Data and Computer Communications Part 3 - Wide area Networks - Concepts

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Data and Computer CommunicationsPart 3 - Wide
area Networks - Concepts

• Chapter 9
• Circuit Switching

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Switching Networks

• Long distance transmission is typically done over
  a network of switched nodes
• Nodes are not concerned with content of data
• End devices are referred to as stations
• Computer, terminal, phone, etc.
• A collection of nodes and connections is a
  communications network
• Data is routed by being switched from node to node

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Switching Networks

• Communication Network
• Switching Network (used in WAN)
• Broadcast Network (used in LAN)
• Switching Networks
• Circuit-switching networks
• Packet-switching networks

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Nodes

• Nodes may connect to other nodes only, or to
  stations and other nodes
• Node to node links are usually multiplexed
• A network is usually partially connected
• But some redundant connections are desirable for
  reliability
• Two different switching technologies exist
• Circuit switching
• Packet switching

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Simple Switched Network
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Simple switching network
Communication Network Node

• provides switching facility (routing)

Network Station

• end node (source destination)

Communication is achieved by transmitting data
from source to destination through a network of
switching nodes
Communication Network
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Switching technology

• Circuit switching
• need a connection established between end nodes
• connection is maintained until one of end nodes
  terminates
• Connection is dedicated to the communication
  between two nodes
• Example Public Switch Telephone Network (PSTN)
• Packet switching
• data are transmitted in short messages called
  packets
• a connection between the two end-nodes is not
  maintained
• a node-to-node link can be dynamically shared by
  many packets
• Example Public Data Network (PDN) like X.25,
  Frame Relay

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Key differences
Keys Circuit switching Packet
switching data dedicated single
route different routes node status both
must be ready sender ready to send is
enough connection dedicated
shared utilization poor
good data rate
fixed
varies Prioritization not supported
supported
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Public Switching Telephone Network
end office
subscriber loop
Long-distance office
subscriber loop (local loop) link between
subscriber and network
Intercity trunk
connecting trunk (exchanges) switching
center localized support for subscriber
Intercity trunk (trunks) branches between
exchanges carry multiple voice circuits
connecting trunk
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PSDN

• PSDN is designed specifically for the
  transmission of data rather than voice
• Communication is shared

LAN
PSDN Public Switched Data Network
LAN
PSDN
LAN
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Packets
data
header data
header data
header data
packet
packet
packet

• Messages are broken up into a series of packets
• Header is used to route the packet through the
  network

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PSDN switching approach

• Virtual Circuit
• network protocols establish a logical route
  called virtual circuit
• packets use the same route data arrive in order
• similar to circuit switching ! but the route is
  not dedicated
• need call setup
• Datagram
• each packet is transmitted independently
• network protocols route each packet as though it
  were a separate message
• packet may not arrive in order need protocol to
  ensure ordering
• call setup is not required

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Virtual circuit
B

• A logical connection known as
• virtual circuit (VC) is set up
• between two stations.
• Packets are labeled with
• a virtual circuit number and
• a sequence number

A
C
virtual circuit 1
B
A
C
virtual circuit 2
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Datagram
B

• Each packet is transmitted
• independently.
• Packets are labeled with
• a destination address and
• may arrive out of sequence

A
C
B
A
C
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Routing methods in packet switching

• Fixed routing
• each packet is transmitted independently
• Flooding
• network protocol establishes a logical route
  called virtual circuit
• packets use the same route data arrive in order
• Random routing
• the route taken is random
• Adaptive routing
• the route reacts to changing conditions within
  the network

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Circuit-Switching Networks

• During communication, a dedicated communication
  path exists between sender and receiver e.g.,
  telephone system.
• Communication involves 3 phases
• Circuit establishment Call request propagates,
  hop by hop through the network, to establish a
  dedicated link (channels in each component link
  in some path, from sender to receiver)
• Data transfer continuous transfer either analog
  or digital signal
• Circuit disconnect circuit disconnect signal
  to deallocate the links

