Data communications


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Data communications

  1. 1. Data Communications Chapter 6
  2. 2. What is a Computer Network? <ul><li>A computer network is a collection of computers that communicate with one another over transmission lines. </li></ul><ul><li>Three basic types of networks are : </li></ul><ul><ul><li>Local area networks (LANs)–connects computers that reside in a single geographic location on the premises of the company that operates the LAN. </li></ul></ul><ul><ul><li>Wide area networks (WANs)–connects computers at different geographic sites. </li></ul></ul><ul><ul><li>Internets –a network of networks </li></ul></ul>
  3. 3. Major Network Types
  4. 4. Local Area Networks <ul><li>A local area network (LAN) is a group of computers connected together on a single company site. </li></ul><ul><li>Usually the computers are located within a half mile or so of each other, although longer distances are possible. </li></ul><ul><ul><li>The key distinction, however, is that all of the computers are located on property controlled by the company that operates the LAN. </li></ul></ul><ul><li>Computers and printers are connected via a switch , which is a special-purpose computer that receives and transmits messages on the LAN. </li></ul>
  5. 5. Local Area Networks (Continued) <ul><li>Each device on a LAN (computer, printer, etc.) has a hardware component called a network interface card (NIC) that connects the device’s circuitry to the cable. </li></ul><ul><ul><li>The NIC works with programs in each device to implement Layer 1 and Layer 2 protocols. </li></ul></ul><ul><ul><li>Each NIC has a unique identifier, which is called the (MAC) media access control address. </li></ul></ul><ul><li>The computers, printers, switches, and other devices on a LAN are connected using one of two media. </li></ul><ul><ul><li>Most connections are made using unshielded, twisted pair (UTP) cable. </li></ul></ul><ul><ul><li>A device called an RJ-45 connector is used to connect the UTP cable into NIC devices on the LAN. </li></ul></ul>© 2007 Prentice Hall, Inc.
  6. 6. Local Area Networks (Continued) <ul><li>The connection between switches can use UTP cable, but if they carry a lot of traffic or are far apart UTP cable may be replaced by optical fiber cables. </li></ul><ul><ul><li>The signals on such cables are light rays, and they are reflected inside the glass core of the optical fiber cable. </li></ul></ul><ul><ul><li>Optical fiber cable uses special connectors called ST and SC connectors. </li></ul></ul>© 2007 Prentice Hall, Inc.
  7. 7. Figure 5-6 Local Area Network
  8. 8. IEEE 802.3 or Ethernet Protocol <ul><li>The committee that addresses LAN standards is called the IEEE 802 Committee. </li></ul><ul><ul><li>Thus, IEEE LAN protocols always start with the number 802. </li></ul></ul><ul><li>Today, the world’s most popular protocol for LAN is the IEEE 802.3 protocol. </li></ul><ul><ul><li>This protocol standard, also called Ethernet , specifies hardware characteristics such as which wire carries which signals. </li></ul></ul><ul><ul><li>It also describes how messages are to be packaged and processed for transmission over the LAN. </li></ul></ul><ul><ul><li>Ethernet operates at Layers 1 and 2 of the TCP/IP-OSI architecture. </li></ul></ul>© 2007 Prentice Hall, Inc.
  9. 9. IEE 802.3 or Ethernet Protocol (Continued) <ul><li>Most personal computers today are equipped with an onboard NIC that supports what is called 10/100/1000 Ethernet . </li></ul><ul><ul><li>These products conform to the 802.3 specification and allow for transmission at a rate of 10, 100, or 1,000 Mbps. </li></ul></ul><ul><ul><li>Communications speeds are expressed in bits , whereas memory sizes are expressed in bytes. </li></ul></ul>© 2007 Prentice Hall, Inc.
