William Stallings Data and Computer Communications 7th Edition

1
William StallingsData and Computer
Communications7th Edition

• Chapter 10
• Circuit Switching and Packet Switching

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Switching Networks

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

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Nodes

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

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Simple Switched Network
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Switching Activities

• Some nodes connect only to other nodes
  (intermediary nodes). Sole purpose is to switch
  data
• Some nodes have one or more stations attached.
  They accept from and deliver data to the attached
  station.
• Node-to-node links are usually multiplexed
• Multiple paths enhance reliability

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Circuit Switching

• Originated in public telephone networks
• Well suited to analog transmission of voice
  signal
• 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 - Applications

• Inefficient
• Channel capacity dedicated for duration of
  connection
• If 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
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Telecom Components

• Subscriber
• Devices attached to network
• Subscriber line
• Link between subscriber and network
• Also called Local Loop or Subscriber Loop
• Almost all Local Loops are TPW
• Range from Few km up to tens of km
• Exchange
• Switching center in the network
• End office specific switching center that
  supports subscribers
• Trunks
• Branches between exchanges
• Multiplexed

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Circuit Establishment
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Circuit Switching Concepts

• Digital Switch
• Provide transparent signal path between devices
• Typically allows full duplex transmission
• Network Interface
• Functions and h/w needed to connect digital
  devices to the network
• Control Unit
• Establish connections - Generally on demand,
  Handle and acknowledge requests, Determine if
  destination is free,construct path
• Maintain connection
• Disconnect

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Blocking or Non-blocking Circuit Switching

• Blocking
• A network may not be able to connect stations
  because all paths are in use (more stations than
  path)
• Used on voice systems
• Short duration calls
• Non-blocking
• Permits all stations to connect (in pairs) at
  once (at least as many paths as stations)
• Used for some data connections

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Space Division Switching

• Developed for analog environment, but carried
  over into digital
• Signal paths are physically separate (slide 15)
• Each connection requires dedicated path (crossbar
  switch)

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Crossbar switch

• Number of crosspoints grows as square of number
  of stations
• Loss of crosspoint prevents connection
• Inefficient use of crosspoints
• If all stations connected, only a few crosspoints
  in use
• Non-blocking

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Space Division Switch
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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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Three Stage Space Division Switch
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Time Division Switching

• Modern digital systems rely on intelligent
  control of space and time division elements
• Use digital time division techniques to set up
  and maintain virtual circuits
• Partition low speed bit stream into pieces that
  share higher speed stream

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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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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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Location of Signaling

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

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In Channel Signaling

• Use same channel for signaling control and call
• Requires no additional transmission facilities
• Inband
• Control signals have same electromagnetic
  properties (frequency) 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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CommonChannelSignaling Modes
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Signaling System Number 7

• SS7
• Common channel signaling scheme
• ISDN
• Overall purpose to provide international
  standardized common channel signaling system
• Performs call management (setup, maintenance,
  termination) and network management functions
• Network is circuit switched, but control is
  packet switched

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Softswitch Architecture

• Latest trend in circuit-switching technology
• General purpose computer running software to make
  it a smart phone switch
• Lower cost, greater functionality
• Can packetize digitized voice data, allowing
  voice over IP
• Performs call routing
• Separates call processing from hardware function
  of switch

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Traditional Circuit Switching
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Softswitch
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Circuit Switching Shortcomings

• Inefficient for data because of idle time
• Provides for transmission at constant rate must
  transmit and receive at same data rate. Limits
  versatility

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Packet Switching Basic Operation

• Data transmitted in small packets
• Typically 1000 octets (8 bit byte)
• Longer messages split into series of packets
• Each packet contains a portion of user data plus
  some control info
• Control info
• Routing (addressing) info
• Packets are received, stored briefly (buffered)
  and passed on to the next node
• Store and forward

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Use of Packets
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Advantages

• Line efficiency
• Single node to node link can be shared by many
  packets over time
• Packets queued and transmitted as fast as
  possible
• Data rate conversion
• Each station connects to the local node at its
  own speed
• Nodes buffer data if required to equalize rates
• Packets are accepted even when network is busy
• Delivery may slow down
• Priorities can be used

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Switching Technique

• Station breaks long message into packets
• Packets sent one at a time to the network
• Packets handled in two ways
• Datagram
• Virtual circuit

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Datagram

• Each packet treated independently
• Packets can take any practical route
• Packets may arrive out of order
• Packets may go missing
• Up to receiver to re-order packets and recover
  from missing packets

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DatagramDiagram
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Virtual Circuit

• Preplanned route established before any packets
  sent
• Call request and call accept packets establish
  connection (handshake)
• Each packet contains a virtual circuit identifier
  instead of destination address
• No routing decisions required for each packet
• Clear request to drop circuit
• Not a dedicated path

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VirtualCircuitDiagram
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Virtual Circuits v Datagram

• Virtual circuits
• Network can provide sequencing and error control
• Packets are forwarded more quickly
• No routing decisions to make
• Less reliable
• Loss of a node loses all circuits through that
  node
• Datagram
• No call setup phase
• Better if few packets
• More flexible
• Routing can be used to avoid congested parts of
  the network

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Circuit vs. Packet Switching

• Circuit Switched
• Bandwidth guaranteed
• Circuit capacity not reduced by other network
  traffic
• Circuit costs independent of amount of data
  transmitted, resulting in wasted bandwidth