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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

• Dedicated communication path between two stations
• Three phases
• Establish
• Transfer
• Disconnect
• Must have switching capacity and channel capacity
  to establish connection
• Must have intelligence to work out routing

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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 Networks

• Disadvantages
• Both stations need to be simultaneously ready
• Inefficient use of link capacities, especially
  with bursty intermittent traffic
• Advantages
• Low delay once circuit is established

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Circuit-Switching Networks

• Public telephone network
• Best-known example of a circuit-switching network
• Four generic architectural components
• Subscribers - Devices that attach to the network
• Local loop - The link between the subscriber and
  the network (end office). Also called subscriber
  loop
• Exchanges - Switching centers in the network. End
  offices, long-distance offices,
• Trunks - Links between exchanges. Carry multiple
  voice channels by using FDM or synchronous TDM

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Circuit Switching - Applications

• Inefficient
• Channel capacity dedicated for duration of
  connection
• If there is no data, capacity wasted
• Set up (connection) takes time
• Once connected, transfer is transparent
• Developed for voice traffic (phone)

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Public Circuit Switched Network
Connecting Trunk
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Circuit-Switching Networks
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ATT telephone hierarchy
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Switching Concepts

• Single circuit-switched node network
• A collection of stations attached to a central
  switching unit
• The central switch establishes a dedicated path
  between any two devices that wish to communicate

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Switching Concepts

• Digital switch
• Establishes a dedicated path between any two
  devices
• Control unit
• Establishes, maintains, and tears down the
  connection
• Network interface
• (circuit) switch
• Blocking
• Nonblocking

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Telecommunication Components

• Subscriber
• Devices attached to network
• Local Loop
• Subscriber loop
• Connection to network
• Exchange
• Switching centers
• End office - supports subscribers
• Trunks
• Branches between exchanges
• Multiplexed

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Circuit Switching Node's Elements
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Circuit Switching Node's Elements

• Digital Switch
• Provides transparent signal path between devices
• Network Interface (functions hardware needed to
  connect devices to the network)
• Control Unit its function is to
• Establish connections
• Generally on demand
• Handle and acknowledge requests
• Determine if destination is free
• construct path
• Maintain connection
• Disconnect

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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 or Non-blocking

• An important characteristic of a
  circuit-switching device is whether it is
  blocking or non-blocking.
• Blocking
• A network is unable to connect stations because
  all paths are in use
• Used on voice systems
• example Short duration calls
• Non-blocking
• Permits all stations to connect (in pairs) at
  once
• Used for some data connections

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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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There are 2 Types of Switching Techniques
Internal to a Single Circuit-Switching Node 1.
Space-Division Switching 2. Time-Division
Switching
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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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Space Division Switch

• Developed for the analog environment (but is now
  used for both analog digital signals)
• Separate physical paths for every channel (signal
  path)
• Its basic device is the Crossbar switch
• Number of crosspoints grows as square of number
  of stations
• Loss of crosspoint prevents connection
• Inefficient use of crosspoints
• All stations connected, only a few crosspoints in
  use
• Non-blocking

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Crossbar Matrix (switch)
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Multistage Switch

• Reduced number of crosspoints
• More than one path through network
• Results in increased reliability
• More complex control
• May be blocking

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Three Stage Switch
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Time Division Switching

• Partition low speed bit stream into pieces that
  share higher speed stream
• e.g. TDM bus switching
• based on synchronous time division multiplexing
• Each station connects through controlled gates to
  high speed bus
• Time slot allows small amount of data onto bus
• Another lines gate is enabled for output at the
  same time

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Routing in Circuit-Switched Networks

• 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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Routing in Circuit-Switching Networks

• Many connections will need paths through more
  than one switch
• Therefore, the network needs to find an
  appropriate route. Important factors are
• Efficiency
• Resilience
• Public telephone switches are a tree structure
• Static routing uses the same approach all the
  time
• Dynamic routing allows for changes in routing
  depending on traffic

