP consists of five layers of protocols that can be related to the seven layers of the OSI architecture. TCP/IP is used by the Internet and by all intranets and extranets. Many companies and other organizations are thus converting their client/server networks to TCP/IP technology, which are now commonly called IP networks. Examples of telecommunications transmission speeds by type of media and network technology Type of Media Maximum BPS Twisted pair unshielded/shielded 2M/100M Coaxial cable baseband/broadband 264M/550M Satellite/terrestrial microwave 200M Wireless LAN radio 3.3M Infrared EAN 4M Fiber optic cable 320G Network Technologies Typical-Maximum BPS Standard Ethernet or token ring 10-16M Telecommunications and Networks 17

18 High-speed Ethernet FDDI: fiber distributed data interface DDN: digital data network PSN: packet switching network Frame relay network ISDN; integrated services digital network ATM: asynchronous transfer mode OC: Optical Carrier 100M-1G 100M 2.4K-2M 2.4K-64K 1.5M-45M 64K/128K-2M 25/155M-2.4G 52M-10G KBPS = thousand BPS or kilobits per second MBPS = million BPS or megabits per second. GBPS = billion BPS or gigabits per second. Bandwidth Alternatives The communications speed and capacity of telecommunications networks can be classified by bandwidth. This is the frequency range of a telecommunications channel; it determines the channel's maximum transmission rate. The speed and capacity of data transmission rates are typically measured in bits per second (BPS). This is sometimes referred to as the baud rate., though baud is more correctly a measure of signal changes in a transmission line. Narrow-band channels typically provide low-speed transmission rates up to 64K BPS, but can now handle up to 2 million BPS. They are usually unshielded twisted pair lines commonly used for telephone voice communications, and for data communications by the modems of PCs and other devices. Medium-speed channels (medium-band) use shielded twisted-pair lines for transmission speeds up to 100 MBPS. Broadband channels provide high-speed transmission rates at intervals from 256,000 BPS to several billion BPS. Typically, they use microwave, fiber optics, or satellite transmission. Examples are 1.54 million BPS for Tl and 45M BPS for T3 communications channels, up to 100 MBPS for communications satellite channels, and between 52 MBPS and 10 GBPS for fiber optic lines. Switching Alternatives Regular telephone service relies on circuit switching, in which a switch opens a circuit to establish a link between a sender and receiver; it remains open until the communication session is completed. In message switching, a message is transmitted a block at a time from one switching device to another. Packet switching involves subdividing communications messages into fixed or variable groups called packets. For example, in the X.25 protocol, packets are 128 characters long, while they are of variable length in the frame relay technology. Packet switching networks are frequently operated by valueadded carriers who use computers and other communications processors to control the packet switching process and transmit the packets of various users over their networks. Early packet switching networks were X.25 networks. The X.25 protocol is an international set of standards governing the operations of widely used, but relatively slow, packet switching networks. Frame relay is another popular packet switching protocol, and is used by many large companies for their wide area networks. Frame relay is considerably faster than X.25, and is better able to handle the heavy telecommunications traffic of interconnected local area networks within a company's wide area client/server network. ATM (asynchronous transfer mode) is an emerging high-capacity cell switching technology. An ATM switch breaks voice, video, and other data into fixed cells of 53 bytes (48 bytes of data and 5 bytes of control information), and routes them to their next destination in the network. ATM networks are being developed by many companies needing its fast, high-capacity multimedia capabilities for voice, video, and data communications. Telecommunications and Networks 18