Network Layer and Circuit Switching

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Network Layer and Circuit Switching

• Networks provide for communication between
  devices without direct connections. Circuit
  Switching is the Oldest Networking Technology

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Why Networks?

• Until now we talked about situations where
  devices have been directly connected either by
  point to point links or multipont links. This is
  not possible for every pair of devices that wish
  to communicate in the world e.g. every phone in
  the world in connecting to every other phone in
  the world. This is obvious for two reasons
• The direct connection of every device with every
  other device would require N(N-1) full duplex
  links. This would be prohibitively expensive.
• Each communication link would require a port on
  the device. The cost of the ports would be a
  limiting factor and also their physical size.

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What is a Network?
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Kinds of Networks

• By Communication Technique
• Switched Networks
• Broadcast Networks e.g. LANs
• By Geographical Coverage
• Wide Area Network
• Cover large geographical areas, often crossing
  public right-of-ways
• Usually consist of several interconnected
  switching points
• Local Area Network
• Small scope, usually a building or cluster
• Typically owned by the same organization that
  owns the equipment

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Switched Network Types

• Circuit Switched Networks
• Packet Switched 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
• Oldest Networking Technology - more than a
  hundred years old
• 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

• In circuit switching network any two stations
  wishing to communicate first establish a
  connection by requesting to the network.
• The network responds by arranging a connection in
  such a way that a dedicated/physical path is
  established between the two parties
• For the duration of the connection the path is
  dedicated to the connection and cannot be used
  for other connections
• After the connection has been established the
  network becomes pretty much transparent to the
  two parties. Data can be exchanged transparently
  over the connection
• After the parties are done communicating the
  connection is released by making a request to the
  network
• The network responds by de-allocating the
  resources of the network that were dedicated to
  the connection

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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 Establishment

• Station requests connection from node
• Node determines best route, sends message to next
  link
• Each subsequent node continues the establishment
  of a path
• Once nodes have established connection, test
  message is sent to determine if receiver is
  ready/able to accept message

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Information Transfer

• Point-to-point transfer from source to node
• Internal switching and multiplexed transfer from
  node to node
• Point-to-point transfer from node to receiver
• Usually a full-duplex connection throughout

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Circuit Disconnect

• When transfer is complete, one station initiates
  termination
• Signals must be propagated to all nodes used in
  transit in order to free up resources

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Circuit Switching Application

• Circuit switching is well suited for analog voice
  communications as in the telephone network.
• Circuit switching turns out to be rather
  in-efficient for data networks due to its
  resource allocation nature.
• Circuit Switching is ill-suited to data
  communication because data traffic is BAD

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Examples of Circuit Switching

• Public Switched Telephone Network - PSTN
• Private Automatic Branch Exchange - PABX
• Integrated Services Digital Network - ISDN

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Public Switched Telephone Network (PSTN)

• PSTN is short for Public Switched Telephone
  Network, which refers to the international
  telephone system based on copper wires carrying
  analog voice data. This is in contrast to newer
  telephone networks base on digital technologies,
  such as ISDN and FDDI.
• Telephone service carried by the PSTN is often
  called plain old telephone service (POTS).

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POTS

• POTS is short for plain old telephone service,
  which refers to the standard telephone service
  that most homes use. In contrast, telephone
  services based on high-speed, digital
  communications lines, such as ISDN and FDDI, are
  not POTS.
• The main distinctions between POTS and non-POTS
  services are speed and bandwidth. POTS is
  generally restricted to about 33.6 kbps (33,600
  bits per second) though several modem
  manufacturers have developed technologies that
  would enable rates of 56.6 kbps.

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Public Switched Telephone Network (PSTN) Elements

• Subscribers
• Local loop
• Connects subscriber to local telco exchange
• Exchanges
• Telco switching centers
• Also known as end office
• gt19,000 in US

• Trunks
• Connections between exchanges
• Carry multiple voice circuits using FDM or
  synchronous TDM
• Managed by IXCs (inter-exchange carriers)

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Telephone Network Structure
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Telephone Network
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Typical Circuit Route for Medium Distance Calls-
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Circuit Switching Connection
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PSTN Connectivity
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Is PSTN Analog or Digital?
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About the Local Loop

• The local loop is still predominantly twisted
  pair copper wire
• Analog signaling is used on the local loop in
  PSTN
• Digital signaling is used on the local loop in
  ISDN
• Local loop is the bottleneck to high speed
  connectivity
• Several technologies have been developed to give
  high speed data communication on the local loop
• Examples - xDSL, FTTC, FTTH

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PABX

• PBX Private Branch Exchange
• A Customer Premise Communication Switch used to
  connect customer telephones (and related
  equipment) to LEC central office lines (trunks),
  and to switch internal calls within the
  customer's telephone system. Modern PBXs offer
  numerous software-controlled features such as
  call forwarding and call pickup. A PBX uses
  technology similar to that used by a central
  office switch (on a smaller scale). (The acronym
  PBX originally stood for "Plug Board Exchange".)