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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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Alternate Routing Diagram
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Control Signaling

• Control unit manages 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 (e.g. SS7)

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Control Signaling Functions

• The means by which the network is managed, calls
  are established, maintained, and terminated
• Audible communication with subscriber
• Transmission of dialed number
• Call cannot be completed indication
• Call ended indication
• Signal to ring phone
• Billing info
• Equipment and trunk status info
• Diagnostic info
• Control of special equipment

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Control Signal Sequence

• Both phones on hook
• Subscriber lifts receiver (off hook)
• End office switch signaled
• Switch responds with dial tone
• Caller dials number
• If target is not busy, ringer signal is sent to
  target subscriber
• Feedback to caller
• Ringing tone, engaged tone, unobtainable
  (disconnected line, etc.)
• Target accepts call by lifting receiver
• Switch terminates ringing signal and ringing tone
• Switch establishes connection
• Connection is released when Source subscriber
  hangs up

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Switch to Switch Signaling

• Subscribers connected to different switches
• Originating switch seizes inter-switch trunk
• Off hook signal is sent on trunk, requesting a
  digit register at target switch (so that address
  may be communicated)
• Terminating switch sends off hook followed by on
  hook (known as wink) to show register-ready
  status
• Originating switch sends address

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Control Signals through a Circuit-Switching
Telephone Network
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Location of Signaling

• Subscriber to network
• Depends on subscriber device and switch
• Within network
• is concerned with management of subscriber calls
  and network
• More complex
• Two types of control signaling are used in
  circuit switching networks
• In-channel signaling
• Common channel signaling

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In-Channel Signaling

• Use same channel for signaling and call
• Requires no additional transmission facilities
• Inband
• Uses same frequencies as voice signal
• Can go anywhere a voice signal can
• Impossible to set up a call on a faulty speech
  path (because the control signals used to set up
  the path have to follow same path)
• Out of band
• Voice signals do not use full 4kHz bandwidth
• Narrow signal band within 4kHz used for control
• Can be sent whether or not voice signals are
  present
• Requires extra electronics
• Slower signal rate (narrow bandwidth)

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Drawbacks of In Channel Signaling

• Limited transfer rate
• Delay between entering address (dialing) and
  connection
• This is overcome by use of common channel
  signaling

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Common Channel Signaling

• Control signals carried over paths independent of
  voice channel
• One control signal channel (path) can carry
  signals for a number of subscriber channels and
  is therefore the common control channel for these
  subscriber lines (channels)
• Two modes of operation are used in Common channel
  signaling
• Associated Mode
• Common channel closely tracks inter-switch trunks
  
• Disassociated Mode
• Additional nodes (signal transfer points)
• Effectively two separate networks

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Common v. In Channel Signaling
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Common Channel Signaling Modes
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Signaling System Number 7

• SS7
• It is the most widely used common channel
  signaling scheme
• Used in ISDN
• Optimized for 64kbps digital channel network
• Call control, remote control, management and
  maintenance
• Reliable means of transfer of info in sequence
• Will operate over analog and below 64k
• Point to point terrestrial and satellite links
• Although the network being controlled is a
  circuit switched network, the control signaling
  is implemented using packet switching technology

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SS7 Signaling Network Elements

• Signaling point (SP)
• Any point in the network capable of handling SS7
  control messages
• Signal transfer point (STP)
• A signaling point capable of routing control
  messages
• Control plane
• Responsible for establishing and managing
  connections
• Information plane
• Once a connection is set up, info is transferred
  in the information plane

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Transfer Points
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Signaling Network Structures

• STP capacities
• Number of signaling links that can be handled
• Message transfer time
• Throughput capacity
• Network performance
• Number of SPs
• Signaling delays
• Availability and reliability
• Ability of network to provide services in the
  face of STP failures