Transcript William Stallings Data and Computer Communications

Data and Computer
Communications
Part 3 - Wide area Networks Concepts
Chapter 9
Circuit Switching
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
node status
connection
utilization
data rate
Prioritization
dedicated single route
different routes
both must be ready
sender ready to send is enough
dedicated
shared
poor
good
fixed
varies
not supported
supported
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Public Switching Telephone
Network
end office
subscriber loop
Long-distance office
subscriber loop (local loop)
Intercity trunk
link between subscriber and network
connecting trunk (exchanges)
switching center; localized support for
subscriber
Intercity trunk (trunks)
connecting trunk
branches between exchanges; carry
multiple voice circuits
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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
packet
data
header
data
header
packet

Messages are broken up into a series of packets

Header is used to route the packet through the network
data
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
B
1.3
1.2
1.1
2.3
2.2
2.1
 A logical connection known as
virtual circuit (VC) is set up
between two stations.
A
C
 Packets are labeled with
a virtual circuit number and
a sequence number
virtual circuit #1
B
A
2
4
3
5
1
C
virtual circuit #2
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Datagram
B
B.3
B.2 B.1
C.3
C.2 C.1
 Each packet is transmitted
independently.
A
C
B.3
B
B.2
4
2
A
 Packets are labeled with
a destination address and
may arrive out of sequence
C.1
1
B.1
3
C.3
C.2
5
C
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Routing methods in packet
switching

Fixed routing


Flooding



network protocol establishes a logical route called virtual circuit
packets use the same route; data arrive in order
Random routing


each packet is transmitted independently
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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AT&T 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 Node:
Digital Switch
Provides transparent signal path between any
pair of attached devices
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
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Circuit-Switching Node:
Control 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 line’s 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 49
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
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using packet switching technology
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
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