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ISDN

• Abbreviation of integrated services digital
  network, an international communications standard
  for sending voice, video, and data over digital
  telephone lines. ISDN requires special metal
  wires and supports data transfer rates of 64 Kbps
  (64,000 bits per second). Most ISDN lines offered
  by telephone companies give you two lines at
  once, called B channels. You can use one line for
  voice and the other for data, or you can use both
  lines for data to give you data rates of 128
  Kbps, four or five times the data rate provided
  by today's fastest modems.

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B-ISDN

• The original version of ISDN employs baseband
  transmission. Another version, called B-ISDN,
  uses broadband transmission and is able to
  support transmission rates of 1.5 million bits
  per second and higher. B-ISDN requires fiber
  optic cables and is not widely available.

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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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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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Blocking or Non-blocking

• Blocking
• A network is unable to connect stations because
  all paths are in use
• A blocking network allows this
• Used on voice systems
• Short duration calls
• Non-blocking
• Permits all stations to connect (in pairs) at
  once
• Used for some data connections

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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 Switch Design - Cross Bar Switch

• 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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Circuit-Switching NodeControl Unit

• Control Unit
• Establish connections
• Generally on demand
• Handle and acknowledge requests
• Determine if destination is free
• construct path
• Maintain connection -while needed
• Disconnect - Breaks down connection on completion

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Circuit Switch Design - TDM Bus Switch

• 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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Multistage Switch

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

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Issues in Circuit Switched Networks

• Routing
• Control Signalling

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Routing

• Routing in the network tries to determine the
  path from a given source to a given destination
• Many connections will need paths through more
  than one switch
• Need to find a route
• 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
• Uses a peer structure for nodes

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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 - almost like packet
  switching

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Static Hierarchical Routing

• Trace common node in hierarchy
• Resiliency provided by cross-cutting trunk lines
• Minimum switching costs, flexibility only via
  alternate trunks
• Not able to adapt to changing conditions
• Not able to adapt to node failures easily

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Alternate Routing

• Possible routes between two end offices are
  predefined
• Originating switch selects the best route for
  each call
• Routes listed in preference order
• Different sets of routes may be used at different
  times
• Routing paths can be fixed (1 route) or dynamic
  (multiple routes, selected based on current and
  historical traffic)

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Alternate Routing
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Adaptive Routing

• Traffic reporting and analysis with new paths
  computed periodically, adapts to net load, events
• Need to use algorithms to determine paths
  dynamically, based on load/congestion vectors

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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 Control Signalling
• In-channel signaling uses the same channel for
  control signals and calls
• Common Channel Control Signalling
• Common-channel signaling uses independent
  channels for control (SS7)

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Control Signaling Functions

• Audible communication with subscriber
• Transmission of dialed number
• Call can not be completed indication
• Call ended indication
• Signal to ring phone
• Billing info
• Equipment and trunk status info
• Diagnostic info
• Control of specialist equipment

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Location of Signaling

• Subscriber to network
• Depends on subscriber device and switch
• DSS1
• Within network
• Management of subscriber calls and network
• More complex
• SS7

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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 not busy, send ringer signal to target
  subscriber
• Feedback to caller
• Ringing tone, engaged tone, unobtainable
• Target accepts call by lifting receiver
• Switch terminates ringing signal and ringing tone
• Switch establishes connection
• Connection release when Source subscriber hangs
  up

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Switch to Switch Signaling

• Subscribers connected to different switches
• Originating switch seizes interswitch trunk
• Send off hook signal on trunk, requesting digit
  register at target switch (for address)
• Terminating switch sends off hook followed by on
  hook (wink) to show register ready
• Originating switch sends address

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Control Signals
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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
• 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
• Need 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
• 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 can carry signals for
  a number of subscriber channels
• Common control channel for these subscriber lines
• Associated Mode
• Common channel closely tracks interswitch 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
• Common channel signaling scheme
• Used in ISDN and inside PSTN
• Optimized for 64k 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
• Every element of the SS7 is replicated for
  resilience

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SS7 Benefits

• SS7 adds intelligence to a network
• Basis of new end user services e.g.
• 800 and 900 services
• Mobile Telephone Service
• Mobile subscriber authentication
• Caller identification
• Charging calls to a credit card
• Charging calls to a calling card
• SS7 standards include a standard client/server
  transaction protocol - Transaction Capabilities
  Application Part - TCAP

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SS7 Signaling Network Elements

• Signaling point (SP)
• Any point in the network capable of handling SS7
  control message
• 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