1. Slide 4-1 Chapter 4 Data Communications and Networking Considerations Introduction to Information Systems Judith C. Simon

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3. Slide 4-3 Chapter 4 Major Topics ä General concepts ä Characteristics of communications channels ä Data communications media ä Communications carriers ä Data communications hardware ä Network topologies ä Network protocols ä LANs and WANs ä Intranet and extranet communications ä Distributed systems ä Strategic network issues ä Communications infrastructure ä Ubiquitous networks

4. Slide 4-4 General Concepts of Data Communications ä Data communications: computer-based electronic transmission of data ä Components: sender, receiver, medium (“channel”), message ä Network: collection of computers and other hardware connected through communications media, along with programs to provide capabilities of sharing resources

5. Slide 4-5 Characteristics of Communications Channels Characteristics of communications channels (described on next slides) include Characteristics of communications channels (described on next slides) include ä type of service ä direction of communication ä number of data paths ä number of connections ä type of signal ä speed of transmission ä mode of transmission

6. Slide 4-6 Types of Service ä Dedicated channel: assigned exclusively to a particular service, such as PC-to-printer, in which the channel is available full time for communication between the two devices ä Circuit-switched channel: communication circuit is established for a transmission, then released to be available to others, which is less expensive than dedicated (e.g., telephone system) ä Packet-switched channel: store-and-forward design widely used on networks, in which long messages are separated into smaller “packets” for transmission, often relayed through intermediate computers on the way to the destination computer

7. Slide 4-7 Types of Channel Connections Dedicated communications channel (point-to-point)

8. Slide 4-8 Types of Channel Connections Circuit-switched communications channel 

9. Slide 4-9 Types of Channel Connections Network Server Packet-switched communications channel (multipoint)

10. Slide 4-10 Direction of Communication ä Simplex: data may be transmitted in only one direction; e.g., radio and television ä Half duplex: data may be transmitted in one direction or the other, but not in both directions at the same time; e.g., CB radio ä Full duplex (or “duplex”): data may be transmitted in both directions at the same time, e.g., telephone

11. Slide 4-11 Simplex Communication Data is transmitted in one direction only

12. Slide 4-12 Half-Duplex Communication Data can be transmitted in both directions, but not at the same time. OR

13. Slide 4-13 Full-Duplex Communication Data can be transmitted in both directions, at the same time.

14. Slide 4-14 Number of Data Paths ä Serial: one data-carrying path in a channel ä Parallel: several data paths in a channel

15. Slide 4-15 Serial Communications SerialChannel 0 1 0 0 0 0 1 1 1 0 0 0 0 01 1 EBCDIC “B”EBCDIC “A” One data-carrying path in communications channel

16. Slide 4-16 Parallel Communications ParallelChannel Data Carrying Paths EBCDIC “B” EBCDIC “A” 1 1 0 0 0 0 1 0 1 1 0 0 0 0 0 1 Multiple data paths in communications channel, e.g., 8 data-carrying paths

17. Slide 4-17 Number of Connections ä Point-to-point: only two devices connected to the channel, e.g., one computer and one printer ä Multipoint: more than two devices connected to the channel

18. Slide 4-18 Number of Connections Multipoint Lines Point-to-Point Lines

19. Slide 4-19 Type of Signal - and Related Terms ä Digital signal: discrete ä Analog signal: continuous ä Modulation: altering an analog (“carrier”) signal to represent digital data ä Demodulation: changing a digital signal back to analog after transmission ä Modem: device that performs modulation/demodulation ä Codec: device that converts analog video and audio data into digital signals, such as for storage on a CD ä DSL: digital subscriber line, which provides higher-speed transmission of data than traditional telephone lines

20. Slide 4-20 Digital and Analog Signals amplitude time Continuous (Analog) amplitude time Discrete (Digital)

21. Slide 4-21 Speed of Transmission ä Bits per second (bps): how fast data may be sent over a communications channel; bps is the way that speed of transmission is typically measured ä Baud rate: how fast signals may be sent over a communications channel; more than one bit may be sent per signal, so bps may be higher than baud ä Bandwidth: difference between highest and lowest frequencies that may be transmitted over the communications channel; bandwidth affects speed of signal transmission, which affects speed of data transmission

22. Slide 4-22 Mode of Transmission ä Synchronous (same-time) transmission: block-at-a- time transmission, performed by transmitting a timing signal with the data signal so that both sender and receiver use the same timing signal (prevents timing problems that could cause data loss) ä Asynchronous (not-same-time) transmission: character-at-a-time transmission; start and stop bits are used (instead of a timing signal) to identify the beginning and ending of each character

23. Slide 4-23 Data Communications Media (Media = Channels or Means of Transmission) ä Wire pairs: most common medium, widely used by local phone companies; usually twisted-pair wires; relatively inexpensive but low-speed medium ä Coaxial cable: widely used for long-distance connections and in local area networks (LANs); faster data transmission than wire pairs because of greater bandwidth ä Optical fiber: considered the most secure medium because tapping into its signals is more difficult; preferred in locations such as hospital operating rooms with extensive electrical equipment as well as a critical need for accurate data, because its laser light signals are not affected by electrical or magnetic interference

24. Slide 4-24 Data Communications Media (continued) ä Broadcast radio: wireless method used for cellular phone and handheld computer transmissions ä Microwave: both terrestrial and satellite systems use line- of-sight signals (transmitter and receiver must be in a straight line with no barriers between them); signals are subject to distortion by weather; many satellite systems are in geosynchronous orbit 22,000+ miles above the equator, although low- and medium-orbit satellite systems are increasing in use ä Infrared: line-of-sight signals being used in some wireless local area networks; security of transmission is an issue unless data is encrypted (scrambled)

