Case mis ch06


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Case mis ch06

  1. 1. 6장Data Communications
  2. 2. Q1: What Is a Computer Network?• Network – Collection of computers – Communicate with one another over transmission line• Basic types of Network Topologies – Local Area Network (LAN) – Wide Area Network (WAN) – Internet 6-2
  3. 3. LAN (Local Area Network)• Computers connected on single company site• Usually located in small area, such as a home, office building, or group of buildings• Connects two to several hundred computers 6-3
  4. 4. WAN (Wide Area Network)• Connects computers across metropolitan, state, regional, national areas• Uses communication networks from vendors – Licensed by government 6-4
  5. 5. Internet• Global network of networks• Connects LANs, WANs, and other networks• Public and private networks of networks• Uses a variety of communication methods and conventions – Seamless flow of data provided by standardized layered protocol • Protocol: a set of standard rules for data representation, signaling, authentication, and error detection required to send information 6-5
  6. 6. Q2: What Are the Components of a LAN? 6-6
  7. 7. Q2: What Are the Components of a LAN? (cont’d) • Switch/Router – Special purpose computer that receives and transmits messages • Network interface card (NIC) – Hardware that connects each device’s circuitry to the cable – Onboard NIC or card that fits into an expansion slot • MAC (media control address) • UTP (unshielded twisted pair cable) • Optical fiber cable 6-7
  8. 8. Network Interface Card (NIC) 6-8
  9. 9. IEEE 802.3 Protocol(Institute of Electrical and Electronics Engineers)• World’s most popular protocol for LANs• 10/100/1000 Mbps Ethernet• Protocol specifies hardware characteristics, cables, and how messages are packaged and processed 6-9
  10. 10. Q3: What Are the Alternatives for a WAN?• WANs connect computers at separate sites – Use routers and public communications links between sites – Cable connections made through licensed public telecommunications companies• Internet service provider (ISP) – A company that provides customers access to the Internet – Provides legitimate Internet address – Serves as gateway to Internet – Pay for Internet access 6-10
  11. 11. Q3: What Are the Alternatives for a WAN? (cont’d) 1. Connect to an Internet service provider (ISP) 2. Network of Leased Lines 3. Public Switched Data Networks 4. Virtual Private Network (VPN) 6-11
  12. 12. Connecting the Personal Computer to an ISP: Modems 1.Dial-up Modems 2.DSL Modems 3.Cable Modems• Modem – a device to produce a signal that can be transmitted easily and decoded to reproduce the original digital data• Narrowband – less than 56 kbps speed• Broadband – more than 256 kbps speed 6-12
  13. 13. 1. Dial-up Modems– Converts signals between analog and digital so computers can communicate over a regular telephone line– Connection made by dialing the ISP’s phone number– Maximum transmission speed of 56 kbps (53 kbps really)– Modulation standards – V.34, V.90, V.92 specify how digital signals are converted to analog– Point-to-Point Protocol (PPP) – governs how data (messages) are packaged and handled between modem and ISP 6-13
  14. 14. Personal Computer (PC) Internet Access 6-14
  15. 15. 2. DSL ModemsDigital Subscriber Line (DSL) – Operates over telephone lines – Data signals do not interfere with voice telephone service – Faster data transmission than dial-up • 256 kbps to 768 kbps – DSL modems use their own protocols – Download and upload speeds differ • Asymmetric digital subscriber lines (ADSL) • Symmetrical digital subscriber lines (SDSL) – 1.544 mbps both directions 6-15
  16. 16. 3. Cable Modems• Provide high-speed data transmission• Use cable television lines – High-capacity optical fiber cable run to neighborhood distribution substation – Television cables connect at distribution substation and runs to subscriber’s house – Does not interfere with television transmission – Up to 500 subscribers connected at each substation • Performance can vary based on number connected – Download speed up to 10Mbps, upload up to 256 kbps 6-16
