Simon Text Book

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Chapter 4
Data Communications and Networking Considerations
Introduction to Information Systems
Judith C. Simon
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Chapter 4 Major Topics

• General concepts
• Characteristics of communications channels
• Data communications media
• Communications carriers
• Data communications hardware
• Network topologies
• Network protocols
• LANs and WANs
• Intranet and extranet communications
• Distributed systems
• Strategic network issues
• Communications infrastructure
• Ubiquitous networks

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General Concepts of Data Communications

• Data communications computer-based electronic
  transmission of data
• Components sender, receiver, medium
  (channel), message
• Network collection of computers and other
  hardware connected through communications media,
  along with programs to provide capabilities of
  sharing resources

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Characteristics of Communications Channels

• Characteristics of communications channels
  (described on next slides) include
• type of service
• direction of communication
• number of data paths
• number of connections
• type of signal
• speed of transmission
• mode of transmission

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Direction of Communication

• Simplex data may be transmitted in only one
  direction e.g., radio and television
• 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
• Full duplex (or duplex) data may be
  transmitted in both directions at the same time,
  e.g., telephone

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Number of Data Paths

• Serial one data-carrying path in a channel
• Parallel several data paths in a channel

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Number of Connections

• Point-to-point only two devices connected to
  the channel, e.g., one computer and one printer
• Multipoint more than two devices connected to
  the channel

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Type of Signal - and Related Terms

• Digital signal discrete
• Analog signal continuous
• Modulation altering an analog (carrier)
  signal to represent digital data
• Demodulation changing a digital signal back to
  analog after transmission
• Modem device that performs modulation/demodulati
  on
• Codec device that converts analog video and
  audio data into digital signals, such as for
  storage on a CD
• DSL digital subscriber line, which provides
  higher-speed transmission of data than
  traditional telephone lines

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Speed of Transmission

• 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
• 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
• 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

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Mode of Transmission

• 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)
• 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

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Data Communications Media(Media Channels or
Means of Transmission)

• Wire pairs most common medium, widely used by
  local phone companies usually twisted-pair
  wires relatively inexpensive but low-speed
  medium
• Coaxial cable widely used for long-distance
  connections and in local area networks (LANs)
  faster data transmission than wire pairs because
  of greater bandwidth
• 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

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Data Communications Media(continued)

• Broadcast radio wireless method used for
  cellular phone and handheld computer
  transmissions
• 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
• Infrared line-of-sight signals being used in
  some wireless local area networks security of
  transmission is an issue unless data is encrypted
  (scrambled)

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Communications Carriers

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

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Data Communications Hardware

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

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Network Topologies

• Topology interconnection of devices and
  channels many ways exist for connecting
  computers, including
• 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
• 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
• 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

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Network Topologies

• 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
• Ring uses point-to-point channels to connect
  ring repeaters in a configuration similar to a
  loop computers are connected to the ring
  repeaters
• Bus all the computers are connected to a common
  communications channel

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Network Protocols(Rules for Network
Communications)

• ISO-OSI Reference Model guidelines for
  development of protocols so different vendors
  hardware will work together
• 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
• Polling early protocol used on multipoint
  channels which involved a master computer
  checking with subordinate computers to see if
  they had something to transmit

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Network Protocols (continued)

• X.25 protocol widely used today for connecting
  computers to public and private networks with
  packet-switching services
• ISDN protocol that describes how a
  communications channel can be shared to have
  various services simultaneously on one
  communications channel
• 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)

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Network Protocols (continued)

• 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
• TCP/IP used for Internet communications because
  it allows computers to communicate regardless of
  their operating systems and hardware
• ATM allows one high-speed channel to
  simultaneously carry different types of services,
  such as voice, TV, data, etc.

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LANs and WANs

• Local area network (LAN) operates within a
  restricted geographic area such as within one
  building usually owned by a single organization
• 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
• LAN vs. WAN Comparison LAN transmission speed
  is usually greater than WAN, and LAN usually has
  lower error rates than WANs, due to LANs shorter
  travel distance LANs share more than data, e.g.,
  peripherals, programs, processing power

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Intranet and Extranet Communications

• Intranet uses Internet technology for private
  (internal) business operations cost of
  developing and maintaining these systems is often
  lower than for LANs
• 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

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Distributed Systems

• Distributed data and programs are spread out
  over several locations, and computers share the
  workload
• 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

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Strategic Network Issues

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

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Strategic Network Issues, continued

• Security of data
• Flexibility when changes in network are needed
• Backup/recovery provisions in case any computers
  quit working
• Efficiency in use of resources without
  unnecessary waste, such as in transmission time
  and storage space