ICSA 411: Week 5 Wide Area Networks

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ICSA 411 Week 5Wide Area Networks

• Elizabeth Lane Lawley, Instructor

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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-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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Packet-Switching Networks

• Includes X.25, ISDN, ATM and frame-relay
  technologies
• Data is broken into packets, each of which can be
  routed separately
• Advantages better line efficiency, signals can
  always be routed, prioritization option
• Disadvantages transmission delay in nodes,
  variable delays can cause jitter, extra overhead
  for packet addresses

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Packet-Switching Techniques

• Datagram
• each packet treated independently and referred to
  as a datagram
• packets may take different routes, arrive out of
  sequence
• Virtual Circuit
• preplanned route established for all packets
• similar to circuit switching, but the circuit is
  not dedicated

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Packet-Switched Routing

• Adaptive routing changes based on network
  conditions
• Factors influencing routing are failure and
  congestion
• Nodes must exchange information on network status
• Tradeoff between quality and amount of overhead

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Packet-Switched Congestion Control

• When line utilization is gt80, queue length grows
  too quickly
• Congestion control limits queue length to avoid
  througput problems
• Status information exchanged among nodes
• Control signals regulate data flow using
  interface protocols (usually X.25)

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X.25 Interface Standard

• ITU-T standard for interface between host and
  packet-switched network
• Physical level handles physical connection
  between host and link to the node
• Technically X.21, but other standards can be
  substituted, including RS-232
• Link level provides for reliable data transfer
• Uses LAPB, which is a subset of HDLC
• Packet level provides virtual circuits between
  subscribers

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Virtual-Circuit Service

• External virtual circuit logical connection
  between two stations on the network
• Internal virtual circuit specific preplanned
  route through the network
• X.25 usually has a 11 relationship between
  external and internal circuits
• In some cases, X.25 can be implemented as a
  packet-switched network

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ISDN

• First generation is narrowband ISDN
• uses 64kbps channels
• circuit-switched
• Second generation is broadband ISDN (B-ISDN
• uses higher data rates (hundreds of mbps)
• packet-switched network
• development effort led to ATM/cell relay

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ISDN Principles

• Support of voice and nonvoice using limited set
  of standard facilities
• Support for switched and nonswitched applications
• Reliance on 64kbps connections
• Intelligence in the networks
• Layered protocol architecture (can be mapped onto
  OSI model)
• Variety of configurations

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ISDN User Interface

• Pipe to users premises has fixed capacity
• Standard physical interface can be used for
  voice, data, etc
• Use of the pipe can be a variable mix of voice
  and data, up to the capacity
• User can be charged based on use rather than time

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ISDN Network Architecture

• Physical path from user to office
• subscriber loop, aka local loop
• full-duplex
• primarily twisted pair, but fibers coming
• Central office connecting subscriber loops
• B channels 64kbps
• D channels 16 or 64kbps
• H channels 384, 1536, or 1920 kbps

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ISDN B Channel

• Basic user channel (aka bearer channel)
• Can carry digital voice, data, or mixture
• Mixed data must have same destination
• Four kinds of connections possible
• Circuit-switched
• Packet-switched
• Frame mode
• Semipermanent

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ISDN D Channel

• Carries signaling information using
  common-channel signaling
• call management
• billing data
• Allows B channels to be used more efficiently
• Can be used for packet switching

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ISDN H Channel

• High speed rates
• Only available over primary interface
• Used in ATM

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ISDN Basic Access

• Basic Rate Interface (BRI)
• Two full-duplex 64kbps B channels
• One full-duplex 16kbps D channel
• Framing, synchronization, and overhead bring
  total data rate to 192kbps
• Can be supported by existing twisted pair local
  loops
• 2BD most common, but 1BD available

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ISDN Primary Access

• Primary Rate Interface (PRI)
• Used when greater capacity required
• No international agreement on rates
• US, Canada, Japan 1.544mbps ( to T1)
• Europe 2.048mbps
• Typically 23 64kbps B 1 64kbps D
• Fractional use of nBD possible
• Can be used to support H channels

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WANs for Voice

• Requires very small and nonvariable delays for
  natural conversation--difficult to provide this
  with packet-switching
• As a result, the preferred method for voice
  transmission is circuit-switching
• Most businesses use public telephone networks,
  but a few organizations have implemented private
  voice networks

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WANs for Data

• Public packet-switched networks (X.25)
• Private packet-switched networks
• Leased lines between sites (non-switched)
• Public circuit-switched networks
• Private circuit-switched networks (interconnected
  digital PBXs)
• ISDN (integrated X.25 and traditional
  circuit-switching)

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WAN Considerations

• Nature of traffic
• stream generally works best with dedicated
  circuits
• bursty better suited to packet-switching
• Strategic and growth control--limited with public
  networks
• Reliability--greater with packet-switching
• Security--greater with private networks

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High-Speed WAN Alternatives

• Faster multiplexed lines
• T-3
• SONET
• Switched networks
• Frame relay
• SMDS
• ATM

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Common Nonswitched Offerings

• Analog lines (dedicated modems)
• Digital data lines (dedicated DSUs)
• T-1, T-3 leased lines
• Frame relay over dedicated lines
• SONET

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Common Switched Offerings

• Dial-up modems
• X.25 packet switching
• ISDN
• Frame relay
• SMDS
• ATM

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Changes in WAN Structure

• Until recently, most devices were fed through a
  syncrhonous TDM multiplexer (MUX), sent to a
  carrier that split it up into dedicated circuits
• Increasingly, organizations now use a switched
  alternative, allowing better utilization of
  bandwidth, and removing need for a MUX

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Integrated Network withDedicated Channels
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Integrated Network withPublic Switched WAN
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High-Speed WAN Options

• Frame Relay
• ATM (Asynchronous Transfer Mode)
• SMDS (Switched Multimegabit Data Service)
• Broadband ISDN

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Frame Relay

• Designed to eliminate excessive X.25 overhead
• Control signaling takes place on a separate
  logical connection (nodes dont need state tables
  for each call)
• Multiplexing/switching take place at layer 2,
  eliminating a layer of processing
• No hop-by-hop flow/error control

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Packet Switching Diagram
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Frame Relay Diagram