  10. 10. LANs with Wireless Connections <ul><li>Wireless connections have become popular with LANs. </li></ul><ul><li>The NIC for wireless devices have been replaced by wireless NIC (WNIC). </li></ul><ul><ul><li>For laptop computers, such devices can be cards that slide into the PCMA slot or they can be built-in, onboard devices. </li></ul></ul><ul><li>Several different wireless standard exist </li></ul><ul><ul><li>As of 2005, the most popular is IEEE 802.11g </li></ul></ul><ul><ul><li>The current standard, 802.11g allows speeds of up to 54 Mbps. </li></ul></ul><ul><ul><li>The WNICs operate according to the 802.11 protocol and connect to an access point (AP). </li></ul></ul>© 2007 Prentice Hall, Inc.
  11. 11. Figure 5-10 © 2007 Prentice Hall, Inc.
  12. 12. Figure 5-12 Summary of LAN and WAN Networks © 2007 Prentice Hall, Inc.
  13. 13. Wide Area Networks <ul><li>A wide area network (WAN) connects computers located at physically separated sites. </li></ul><ul><ul><li>A company with offices in Detroit and Atlanta must use a WAN to connect the computers together. </li></ul></ul><ul><ul><li>Because the sites are physically separated, the company cannot string wire from one site to another. </li></ul></ul><ul><li>An Internet service provider (ISP) has three important functions: </li></ul><ul><ul><li>It provides you with a legitimate Internet address. </li></ul></ul><ul><ul><li>It serves as your gateway to the Internet. </li></ul></ul><ul><ul><li>It receives the communication from your computer and passes them on to the Internet, and it receives communication from the Internet and passes them on to you. </li></ul></ul>© 2007 Prentice Hall, Inc.
  14. 14. Connecting the Personal Computer to an ISP: Modems <ul><li>Home computers and those of small businesses are commonly connected to an ISP in one of three ways: </li></ul><ul><ul><li>Using a regular telephone line </li></ul></ul><ul><ul><li>Using a special telephone line called a DSL line </li></ul></ul><ul><ul><li>Using cable TV line </li></ul></ul><ul><li>All three ways require that the digital data in the computer be converted to an analog , or wavy, signal. </li></ul><ul><li>A device called a modem , or modulator/demodulator performs this conversion. </li></ul>© 2007 Prentice Hall, Inc.
  15. 15. Figure 5-14 Personal Computer (PC) Internet Access
  16. 16. DSL Modems <ul><li>A DSL modem is the second modem type. </li></ul><ul><li>DSL stands for digital subscriber line. </li></ul><ul><li>DSL modems operate on the same lines as voice telephones and dial-up modems. </li></ul><ul><ul><li>They operate so that their signals do not interfere with voice telephone service. </li></ul></ul><ul><ul><li>They provide much faster data transmission speeds than dial up modems. </li></ul></ul><ul><ul><li>They always maintain a connection. </li></ul></ul>© 2007 Prentice Hall, Inc.
  17. 17. DSL Modems (Continued) <ul><li>DSL data transmission and telephone conversations can occur simultaneously. </li></ul><ul><li>DSL modems use their own Layer 1 and Layer 2 protocols for data transmission. </li></ul><ul><li>DSL lines that have different upload and download speeds are called asymmetric digital subscriber lines (ADSL). </li></ul><ul><li>Symmetrical digital subscriber lines (SDSL) offers the same speed in both directions. </li></ul>© 2007 Prentice Hall, Inc.
  18. 18. Cable Modems <ul><li>A cable modem is the third modem type. </li></ul><ul><li>Cable modems provide high-speed data transmission using cable television lines. </li></ul><ul><li>At the maximum, users can download data up to 10 Mbps and can upload data at 256 kbps. </li></ul><ul><li>Narrowband lines typically have transmission speeds less than 56 kbps. </li></ul><ul><li>Broadband lines have speeds in excess of 256 kbps. </li></ul>© 2007 Prentice Hall, Inc.
  19. 19. Figure 5-15 Wide Area Network Using Leased Lines © 2007 Prentice Hall, Inc.
  20. 20. Networks of Leased Lines <ul><li>A WAN connects computers located at geographically distributed company sites. </li></ul><ul><li>The lines that connect these sites are leased from telecommunication companies that are licensed to provide them. </li></ul><ul><li>A variety of access devices connect each site to the transmission. </li></ul><ul><ul><li>These devices are typically special-purposed computers. </li></ul></ul><ul><ul><li>The particular devices required depend on the line used and other factors </li></ul></ul><ul><ul><li>Sometimes switches and routers are employed. </li></ul></ul>© 2007 Prentice Hall, Inc.
