Data and Computer CommunicationsPart 3 - Wide area Networks - Concepts
Chapter 9Circuit Switching
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Switching NetworksLong distance transmission is typically
done over a network of switched nodesNodes are not concerned with content of
dataEnd 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 NetworksCommunication Network
Switching Network (used in WAN) Broadcast Network (used in LAN)
Switching Networks Circuit-switching networks Packet-switching networks
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NodesNodes may connect to other nodes only,
or to stations and other nodesNode to node links are usually multiplexedA 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
Communication Network Node
Network Station
end node (source & destination)
provides switching facility (routing)
Communication is achieved by transmittingdata from source to destination through anetwork of switching nodes
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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
Long-distance office
connecting trunk
subscriber loop
Intercity trunk subscriber loop (local loop) link between subscriber and network
connecting trunk (exchanges) switching center; localized support for subscriber
Intercity trunk (trunks) branches between exchanges; carry multiple voice circuits
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PSDN
PSDN= Public Switched Data Network
LAN
LAN
LAN
PSDN
• PSDN is designed specifically for the transmission of data rather than voice• Communication is shared
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Packets
header data
packet
Messages are broken up into a series of packets
Header is used to route the packet through the network
header data header data
data
packet packet
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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
1.3 1.2 1.1
2.3 2.2 2.1
2.32.2 2.1
1.31.2
1.1
A
B
C
A 1
2
3
4
5
B
C
virtual circuit #1
virtual circuit #2
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
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Datagram
B.3 B.2 B.1
C.3 C.2 C.1
C.3 C.2 C.1
B.3B.2
B.1
A
B
C
A 1
2
3
4
5
B
C
B.3 B.2
B.1
C.3 C.2
C.1
Each packet is transmitted independently. Packets are labeled with a destination address and may arrive out of sequence
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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 SwitchingDedicated communication path between
two stationsThree 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 StagesCircuit establishmentTransfer of information
point-to-point from endpoints to node internal switching/multiplexing among nodes
Circuit disconnect
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Circuit-Switching NetworksDisadvantages:
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 networkLocal 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 - ApplicationsInefficient
Channel capacity dedicated for duration of connection
If there is no data, capacity wasted
Set up (connection) takes timeOnce connected, transfer is transparentDeveloped 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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AT&T telephone hierarchy
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Switching ConceptsSingle 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 ComponentsSubscriber
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 demandHandle and acknowledge requestsDetermine if destination is freeconstruct path
Maintain connection Disconnect
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Circuit Switching Node:Digital SwitchProvides transparent signal path between
any pair of attached devicesTypically full-duplex
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Circuit-Switching Node:Network InterfaceProvides hardware and functions to
connect digital devices to switchAnalog devices can be connected if
interface includes CODEC functionsTypically full-duplex
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Circuit-Switching Node:Control UnitEstablishes on-demand connectionsMaintains connection while neededBreaks 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 NetworksBlocking: 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 SwitchDeveloped 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 SwitchReduced number of crosspointsMore than one path through network
Results in increased reliability
More complex controlMay be blocking
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Three Stage Switch
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Time Division SwitchingPartition low speed bit stream into pieces
that share higher speed streame.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 line’s gate is enabled for output at the
same time
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Routing in Circuit-Switched NetworksTraditional 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 NetworksMany connections will need paths through
more than one switchTherefore, 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 RoutingPossible routes between two end offices
are predefinedOriginating switch selects the best route
for each callRouting 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 SignalingControl 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 SignalingSubscribers connected to different switchesOriginating switch seizes inter-switch trunkOff 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 SignalingSubscriber 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 SignalingUse 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 SignalingLimited transfer rateDelay between entering address (dialing)
and connectionThis 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 ElementsSignaling 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 StructuresSTP 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