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Bis Chapter5


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Bis Chapter5

  1. 1. Chapter 5 Networks, telecommunications and the Internet
  2. 2. TOPICS <ul><li>Introduction to computer networks </li></ul><ul><li>Network components </li></ul><ul><li>The evolution of networking technology </li></ul><ul><li>The Internet </li></ul><ul><li>How the Internet works </li></ul><ul><li>Other wide-area networks </li></ul><ul><li>Smaller-scale networks </li></ul>
  3. 3. Computer networks defined <ul><li>A computer network can be defined as: ‘a communications system that links two or more computers and peripheral devices and enables transfer of data between the components’. </li></ul><ul><li>Small-scale networks within a workgroup or single office are known as local area networks (LAN). Larger-scale networks which are national or international are known as wide area networks (WAN). The Internet is the best known example of a Wide Area Network. </li></ul>
  4. 4. Figure 5.1 Communications links between different stakeholders in an industry
  5. 5. Business benefits of networks <ul><li>Reduce cost compared with traditional communications. </li></ul><ul><li>Reduces time for information transfer </li></ul><ul><li>Enables sharing and dissemination of company information </li></ul><ul><li>Enables sharing of hardware resources such as printers, backup, processing power. </li></ul><ul><li>Promotes new ways of working </li></ul><ul><li>Operates geographically separate businesses as one. </li></ul><ul><li>Restructures relationships with partners </li></ul>
  6. 6. Disadvantages of networks <ul><li>1 The initial setup cost may be high, and there may be a considerable period before the costs are paid off. </li></ul><ul><li>2 When implementing or updating the network there may be considerable practical difficulties. Deploying cabling can be very disruptive to staff doing their daily work. </li></ul><ul><li>3 In the long term, companies become reliant on networks and breaks in service can be very disruptive. For this reason investment in network maintenance is vital. </li></ul><ul><li>4 Security is reduced through introducing a network, since there are more access points to sensitive data. Data may also be intercepted when it is transferred from one site to another. </li></ul>
  7. 7. TOPICS <ul><li>Introduction to computer networks </li></ul><ul><li>Network components </li></ul><ul><li>The evolution of networking technology </li></ul><ul><li>The Internet </li></ul><ul><li>How the Internet works </li></ul><ul><li>Other wide-area networks </li></ul><ul><li>Smaller-scale networks </li></ul>
  8. 8. Client/server model of computing <ul><li>The client/server architecture consists of client computers such as PCs sharing resources such as a database stored on more powerful server computers. </li></ul><ul><li>Processing can be shared between the clients and the servers. </li></ul><ul><li>Client/server architecture is significant since most modern networked information systems are based on this structure. </li></ul>
  9. 9. Figure 5.3 Components of a client/server system
  10. 10. Why use client / server ? <ul><li>The adoption of C/S architecture is to “downsize” from large mainframes with of user terminals which had limited functionality. </li></ul><ul><li>Rather than all the tasks involved in program execution (other than display) occurring on the mainframe, client/server gives the opportunity for them to be shared between a central server and clients. </li></ul><ul><li>This gives the potential for faster execution, as processing is distributed across many clients. </li></ul>
  11. 11. Problems to be managed with client/server <ul><li>High cost of ownership – Although price of PCs is relatively low, support and maintenance cost could be high. </li></ul><ul><li>Instability --C/S technology is often complex and involves integrating different hardware and software components from many different companies. </li></ul><ul><li>Performance -- For some mission-critical applications, a smaller server cannot deliver the power required. </li></ul><ul><li>Lack of worker focus – The freedom of choice can lead to non-productive time-wasting, as users rearrange the colours and wallpaper on their desktop rather than performing their regular tasks. </li></ul>
  12. 12. Servers <ul><li>Servers are vital to an information system, since they regulate the flow of information around the network. </li></ul><ul><li>Network servers run the network operating system (NOS) , the software that is used to manage the network, and are often used to store large volumes of data. </li></ul><ul><li>The server and NOS together perform the following functions: </li></ul><ul><ul><li>Maintain security </li></ul></ul><ul><ul><li>Sharing of peripheral devices </li></ul></ul><ul><ul><li>Sharing of applications </li></ul></ul><ul><ul><li>Sharing of information </li></ul></ul>
  13. 13. Servers (Continued) <ul><li>Maintain security – access to information in files is restricted according to the user name and password issued to users of the network. </li></ul><ul><li>Sharing of peripheral devices connected to the network, such as printers and tape drivers. These are often attached directly to the server. </li></ul><ul><li>Sharing of applications such as word processors, which do not then need to be stored on the hard drive of the end-user’s computer. The cost of buying applications can be reduced through buying a “site licence”. </li></ul><ul><li>Sharing of information – access to this data is maintained by the NOS and it is stored within the hard drive of a server as files or as part of a database. </li></ul>
  14. 14. Examples of different servers Network Contains functions to manage the network resources and control user access File This term is sometimes used to refer to network server functions. It can also indicate that users’ files such as documents and spreadsheets are stored on the network server Print Dedicated print servers have a queue of all documents for which print requests have been made, often combined with file or network servers Fax Used to route incoming and outgoing faxes received and sent from the users’ desktop Mail Stores and forwards e-mail messages Database Used to store data and provide the software to process data queries supplied by users, often accessed by Structured Query Language (SQL) Application Used to store programs such as spreadsheet or bespoke applications run by end-users on their PCs. This removes the need to store each application on every user’s hard disk Communications Manages connections with other networks in a WAN configuration. Sometimes known as gateways and attached to other gateway devices such as routers and firewall servers.
