Network Characteristics: Ownership Service Paradigms Performance

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Network CharacteristicsOwnershipService
ParadigmsPerformance
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Network Ownership

• Private network owned and used by single entity
• Most LANs
• Many corporations have their own WANs
• Use leased lines, transmissions do not occur over
  common carriers

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Network Ownership Continued

• Public network owned by common carrier and used
  by many
• Universal service any subscriber can communicate
  to any other
• Public in the sense that transmissions from all
  go through same hardware
• Transmissions not broadcast

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Private Pros/Cons

• Complete hardware control
• Capacity, etc.
• Policy control
• Isolation/security
• Expense
• Equipment, personnel
• Have to track technology

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Public Pros/Cons

• Much less expensive
• Rely on common carrier to keep up to date
• Data not private
• Less secure
• Less direct control!

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VPNs

• Virtual Private Network (VPN) provides service
  where a public network appears to be private
• Entity has multiple sites
• Use special hardware/software to encrypt all
  transmissions between sites

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VPNs Continued

• Can disallow all other traffic or encrypt only
  traffic to certain sites
• Now if anyone sees transmissions they cant
  decode them

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Transmission Paradigms

• Two major forms connection-oriented (CO) and
  connectionless (CL)
• CO like a phone call
• Connection must be initialized
• Connection used in accordance to agreed upon
  parameters for transmission
• Connection released

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Connection Oriented

• Can be engineered for fixed or variable data
  rates
• Connection is not dropped if no data transferred
• Connection can either be simplex (two connections
  required) or full duplex
• Can be either permanent or switched

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CO Continued

• Can guarantee a performance level and have
  other QoS aspects easier measurement and data
  capture
• Might have either stream or message interface
• Stream no boundaries on transmissions
• Message destination receives exactly same size
  blocks as sent

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Connection Less

• Like the post office
• Each message addressed
• No setup or tear down
• Typically no service guarantee
• Good for short transmissions
• CO typically has abbreviated destination address

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Comments

• CO has immediate feedback for network failure
• Note CO vs. CL can occur at many different
  levels of network hierarchy

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Examples
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Performance Characteristics

• A number of ways to measure effectiveness of
  network
• Might want different metrics for different levels
  or types of connections

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Delay

• How long does one bit take to transit network?
• Measured in fractions of a second
• May be expressed in maximum and average delay

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Types of Delay

• Propagation proportional to distance traveled
• Speed of light
• Switching electronics in hubs, bridges, etc.
  time to do the basic message switching job
• Access time it takes to gain medium
• CSMA/CD
• Queuing time spent in router queue time cost
  due to other messages

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Throughput A Capacity Measure

• Rate at which data transferred
• Really how many bits can enter network per unit
  of time how much can the network hold?
• Measured in bits per second
• Not exactly equal to bandwidth
• Bandwidth theoretical hardware limit
• Throughput will be less due to headers and other
  losses of bandwidth
• Not speed as it measures rate not delay

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The Need for Speed

• Think of I-25 from Denver to Fort Collins as the
  network with one big lane
• Takes one car 60 minutes to reach Fort Collins -
  This is speed or delay
• Two cars can enter I-25 a minute,
• Capacity, bandwidth 2 cars per minute
• One lane 120 cph
• If we add bandwidth (another lane) we could get
  up to 4 cars per minute

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Congestion

• Utilization (U) of given throughput
• Proportion between 0 and 1 of total capacity in
  use
• 0 means none used (idle). 1 means fully used.
• Too many packets on link at a time
• Increases delay
• Delay can be estimated from percentage of
  congestion
• DD0/(1-U)
• D0idle delay
• Upercentage utilized

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Utilization For 250 ms Delay (D0)
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Congestion Continued

• One chooses acceptable utilization before
  upgrading
• 50 -- 80 typical

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Delay-Throughput Product

• Another figure of merit is delay times throughput
• Might be a better measurement than either alone
• Measure of how much data on link at a time
• I-25 model (as above)
• Product
• (2 cars per minute) ? (60 minutes) 120 cars
• What adding bandwidth will do

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Jitter

• Variance in delay
• Regularity of delivery
• If always at same time ? zero jitter
• Low jitter important for audio and video
• Isochronous networks have a jitter of zero
• Asynchronous networks more and more carrying
  voice and video and using algorithms to mitigate
  jitter

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Exercises

• Problems 15.3, 15.4, 15.6, 15.7