Transcript Class 13 - Oct. 16 - Circuit Switching

CSCI 465
Data Communications and Networks
Lecture 13
Martin van Bommel
CSCI 465
Data Communications & Networks
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Switching Techniques
• Transmission beyond local area requires
intermediate switching nodes
• Nodes concerned not with content but with
movement of data
• Two technologies of switching differ in way
data is switched from one link to another
– circuit switching versus packet switching
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Data Communications & Networks
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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 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
– end-to-end circuit established before any signals
can be transmitted
• Data transfer
– point-to-point from endpoints to nodes
– internal switching/multiplexing among nodes
• Circuit disconnect
– deallocate resources dedicated to circuit
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PSTN
• Public Switched Telephone Network
• Four generic components
– Subscribers - devices that attach to network
– Local loop - link from subscriber to network
– Exchanges - switching centers
(end office - connected to subscribers)
– Trunks - branches between exchanges
(FDM or synchronous TDM)
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Circuit-Switched Node
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Circuit-Switching Node:
Digital Switch
• Provides transparent signal path between any
pair of attached devices
• Appears to devices as direct connection
• Typically full-duplex
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Circuit-Switching Node:
Network Interface
• Provides hardware and functions to connect
digital devices to switch
• Analog devices can be connected if interface
includes CODEC functions
• Typically full-duplex
• Trunks to other switches carry TDM signals to
provide links for multiple node networks
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Data Communications & Networks
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Circuit-Switching Node:
Control Unit
• Establishes on-demand connections
– handle and acknowledge request
– determine if destination is free
– construct path through switch
• Maintains connection while needed
– maintain time-division for connection
• Breaks down connection on completion
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Data Communications & Networks
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Blocking/Non-blocking Networks
• Blocking
– network is unable to connect two stations
because all possible paths are already in use
(acceptable for voice only)
• Non-blocking:
– permits all possible connection requests because
any two stations can be connected (and may stay
that way for a long time)
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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
common transmission
path
– Bus must have higher
data rate than the
individual I/O lines
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Circuit-Switched Networks:
Routing
• 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-topeer, 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 (one 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 – Signaling System Number 7
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Traditional Circuit Switching
Softswitch Architecture
• latest trend in circuit-switching technology
• computer running specialized software that
turns it into a smart phone switch
• costs less and provides more functionality
• Media gateway (MG) – physical switching
• Media gateway controller (MGC) – call
processing logic
CSCI 465
Data Communications & Networks
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Softswitch