  15. 15. Servers (Continued) <ul><li>When creating an information system, there are a number of critical functions which must be designed in to the server. </li></ul><ul><li>These are important requirements which must be checked with server vendors, database vendors and operating systems vendors. </li></ul><ul><li>They are: </li></ul><ul><ul><li>Performance </li></ul></ul><ul><ul><li>Capacity </li></ul></ul><ul><ul><li>Resilience/fault tolerance </li></ul></ul><ul><ul><li>Clustering </li></ul></ul>
  16. 16. Servers (Continued) <ul><li>Performance </li></ul><ul><ul><li>The server should be fast enough to handle all the requests from users attached to the network. </li></ul></ul><ul><ul><li>A margin should be build in to accommodate future growth in users and network traffic. </li></ul></ul><ul><ul><li>This means specifying a suitable amount of memory, a fast hard disk and, less importantly, a fast processor. </li></ul></ul><ul><li>Capacity </li></ul><ul><ul><li>The hard disk capacity should be large enough that it will not need to be upgraded in the near future. </li></ul></ul>
  17. 17. Servers (Continued) <ul><li>Resilience/fault tolerance </li></ul><ul><ul><li>If there is a problem affecting the hardware, such as a power surge or a problem with the hard disk, it is important that the whole network does not “crash” because of this. </li></ul></ul><ul><ul><li>Preventive measures should be taken, such as installing an uninterruptible power supply or running two disks in parallel (disk mirroring or RAID – redundant array of inexpensive disks). </li></ul></ul><ul><li>Clustering </li></ul><ul><ul><li>It is used to spread the load across different servers, so improving reliability and performance. </li></ul></ul><ul><ul><li>It involves linking several servers together via a high-speed link such as fibre-optic cabling. This can enable parallel processing, where tasks are shared between processors, and also storage mirroring, where duplicate copies of data are stored on different servers to improve performance and reduce the risk of one server failing. </li></ul></ul>
  18. 18. Servers (Continued) <ul><li>Servers can be specified as powerful PCs running an operating system such as Windows NT or Novell Netware, or they can run the UNIX operating system, from companies such as SUN, IBM or Hewlett-Packard. </li></ul>
  19. 19. Data communications equipment or telecommunications processors <ul><li>Modem (modulator-demodulator) allows users to send and receive data via an ordinary telephone line. </li></ul><ul><ul><li>The modem receives analogue data transmitted via a telephone line and converts this into digital data so that the computer can make use of it. </li></ul></ul><ul><ul><li>The modem converts outgoing digital data into an analogue signal before transmitting them. </li></ul></ul><ul><ul><li>The speed of a modem is measured in bauds or bits per second (bps). </li></ul></ul><ul><ul><li>Early modems operated at speeds of 1200 baud (approx. 100 characters). </li></ul></ul><ul><ul><li>Typical modem is 56,600 bps (approx. 4,700 characters) </li></ul></ul>
  20. 20. Alternatives to the modem and conventional telephone line <ul><li>Some of the alternatives to the modem and conventional telephone line include the following: </li></ul><ul><li>Digital telephone exchanges support an integrated services digital network (ISDN) standard that allows data transfer rates that are up to five times faster than a 56,600 bps modem. </li></ul><ul><ul><li>An ISDN telephone line provides two separate “channels”, allowing simultaneous voice and data transmissions. </li></ul></ul><ul><ul><li>Since ISDN lines transmit digital data, a modem is not required to make use of the service. </li></ul></ul><ul><ul><li>Instead, a special terminal adaptor (often called an ISDN modem) is used to pass data between the computer and the ISDN line. </li></ul></ul>
  21. 21. Alternatives to the modem and conventional telephone line <ul><li>Asymmetric Digital Subscriber Line (ASDL) </li></ul><ul><ul><li>The ASDL services make use of existing telephone lines to provide very high data transfer rates. </li></ul></ul><ul><ul><li>ADSL is usually referred to as “broadband” Internet access. </li></ul></ul><ul><ul><li>Although the bandwidth offered by such services is usually shared by a number of users, ASDL offers many of the benefits associated with ISDN and the potential of data transfer rates of up to 6 Mbps. </li></ul></ul><ul><li>Recent development </li></ul><ul><ul><li>Introduction of systems that make use of satellite communications in order to receive data at very high speeds. </li></ul></ul><ul><ul><li>Many services make use of a conventional modem to send requests for data via a satellite link at speeds of up to 4000 kbps, approximately four times faster than an ISDN link. </li></ul></ul>
