Data communications
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Data communications Data communications Presentation Transcript

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