• Packet Switched
• Bandwidth dynamically allocated on as-needed
  basis
• May have concurrent transmissions over physical
  channel
• May have delays and congestion
• More cost-effective, offer better performance

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X.25

• Old ITU standard
• Interface between host and packet switched
  network
• Almost universal on packet switched networks and
  packet switching in ISDN
• Defines three layers
• Physical
• Link
• Packet

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X.25 Use of Virtual Circuits
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Virtual Circuit Service

• Logical connection between two stations
• External virtual circuit
• Specific preplanned route through network
• Internal virtual circuit
• Typically one to one relationship between
  external and internal virtual circuits
• Can employ X.25 with datagram style network
• External virtual circuits require logical channel
  
• All data considered part of stream

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Frame Relay

• Designed to be more efficient than X.25
• Developed before ATM
• Larger installed base than ATM
• ATM now of more interest on high speed networks

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Frame Relay

• Public WAN packet-switching protocol
• Provides LAN-LAN connectivity
• Relays frames across a network from source to
  destination
• Connection-oriented protocol must first
  establish a connection before two nodes can
  communicate

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LAN-LAN ConnectivityPrior to Frame Relay

• Conventional dial-up circuit switching
• Dedicated leased line using point-to-point
  protocols or X.25 Packet Switching
• (Both have significant problems in todays
  technology)

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Frame Relay Circuits

• Connection-oriented protocol
• Relies on permanent virtual circuit (PVC)
  provide non-dedicated connections through a
  shared medium (bandwidth is shared among multiple
  sites (simplex lines)
• Can also support switched virtual circuit (SVC)

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Frame Relay Permanent Virtual Circuit

• Pre-determined link between source and
  destination
• Bandwidth is shared among multiple sites, not
  dedicated
• Bandwidth cost based on Committed Information
  Rate (CIR)
• Frames encapsulate data contained in layers 3-7

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CIR

• If Leased line, a fixed amount of bandwidth is
  provided, regardless of use.
• If CIR, frame relay provider guarantees a given
  throughput (bps) during normal workloads - user
  hedges bet and hopes for more
• CIR can be symmetric or asymmetric (as in ADSL)
• If CIR is too small, network becomes congested,
  and frames may be dropped
• If CIR is too high, you are paying for excessive
  bandwidth (think of the Sprint commercials)
• Service provider does not guarantee delivery
  above CIR

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BURST

• A data transmission that is equal to, or exceeds
  the CIR
• Committed Burst Maximum amount of data the
  provider guarantees to deliver within a specified
  time period (CIR/time) In general, CIR should
  not exceed 70 of port speed
• Excessive Burst Maximum amount of uncommitted
  data a provider will attempt to deliver within a
  specified time period

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Committed plus Excessive Burst
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Switched Virtual Circuit

• Frame Relay also supports switched virtual
  circuits (SVC)
• SVCs also support CIRs
• Circuits between source and destination are
  established when needed (logical dynamic, rather
  than logical permanent)
• Analogous to PSTN - paths change between
  connections

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PVC Advantages and Disadvantages

• Widespread availability
• Less complex network design
• Less expensive equipment
• Permanent connections - always paying for a
  certain amount of bandwidth, regardless of use
• Every time a new connection is required, a new
  permanent circuit must be established

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Frame Relay Basic Operation

• Packet switching every frame carries source and
  destination address
• Data link layer protocol, but does not support
  flow control, error detection, frame sequencing
  or ACK (all performed at end nodes (routers))
• Since frame integrity performed by end nodes, FR
  is fast and efficient
• Statistical multiplexing multiple subscribers
  share same backbone

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Congestion Control and Management

• If no ACK within a given time period, sending
  node assumes frame was discarded
• Discarded frames must be retransmitted,
  increasing network traffic
• Provider responsibilities
• Design networks to provide sufficient bandwidth
• Ensure links are error free
• Prevent any node from monopolizing the system
• Distribute resources in fair and equitable manner
• Refer to Lucent White Paper http//www.lucent.com/
  livelink/09009403800049af_White_paper.pdf

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Advantages and Disadvantages

• Lost link by link error and flow control
• Increased reliability makes this less of a
  problem
• Streamlined communications process
• Lower delay
• Higher throughput

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Required Reading

• Stallings Chapter 10
• http//www.lucent.com/livelink/09009403800049af_Wh
  ite_paper.pdf

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Chapter 10 Review Questions

• Why is it useful to have more than one possible
  path through a network for each pair of stations?
• Describe a simple switched network describe a
  circuit switched network
• What is a dedicated path?
• Describe the differences between blocking and
  non-blocking circuit switching
• Describe how circuit switching is inefficient.
• Describe SS7.
• Describe packet switching. What are the
  advantages of packet switching compared to
  circuit switching?
• Explain the difference between datagram and
  virtual circuit operation.
• Continued on next page!

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Chapter 10 Review Questions (cont.)

• What is the significance of packet size in a
  packet-switching network?
• What is the difference between inchannel and
  common channel signaling? Which is preferred,
  and why?
• Describe frame relay. What are the relative
  advantages and disadvantages of frame relay?
• Compare and contrast PVC and SVC
• Explain the concept of the committed information
  rate (CIR)
• Describe frame relays method of congestion
  control.