  22. 22. Data communications equipment or telecommunications processors <ul><li>Hubs – are used to connect up to 20 PCs to a network using patch cables running between the back of each PC and the hub. </li></ul><ul><li>Bridges and routers – are used to connect different LANs and transfer data packets from one network to the next. Routers can select the best route for packets to be transmitted and are used on the Internet backbones and wide-area network to achieve this. </li></ul><ul><li>Repeaters </li></ul><ul><ul><li>Over a long transmission distance, signal distortion may occur. </li></ul></ul><ul><ul><li>Repeaters are necessary to increase transmission distances by regenerating signals and re-transmitting them. </li></ul></ul>
  23. 23. A Simple Computer network
  24. 24. Telecommunications channels <ul><li>Telecommunication channels are the different media used to carry information between different locations. </li></ul><ul><li>These include traditional cables and wires known as guided media , and wireless and more recent innovations such as satellite and microwave which are unguided media . </li></ul>
  25. 25. Guided Media <ul><li>The main types of cabling used in LANs are based on copper cabling. </li></ul><ul><li>Data are transmitted along this by applying at one end a voltage, which is received at the other. </li></ul><ul><li>A positive voltage represents a binary “1” and a negative voltage represents a binary “0”. </li></ul><ul><li>There are two main types of twisted copper cabling used in networks. </li></ul><ul><li>Insulated straight cabling is not suitable except for very short distances. </li></ul><ul><li>The two types are: twisted pair ; co-axial </li></ul><ul><li>Fibre-optic is a relatively new transmission medium. </li></ul><ul><li>Packets are transmitted along fibre-optic cables using light or photons emitted from a light-emitting diode at one end of the cable; detection is by a photo-sensitive cell at the other end. </li></ul>
  26. 26. Unguided or wireless media <ul><ul><li>Wireless transmissions can be used for different </li></ul></ul><ul><ul><li>business applications at different scales: </li></ul></ul><ul><ul><li>Wireless infra-red transmission can be used for sending data between a portable PC or personal digital assistant (PDA) and a desktop computer. </li></ul></ul><ul><ul><li>Wireless transmission can also be used locally to form a wireless LAN. Here a microwave or narrow-band radio transmitter and receiver may be used to connect different buildings. </li></ul></ul><ul><ul><li>Microwave transmission can be used to beam information through the atmosphere. The maximum distance that can separate microwave transmitters is 45km. </li></ul></ul><ul><ul><li>Satellite transmission: Messages are sent from a transceiver at one location on the earth’s surface and are bounced off the satellite to another transceiver. </li></ul></ul>
  27. 27. Signal type <ul><li>Analogue Analogue transmissions send data in a continuous wave-form. </li></ul><ul><li>Digital Digital transmissions send binary data as a series of ones and zeros. </li></ul><ul><ul><li>Packets are units of data are exchanged between different devices over communications media. An entire message is broken down into smaller packets. </li></ul></ul>
  28. 28. Transmission speed <ul><li>The speed at which data can be transferred from A to B is governed by the channel capacity, which is measured in bits per second (bps). Transmission of a single bit in a second is equivalent to one baud, a binary event </li></ul><ul><li>Rates are usually measured in terms of thousands of bits/second (Kbit/sec), millions of bits/second (Mbit/sec) or billions of bits per second (Gbit/sec). </li></ul><ul><li>A general term often used for describing capacity is bandwidth . The technical definition of bandwidth is that it is a measure of capacity given by the difference between the lowest and highest frequencies available for a given medium. </li></ul>
