ICSA 411: Week 5a Wide Area Networks

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ICSA 411 Week 5aWide Area Networks

• Elizabeth Lane Lawley, Instructor

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WANs vs LANs

• Wide Area Network
• Cover large geographical areas, often crossing
  public right-of-ways
• Usually consist of several interconnected
  switching points
• Local Area Network
• Small scope, usually a building or cluster
• Typically owned by the same organization that
  owns the equipment

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Types of WANs

• Circuit-switched (todays lecture)
• Packet-switched (Thursdays lecture)

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Circuit-Switching

• Definition Communication in which a dedicated
  communications path is established between two
  devices through one or more intermediate
  switching nodes
• Dominant in both voice and data communications
  today
• e.g. PSTN is a circuit-switched network
• Relatively inefficient (100 dedication even
  without 100 utilization)

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Circuit-Switching Stages

• Circuit establishment
• Transfer of information
• point-to-point from endpoints to node
• internal switching/multiplexing among nodes
• Circuit disconnect

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Circuit Establishment

• Station requests connection from node
• Node determines best route, sends message to next
  link
• Each subsequent node continues the establishment
  of a path
• Once nodes have established connection, test
  message is sent to determine if receiver is
  ready/able to accept message

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Information Transfer

• Point-to-point transfer from source to node
• Internal switching and multiplexed transfer from
  node to node
• Point-to-point transfer from node to receiver
• Usually a full-duplex connection throughout

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Circuit Disconnect

• When transfer is complete, one station initiates
  termination
• Signals must be propagated to all nodes used in
  transit in order to free up resources

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Public Switched Telephone Network (PSTN)

• Subscribers
• Local loop
• Connects subscriber to local telco exchange
• Exchanges
• Telco switching centers
• Also known as end office
• gt19,000 in US

• Trunks
• Connections between exchanges
• Carry multiple voice circuits using FDM or
  synchronous TDM
• Managed by IXCs (inter-exchange carriers)

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Digital Circuit-Switching Node
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Circuit Switching NodeDigital Switch

• Provides transparent signal path between any pair
  of attached devices
• Typically full-duplex

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Circuit-Switching NodeNetwork Interface

• Provides hardware and functions to connect
  digital devices to switch
• Analog devices can be connected if interface
  includes CODEC functions
• Typically full-duplex

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Circuit-Switching NodeControl Unit

• Establishes on-demand connections
• Maintains connection while needed
• Breaks down connection on completion

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Blocking/Nonblocking Networks

• Blocking network is unable to connect two
  stations because all possible paths are already
  in use
• Nonblocking permits all possible connection
  requests because any two stations can be connected

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Switching Techniques

• Space-Division Switching
• Developed for analog environment, but has been
  carried over into digital communication
• Requires separate physical paths for each signal
  connection
• Uses metallic or semiconductor gates

• Time-Division Switching
• Used in digital transmission
• Utilizes multiplexing to place all signals onto a
  common transmission path
• Bus must have higher data rate than individual
  I/O lines

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Routing in Circuit-Switched Networks

• Requires balancing efficiency and resiliency
• Traditional circuit-switched model is
  hierarchical, sometimes supplemented with
  peer-to-peer trunks
• Newer circuit-switched networks are dynamically
  routed all nodes are peer-to-peer, making
  routing more complex

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Alternate Routing

• Possible routes between two end offices are
  predefined
• Originating switch selects the best route for
  each call
• Routing paths can be fixed (1 route) or dynamic
  (multiple routes, selected based on current and
  historical traffic)

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Control Signaling

• Manage the establishment, maintenance, and
  termination of signal paths
• Includes signaling from subscriber to network,
  and signals within network
• In-channel signaling uses the same channel for
  control signals and calls
• Common-channel signaling uses independent
  channels for controls (SS7)

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Advantages of Leased (or Dedicated) Lines

• permanently or semi-permanently connect between
  two points
• economical in high volume calls between two
  points
• no delay associated with switching times
• can assure consistently high-quality connections

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Uses of Leased (or Dedicated) Lines

• Voice grade channels
• normal telephone lines
• in the range of 300 Hertz to 3300 Hertz
• Data grade channels
• use conditioning or equalizing
• reduces the amount of noise on the line,
  providing lower error rates and increased speed
  for data communications

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Types of Leased Lines

• 56K/DS0
• T1/DS1
• Fractional T1
• T3/DS3