  21. 21. Figure 5-16 Transmission Line Types, Uses, and Speeds © 2007 Prentice Hall, Inc.
  22. 22. Public Switched Data Network <ul><li>Another WAN alternative is a public switched network (PSDN) , a network of computers and leased lines that is developed and maintained by a vendor that leases time on the network to other organizations. </li></ul><ul><li>When using a PSDN, each site must lease a line to connect to the PSDN network. </li></ul><ul><ul><li>The location at which this occurs is called a point of presence (POP); it is the access point into the PSDN. </li></ul></ul><ul><ul><li>Once a site has connected to the PSDN POP, the site obtains access to all other sites connected to the PSDN. </li></ul></ul>© 2007 Prentice Hall, Inc.
  23. 23. Public Switched Data Network (Continued) <ul><li>PSDNs save the setup and maintenance activities when using leased lines. </li></ul><ul><ul><li>They also save costs because a company does not have to pay for the entire network. </li></ul></ul><ul><ul><li>The company can just pay for the traffic that it sends. </li></ul></ul><ul><li>Three Layer 1 and 2 protocols are used with PSDNs: </li></ul><ul><ul><li>Frame Relay can process traffic in the range of 56 kbps to 40 Mbps. </li></ul></ul><ul><ul><li>Asynchronous transfer mode (ATM) can process speeds from 1 to 156 Mbps (can handle both voice and data). </li></ul></ul><ul><ul><li>Ethernet, the protocol developed for LANs </li></ul></ul>© 2007 Prentice Hall, Inc.
  24. 24. Figure 5-17 Wide Area Network Using PSDN
  25. 25. Virtual Private Network <ul><li>Virtual private network (VPN) is the fourth WAN alternative. </li></ul><ul><li>A VPN uses the Internet or a private internet to create the appearance of private point-to-point connections. </li></ul><ul><li>A VPN uses the public Internet to create the appearance of a private connection. </li></ul><ul><li>A connection called a tunnel , is a virtual pathway over a public or shared network from the VPN client to the VPN server. </li></ul>© 2007 Prentice Hall, Inc.
  26. 26. Virtual Private Network (Continued) <ul><li>VPN communications are secure. </li></ul><ul><ul><li>The VPN client software encrypts , or codes, the original messages so that its contents are hidden. </li></ul></ul><ul><li>Virtual private networks offer the benefit of point-to-point leased lines, and they enable remote access, both by employees and by any others who have been registered with the VPN server. </li></ul>© 2007 Prentice Hall, Inc.
  27. 27. Example of Remote Access Using a VPN
  28. 28. Example of a WAN using VPN
  29. 29. Criteria for Comparing WANS <ul><li>Many different computer networking alternatives are available, each with different characteristics. </li></ul><ul><li>There are three types of costs that need to be considered. </li></ul><ul><ul><li>Setup costs include the costs of acquiring transmission lines and necessary equipment, such as switches, routers, and access devices. </li></ul></ul><ul><ul><li>Operational costs include lease fees for lines and equipment, charges of the ISP, the cost of ongoing training, etc. </li></ul></ul><ul><ul><li>Maintenance costs include those for periodic maintenance, problem diagnosis and repair, and mandatory upgrades. </li></ul></ul>
  30. 30. Criteria Continued <ul><li>There are six considerations with regard to performance: </li></ul><ul><ul><li>Speed </li></ul></ul><ul><ul><li>Latency (delays during busy periods) </li></ul></ul><ul><ul><li>Availability </li></ul></ul><ul><ul><li>Loss rate </li></ul></ul><ul><ul><li>Transparency (user involvement in operation) </li></ul></ul><ul><ul><li>Performance guarantees </li></ul></ul><ul><li>Other criteria to consider when comparing network alternatives include the growth potential (greater capacity) and the length of contract commitment </li></ul>