  29. 29. Network operating system (NOS) <ul><li>It is the software necessary to control the access to and flow of information around a network. </li></ul><ul><li>The most widely used NOS for a PC-based LAN are Novell Netware and IBM LAN Manager. </li></ul><ul><li>UNIX is used by many medium and large companies operating servers from companies such as Sun Microsystems, Hewlett-Packard and IBM. It is often thought to offer better stability than Windows NT since it is a long-established NOS. </li></ul>
  30. 30. Network operating system (Continued) <ul><li>NOS provides the following functions: </li></ul><ul><li>Access control or security through providing user accounts with user names and passwords. </li></ul><ul><li>File and data sharing of data stored on a database server or file server. </li></ul><ul><li>Communication between users via e-mail, diary systems or workgroup software such as Lotus Notes. </li></ul><ul><li>Sharing of devices, enabling, for example, the backup to tape of data on the server, or printer sharing. </li></ul>
  31. 31. TOPICS <ul><li>Introduction to computer networks </li></ul><ul><li>Network components </li></ul><ul><li>The evolution of networking technology </li></ul><ul><li>The Internet </li></ul><ul><li>How the Internet works </li></ul><ul><li>Other wide-area networks </li></ul><ul><li>Smaller-scale networks </li></ul>
  32. 32. Evolution of networking technology <ul><li>The 1960s – mainframes and dumb terminals </li></ul><ul><li>The 1970s – minicomputers such as DEC PDP-11 and VAX. </li></ul><ul><li>The 1980s – the IBM PC and Apple Macintosh </li></ul><ul><li>The 1990s – the client/server architecture and global networks </li></ul><ul><li>2000s – Ubiquitous IP networks and application service providers (ASPs) </li></ul><ul><li>The Future – wireless access will become the norm on a local- and wide-area level. </li></ul>
  33. 33. TOPICS <ul><li>Introduction to computer networks </li></ul><ul><li>Network components </li></ul><ul><li>The evolution of networking technology </li></ul><ul><li>The Internet </li></ul><ul><li>How the Internet works </li></ul><ul><li>Other wide-area networks </li></ul><ul><li>Smaller-scale networks </li></ul>
  34. 34. What is the Internet ? <ul><li>The Internet refers to the physical network that links computers across the globe. </li></ul><ul><li>It consists of the infrastructure of network servers and communication links between them that are used to hold and transport information between the client PCs and web servers. </li></ul><ul><li>The Internet allows communication between millions of connected computers worldwide. </li></ul><ul><li>Information is transmitted form client PCs whose users request services to server computers that hold information and host business applications that deliver the services in response to requests. </li></ul><ul><li>As such, the Internet is a large-scale client/server system . </li></ul>
  35. 35. The Internet <ul><li>Internet Service Provider (ISP) </li></ul><ul><ul><li>A provider supplying home or business users with a connection to access the Internet. </li></ul></ul><ul><ul><li>They can also host web-based applications. </li></ul></ul><ul><li>Backbones High-speed communications links used to enable Internet communications across a country and internationally. </li></ul>
  36. 36. Figure 5.4 Infrastructure components of the Internet Source: Chaffey (2004).
  37. 37. Business and consumer models of Internet access <ul><li>E-Business opportunities are identified in terms of whether an organization is using the Internet to transact with consumers (business-to-consumer – B2C) or other businesses (business-to-business – B2B). </li></ul><ul><li>Business-to-business transactions predominate over the Internet, in terms of value, if not frequency. </li></ul><ul><li>Figure 5.6 shows that there are many more opportunities for B2B transactions than B2C, both between an organization and its suppliers, together with intermediaries, and through distributors such as agents and wholesalers with customers. There is a higher level of access to the Internet among businesses than among consumers, and a greater propensity to use it for purchasing. </li></ul>
  38. 38. Figure 5.6 B2B and B2C interactions between an organisation, its suppliers and its customers Source: Chaffey (2004).
