Transcript Concepts

Chapter 4
Data Communications
and Networking Considerations
Introduction to Information Systems
Judith C. Simon
Slide 4-1
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Slide 4-2
Chapter 4 Major Topics
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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
Slide 4-3
General Concepts of Data Communications
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Data communications: computer-based electronic
transmission of data
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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
Slide 4-4
Characteristics of Communications Channels
Characteristics of communications channels
(described on next slides) include
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type of service
direction of communication
number of data paths
number of connections
type of signal
speed of transmission
mode of transmission
Slide 4-5
Types of Service
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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
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)
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
Slide 4-6
Types of Channel Connections
Dedicated communications channel
(point-to-point)
Slide 4-7
Types of Channel Connections
Circuit-switched communications channel
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Slide 4-8
Types of Channel Connections
Packet-switched communications channel (multipoint)
Network
Server
Slide 4-9
Direction of Communication
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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
Slide 4-10
Simplex Communication
Data is transmitted in one direction only
Slide 4-11
Half-Duplex Communication
OR
Data can be transmitted in both directions,
but not at the same time.
Slide 4-12
Full-Duplex Communication
Data can be transmitted in both directions,
at the same time.
Slide 4-13
Number of Data Paths
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Serial: one data-carrying path in a channel
Parallel: several data paths in a channel
Slide 4-14
Serial Communications
EBCDIC “B”
Serial
Channel
EBCDIC “A”
0 1 0 0 0 0 1 1 1 0 0 0 0 01 1
One data-carrying path in communications channel
Slide 4-15
Parallel Communications
Data Carrying Paths
EBCDIC “B”
Parallel
Channel
EBCDIC “A”
1
1
0
0
0
0
1
1
1
0
0
0
0
0
0
1
Multiple data paths in communications channel,
e.g., 8 data-carrying paths
Slide 4-16
Number of Connections
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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
Slide 4-17
Number of Connections
Multipoint Lines
Point-to-Point Lines
Slide 4-18
Type of Signal - and Related Terms
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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/demodulation
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
Slide 4-19
Digital and Analog Signals
Discrete (Digital)
amplitude
time
amplitude
Continuous (Analog)
time
Slide 4-20
Speed of Transmission
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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
Slide 4-21
Mode of Transmission
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Synchronous (same-time) transmission: block-at-atime 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
Slide 4-22
Data Communications Media
(Media = Channels or Means of Transmission)
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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
Slide 4-23
Data Communications Media
(continued)
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Broadcast radio: wireless method used for cellular phone
and handheld computer transmissions
Microwave: both terrestrial and satellite systems use lineof-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)
Slide 4-24
Communications Carriers
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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
Slide 4-25
Data Communications Hardware
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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 linesharing capabilities, i.e., multiple computer transmissions
over shared communications channels at the same time
Slide 4-26
Network Topologies
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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
Slide 4-27
Network Topologies
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Star: one central computer is connected by pointto-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
Slide 4-28
Fully Interconnected Topology
Slide 4-29
Chain, Loop & Star Topologies
Loop
Chain
Star
Slide 4-30
Ring & Bus Topologies
Ring
Topology
Bus
Topology
Slide 4-31
Network Protocols
(Rules for Network Communications)
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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
Slide 4-32
Network Protocols (continued)
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X.25: protocol widely used today for connecting
computers to public and private networks with packetswitching 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)
Slide 4-33
Network Protocols (continued)
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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.
Slide 4-34
LANs and WANs
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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 LAN’s shorter
travel distance; LANs share more than data, e.g.,
peripherals, programs, processing power
Slide 4-35
Intranet and Extranet Communications
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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 businessto-business operations, in which one business has
access to data located within another business’
intranet; parts of two business’ intranets may be
linked
Slide 4-36
Distributed Systems
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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
Slide 4-37
Strategic Network Issues
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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
Slide 4-38
Strategic Network Issues, continued
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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
Slide 4-39
Communications Infrastructure
Communications infrastructure refers to the
underlying base of facilities, equipment, software,
and services needed to support applications that
are dependent on communications
Slide 4-40
Ubiquitous Networks
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Ubiquitous: very widespread
Data communications networks are found
worldwide (“ubiquitous”)
Slide 4-41
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