25. Slide 4-25 Communications Carriers ä Communications carriers: companies that provide a common type of communications service to many customers ä POTS: plain old telephone service, available from companies such as ATT, Bell systems (BellSouth, etc.), MCI, and Sprint ä Value-added carrier: typically leases POTS-type services, enhances them, and resells the use of the facilities to someone else

26. Slide 4-26 Data Communications Hardware ä Modem: described earlier; allows users to transmit digital data over analog facilities ä Bridge: device (usually a computer) used to connect two homogeneous networks, i.e., with the same protocols and topologies (described later) ä Gateway: device used to connect two heterogeneous networks, i.e., with different protocols and topologies ä Router: device that makes decisions as to channel (route) that a message will take to its destination ä Multiplexer & concentrator: devices that provide line- sharing capabilities, i.e., multiple computer transmissions over shared communications channels at the same time

27. Slide 4-27 Network Topologies Topology: interconnection of devices and channels; many ways exist for connecting computers, including: Topology: interconnection of devices and channels; many ways exist for connecting computers, including: 4 Fully interconnected (plex): uses point-to-point channels to connect each computer to each other computer on the network; expensive method but provides good backup in case of a failure of any links 4 Chain: first computer has a point-to-point link to the second, second is linked to third, etc.; failure of one link can cease communications from that point on 4 Loop: similar to chain, except that last computer is linked back to first computer through point-to-point channel; failure of one link will not cease communications

28. Slide 4-28 Network Topologies ä Star: one central computer is connected by point- to-point channel to each of the other computers; failure of the central computer would shut down the entire network, although failure of one link affects only the one computer connected to the central computer ä Ring: uses point-to-point channels to connect ring repeaters in a configuration similar to a loop; computers are connected to the ring repeaters ä Bus: all the computers are connected to a common communications channel

29. Slide 4-29 Fully Interconnected Topology

30. Slide 4-30 Chain, Loop & Star Topologies Chain Loop Star

31. Slide 4-31 Ring & Bus Topologies RingTopology BusTopology

32. Slide 4-32 Network Protocols (Rules for Network Communications) ä ISO-OSI Reference Model: guidelines for development of protocols so different vendors’ hardware will work together ä Contention: early protocol used on point-to-point channels for determining when a computer can send information over the channel; each computer “contends” for channel control by sending a set of characters that requests permission to transmit ä Polling: early protocol used on multipoint channels which involved a master computer checking with subordinate computers to see if they had something to transmit

33. Slide 4-33 Network Protocols (continued) ä X.25: protocol widely used today for connecting computers to public and private networks with packet- switching services ä ISDN: protocol that describes how a communications channel can be shared to have various services simultaneously on one communications channel ä CSMA/CD: widely used in LANs, using a bus topology; a computer wanting to transmit data checks the channel to see if another computer is transmitting (looks for a carrier signal); if none is found, it transmits but still checks to be sure no other computer starts transmitting at the same time (a potential collision)

34. Slide 4-34 Network Protocols (continued) ä Token passing: widely used in LANs, usually with a ring topology; a token (string of characters) circulates among the computers, and any computer can take control of it as it passes so that it can transmit and then release the token ä TCP/IP: used for Internet communications because it allows computers to communicate regardless of their operating systems and hardware ä ATM: allows one high-speed channel to simultaneously carry different types of services, such as voice, TV, data, etc.

35. Slide 4-35 LANs and WANs ä Local area network (LAN): operates within a restricted geographic area such as within one building; usually owned by a single organization ä Wide area network (WAN): operates over a large geographic area; usually includes computers owned by the network user and communications channels owned by someone else, such as a telephone company ä LAN vs. WAN Comparison: LAN transmission speed is usually greater than WAN, and LAN usually has lower error rates than WANs, due to LAN’s shorter travel distance; LANs share more than data, e.g., peripherals, programs, processing power

36. Slide 4-36 Intranet and Extranet Communications ä Intranet: uses Internet technology for private (internal) business operations; cost of developing and maintaining these systems is often lower than for LANs ä Extranet: uses Internet technology for business- to-business operations, in which one business has access to data located within another business’ intranet; parts of two business’ intranets may be linked

37. Slide 4-37 Distributed Systems ä Distributed: data and programs are spread out over several locations, and computers share the workload ä Client/server system: distributed system in which a server (usually a combination of special hardware and software) provides services to other computer(s) called clients; server and client may both be PCs, and one computer may be a server for some activities and a client for other activities

38. Slide 4-38 Strategic Network Issues Planning and implementation issues include: Planning and implementation issues include: ä Throughput, i.e., how much data can be sent in a time period ä Response time from user request to receipt of response ä Consistency in speed and quality of service ä Reliability of network system to keep functioning ä Availability of network when needed

39. Slide 4-39 Strategic Network Issues, continued ä Security of data ä Flexibility when changes in network are needed ä Backup/recovery provisions in case any computers quit working ä Efficiency in use of resources without unnecessary waste, such as in transmission time and storage space

40. Slide 4-40 Communications Infrastructure Communications infrastructure refers to the underlying base of facilities, equipment, software, and services needed to support applications that are dependent on communications Communications infrastructure refers to the underlying base of facilities, equipment, software, and services needed to support applications that are dependent on communications

41. Slide 4-41 Ubiquitous Networks ä Ubiquitous: very widespread ä Data communications networks are found worldwide (“ubiquitous”)

42. Slide 4-42