  39. 39. Business and consumer models of Internet access <ul><li>Figure 5.7 gives examples of different companies operating in the business-to-consumer (B2C) and business-to-business (B2B) spheres. </li></ul><ul><li>Figures 5.7 also presents two additional types of transaction : that where consumers transact directly with consumers (C2C) and that where they initiate trading with business (C2B). </li></ul>
  40. 40. Figure 5.7 Summary of transaction alternatives between businesses and consumers Source: Chaffey (2004).
  41. 41. Role of the Internet in restructuring business relationships <ul><li>Figure 5.8 illustrates disintermediation in a graphical form for a simplifies retail channel. </li></ul><ul><li>Further intermediaries such as additional distributors may occur in a business-to-business market. </li></ul><ul><li>Figure 5.8(a) shows the former position where a company markets and sells it products by “pushing” them through a sales channel. </li></ul><ul><li>Figure 5.8 (b) and (c) show two different type of disintermediation in which the wholesale (b) or the wholesaler and retailer (c) are bypassed, allowing the producer to sell and promote direct to the consumer. </li></ul><ul><li>The benefits of disintermediation to the producer are clear – it is able to remove the sales and infra-structure cost of selling through the channel. </li></ul>
  42. 42. Figure 5.8 Disintermediation of a consumer distribution channel showing (a) the original situation, (b) disintermediation omitting the wholesaler and (c) disintermediation omitting both wholesaler and retailer Source: Chaffey (2004).
  43. 43. Role of the Internet in restructuring business relationships (Continued) <ul><li>Figure 5.10(a) starts with the traditional situation in which many sales were through brokers. </li></ul><ul><li>With disintermediation (Figure 5.10(b)) there was the opportunity to sell direct, initially via call centers and then complemented by a transactional website. </li></ul><ul><li>Purchasers still needed assistance in the selection of products and this led to the creation of new intermediaries , a process referred to as re-intermediation (Figure 5.10(c)). </li></ul>
  44. 44. Figure 5.10 The move to re-intermediation: (a) original situation, (b) disintermediation, (c) re-intermediation Source: Chaffey (2004).
  45. 45. Intranets and extranets <ul><li>The majority of Internet services are available to any business or consumer who has access to the Internet. </li></ul><ul><li>However, many e-business applications that access sensitive company information require access to be limited to favoured individuals or third parties. </li></ul><ul><li>If information is limited to those inside an organization, this is an intranet . </li></ul><ul><li>An intranet is a private network within a single company using Internet standards to enable employees to share information using e-mail and web publishing . </li></ul>
  46. 46. Intranets and extranets (Continued) <ul><li>If access to company information is extended to some others, but not everyone beyond the organization, this is an extranet . </li></ul><ul><li>An extranet is formed by extending the intranet beyond a company to customers, suppliers and collaborators. </li></ul><ul><li>The relationship between these terms is illustrated in Figure 5.12. </li></ul><ul><li>Extranets can be accessed by authorized people outside the company such as collaborators, suppliers or major customers, but information is not available to everyone with an Internet connection – only those with password access. </li></ul>
  47. 47. Figure 5.12 The relationship between intranets, extranets and the Internet
  48. 48. Firewalls <ul><li>Firewall is a specialized software application mounted on a server at the point where the company is connected to the Internet. </li></ul><ul><li>Its purpose is to prevent unauthorized access into the company from outsiders. </li></ul><ul><li>Firewalls are necessary when we are creating an intranet or extranet to ensure that the outside access to the confidential information does not occur. </li></ul><ul><li>Firewalls are usually created as software mounted on a separate server at the point where the company is connected to the Internet. </li></ul><ul><li>Firewall software can then be configured to only accept links from trusted domains representing other offices in the company. </li></ul>
  49. 49. Firewalls (Continued) <ul><li>The use of firewalls within the infra-structure of a company is illustrated in Figure 5.14. </li></ul><ul><li>It is evident that multiple firewalls are used to protect information of the company. </li></ul><ul><li>The information made available to third parties over the Internet and extranet is partitioned by another firewall using what is referred to as the demilitarized zone (DMZ). </li></ul><ul><li>Corporate data on the intranet is then mounted on other servers inside the company. </li></ul>
  50. 50. Figure 5.14 An example of the use of firewalls to increase security within an e-business infrastructure Source: Chaffey (2004).