  17. 17. Networks of Leased Lines• WAN connects computers located at geographically distributed sites• Access devices connect sites to transmission – Router or network switch– special-purpose computer to move network traffic from one node to another• Leased line alternatives – T1 line (1.544 Mbps), T3 line (44.736 Mbps), OC-768 (40 Gbps) – Set up as a point-to-point line – Only predefined sites can use leased lines – Requires highly trained specialists to set up – Monthly connection and maintenance costs expensive – Provide low cost per bit transmitted for high traffic networks 6-17
  18. 18. WAN Using Leased Lines 6-18
  19. 19. Public Switched Data Network (PSDN)Public Switched Data Network – Network of computers and leased lines – Developed and maintained by vendor – Time leased on network• Utility that supplies networks for other companies to lease – Each site leases line to connect to PSDN network at access point • Point of presence (POP) • Saves setup and maintenance activities 6-19
  20. 20. WAN Using PSDN 6-20
  21. 21. PSDN Protocols• Frame relay – Process traffic at 56 kbps to 40 Mbps – Simplest and easiest to support – Lower cost than ATM• Asynchronous transfer mode (ATM) – Process speeds from 1 to 156 Mbps – Support voice and data communication• Ethernet – Operates at speeds of 10 to 40 Gbps 6-21
  22. 22. Q4: How Does Encryption Work?• Encryption – Process of transforming clear text into coded text – Used for secure storage or communication – Uses algorithms • DES (Data Encryption Standard) • 3DES (Triple DES) • AES (Advanced Encryption Standard)• Key – Used to encrypt data – Algorithm applies key to produce coded message • Symmetric encryption – same key encrypts and decrypts • Asymmetric encryption – different keys are used – Key used to decrypt message 6-22
  23. 23. Q4: How Does Encryption Work? (cont’d)• Public key/private key cryptography – Public key used to encrypt messages – Public key sent to other party to encode messages to be sent back – Decrypted with private key – Complex and slow• Asymmetric public key encryption — message encrypted using recipients public key. Recipient decrypts with own public key.• HTTPS uses secure socket layer (SSL, a.k.a. TLS) protocol to encrypt data – Fast, secure – See next slide for how it works 6-23
  24. 24. Q4: How Does Encryption Work? (cont’d) Figure 6-14 6-24
  25. 25. Q5: What Is the Purpose of a Firewall?• Computer device that prevents unauthorized network access• May be special-purpose computer or program on a general-purpose computer• Organizations may have multiple firewalls – Perimeter firewalls sit outside organizational network – Internal firewalls are inside network – Packet-filtering firewalls examine each part of a message (source address, destination address, and other bytes) before allowing message to pass – May filter both incoming and outgoing messages• Access control list (ACL) encodes rules stating which IP addresses are allowed into or prohibited from the network 6-25
  26. 26. Q6: What Is a VPN, and Why Is It Important? Virtual private network – Uses Internet or private network to create appearance of point-to-point connections – Uses public Internet to create appearance of private network – Client and server have point-to-point connection called tunnel • Private pathway over shared network – Secure, encrypted communications • VPN client software encrypts data and receiving VPN server decrypts it • Users and their keys must be registered with VPN server 6-26
  27. 27. Remote Access Using VPN: Actual Connections 6-27
  28. 28. How Does the Knowledge in This Chapter Help Dee and You?• Dee and salespeople use thin client computers – Dee creates blog entries on Web pages using Movable Type – Salespeople use pages served by Movable Type• Client computers contain VPN client software – Interact with VPN server via Internet – Secure, private connections – Firewalls stop traffic not addressed to VPN server – Salespeople know how to use VPN 6-28
  29. 29. How Does the Knowledge in This Chapter Help Dee and You? (cont’d)• IT department cannot schedule resources to setup blog server in timely fashion – Will not allow outside person to do setup • Would then have access to system• Consultant can create server on unprotected test server – Can be copied onto network server after acceptance – Requires some internal IT labor – Consultant could include Trojan horse virus or malware • Install only software from known sources • Code should not reside on production server 6-29