  51. 51. What is the World Wide Web ? <ul><li>The World Wide Web (WWW) provides a standard method for exchanging and publishing information on the Internet. </li></ul><ul><li>The medium is based on standard document formats such as HTML (hypertext markup language) which can be thought of as similar to a word-processing format such as that used for Microsoft WORD documents. </li></ul><ul><li>It is the combination of web browsers and HTML that has proved so successful in establishing widespread business use of the Internet. </li></ul>
  52. 52. Web browsers and servers <ul><li>Web browsers are software used to access the information on the WWW that is stored on web servers. </li></ul><ul><li>Web servers are used to store, manage and supply the information on the WWW. </li></ul><ul><li>The main web browsers are Microsoft Internet Explorer and Netscape Navigator or Communicator. </li></ul><ul><li>Browsers display the text and graphics accessed from web sites and provide tools for managing information from web sites. </li></ul><ul><li>Figure 5.15 indicates the process by which web browsers communicate with web servers. </li></ul>
  53. 53. Figure 5.15 Information exchange between a web browser and web server Source: Chaffey (2004).
  54. 54. TOPICS <ul><li>Introduction to computer networks </li></ul><ul><li>Network components </li></ul><ul><li>The evolution of networking technology </li></ul><ul><li>The Internet </li></ul><ul><li>How the Internet works </li></ul><ul><li>Other wide-area networks </li></ul><ul><li>Smaller-scale networks </li></ul>
  55. 55. Networking standards <ul><li>The Internet is a packet-switched network that uses TCP/IP as its protocol. </li></ul><ul><li>This means that, as messages or packets are sent, there is no part of the network that is dedicated to them. </li></ul><ul><li>The alternative type of network is the circuit-switched network such as phone systems where the line is dedicated to the user for the duration of the call. </li></ul><ul><li>The transmission media of the Internet such as telephone lines, satellite links and optical cables are the equivalent of the vans, trains and planes that are used to carry post. </li></ul><ul><li>TCP/IP is the transmission control protocol is a transport-layer protocol that moves data between applications. </li></ul><ul><li>The Internet protocol is a network-layer protocol that moves data between host computers. </li></ul>
  56. 56. Networking standards (Continued) <ul><li>IP address is the unique numerical address of a computer. </li></ul><ul><li>The IP address is in the form of ( ) </li></ul><ul><li>Packet – Each Internet message such as an e-mail or http request is broken down into smaller parts for ease of transmission. </li></ul><ul><li>HTTP (Hypertext transfer protocol) is a standard that defines the way information is transmitted across the Internet between web browsers and web servers. </li></ul><ul><li>Uniform (or Universal) resource locator (URL) is a web address used to locate a web page on a web server. </li></ul><ul><li>Domain name refers to the name of the web server and is usually selected to be the same as the name of the company, and the extension will indicate its type. </li></ul><ul><li>Web page standards – HTML (used for presentation of data) and XML (used for data exchange) </li></ul>
  57. 57. Figure 5.16 The TCP/IP protocol Source: Chaffey (2004).
  58. 58. TOPICS <ul><li>Introduction to computer networks </li></ul><ul><li>Network components </li></ul><ul><li>The evolution of networking technology </li></ul><ul><li>The Internet </li></ul><ul><li>How the Internet works </li></ul><ul><li>Other wide-area networks </li></ul><ul><li>Smaller-scale networks </li></ul>
  59. 59. Wide Area Network (WAN) <ul><li>Wide area network (WAN) is a network covering a large area which connect to businesses in different parts of the same city, different parts of a country, or even different countries (Figure 5.22). </li></ul><ul><li>The WAN will connect many servers at each site. </li></ul><ul><li>If there is a large international coverage, it will be referred to as a global network . </li></ul><ul><li>If the WAN enables communication across the whole company, it is referred to as the “ enterprise network ” or “ enterprise-wide network ”. </li></ul><ul><li>Companies usually pay for their own “ leased lines ” or communications links between different sites. </li></ul><ul><li>Virtual private networks and value-added networks provide cheaper alternatives where the communications links are shared. </li></ul>
  60. 60. WAN (continued) <ul><li>Often the network used to connect remote sites is the public telephone, referred to as POTS or “plain old telephone system”. </li></ul><ul><li>A company can also lease private or dedicated lines from a telecommunications supplier to connect sites, or can set up links using microwave or satellite methods. </li></ul>
  61. 62. Value-added networks (VANs) <ul><li>VANs give a subscription service enabling companies to transmit data securely across a shared network . </li></ul><ul><li>VANs are so named because they allow a company to minimize its investment in wide-area communications while still receiving all the benefits this can bring. </li></ul><ul><li>The cost of setting up and maintaining the network is borne by the service provider , which then rents out the network to a number of companies. </li></ul><ul><li>This works out more cheaply than if a company had leased its own point-to-point private lines, but it is not as secure. </li></ul>
  62. 63. Virtual private network (VPN) <ul><li>A VPN is a data network that makes use of the public telecommunication infrastructure and Internet, but information remains secure by the use of security procedures. </li></ul><ul><li>VPNs are data networks that make use of the public telecommunication infra-structure and Internet, but information remains secure by the use of what is known as tunneling protocol and security procedures such as “firewalls”. </li></ul><ul><li>A virtual private network can again be contrasted with a system of owned or leased point-to-point lines that can only be used by one company. </li></ul>
  63. 64. TOPICS <ul><li>Introduction to computer networks </li></ul><ul><li>Network components </li></ul><ul><li>The evolution of networking technology </li></ul><ul><li>The Internet </li></ul><ul><li>How the Internet works </li></ul><ul><li>Other wide-area networks </li></ul><ul><li>Smaller-scale networks </li></ul>
  64. 65. Local area network (LAN) <ul><ul><li>LAN is a computer network that spans a limited geographic area, typically a single office or building. </li></ul></ul><ul><ul><li>LAN consists of a single network segment or several connected segments that are limited in extent (local). </li></ul></ul><ul><ul><li>A network segment defines a group of clients that are attached to the same hub or network interface card linked to a single server. </li></ul></ul>
  65. 66. Figure 5.24 A small workgroup network connecting a single server to three PCs and a laser printer
  66. 67. Network technology <ul><ul><li>There are a number of different arrangement for connecting clients to the server in a local area network. </li></ul></ul><ul><ul><li>The physical layout of a LAN is known as a network topology </li></ul></ul><ul><ul><li>These are known by the description of the layout or topology: bus, star or ring. </li></ul></ul><ul><ul><li>The layout of the arrangement are shown in Figure 5.25. </li></ul></ul><ul><ul><li>The topology chosen and the media used to implement it will affect the network cost and performance. </li></ul></ul>
  67. 69. Bus or linear <ul><li>Characteristics </li></ul><ul><ul><li>Simple </li></ul></ul><ul><ul><li>Based on co-axial Ethernet cable e.g. twisted pair 10Base-T. </li></ul></ul><ul><li>Advantage </li></ul><ul><ul><li>Easy to install and manage for small workgroup </li></ul></ul><ul><li>Disadvantage </li></ul><ul><ul><li>Breaks in the cable disrupt the whole network. </li></ul></ul>
  68. 70. Star <ul><li>Characteristics </li></ul><ul><ul><li>Each PC is connected via a cable to a central location </li></ul></ul><ul><ul><li>Each PC is not usually connected directly to the server, but via a hub. </li></ul></ul><ul><li>Advantage </li></ul><ul><ul><li>Provide protection from cable breaks </li></ul></ul><ul><li>Disadvantage </li></ul><ul><ul><li>Dependent on central host </li></ul></ul>
  69. 71. Ring <ul><li>Characteristics </li></ul><ul><ul><li>A continuous ring of network cable e.g. token ring </li></ul></ul><ul><ul><li>The word “token” refers to a packet of data which is passed from one node to the next. </li></ul></ul><ul><li>Advantage </li></ul><ul><ul><li>Suitable for large data volumes and mission-critical applications </li></ul></ul><ul><li>Disadvantage </li></ul><ul><ul><li>Higher initial cost and time for installation </li></ul></ul>
  70. 72. Peer-to-peer network <ul><ul><li>A peer-to-peer network is a simple type of local-area network which provides sharing of files and peripherals between PCs. </li></ul></ul><ul><ul><li>The “peer-to-peer” refers to the capability of any computer on a local-area network to share resources, in particular files and peripherals, with others. </li></ul></ul><ul><ul><li>It is particularly appropriate for small workgroups where central control from a server is less necessary. </li></ul></ul><ul><ul><li>For example, a user can, with permission, share across the network a file stored on another user’s hard disk. </li></ul></ul><ul><ul><li>With a peer-to-peer arrangement, data will be distributed and therefore difficult to backup and secure. </li></ul></ul>