Ch. 10 Circuit Switching and Packet Switching

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Ch. 10 Circuit Switching and Packet Switching
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10.1 Switched Communication Networks

• Fig. 10.1 Simple switching network.
• End stations are attached to the "cloud".
• Inside the cloud are communication network nodes
  interconnected with transmission lines.
• The transmission lines often use multiplexing.
• The network is generally not fully connected, but
  alternate paths exist.
• Two technologies for WANs
• Circuit Switching
• Packet Switching

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10.2 Circuit-Switching Networks

• The three phases of a circuit switched connection
  are
• Circuit establishment
• Data transfer
• Circuit disconnect

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10.2 Circuit-Switching Networks (p.2)

• Four generic architectural components of the
  public telecommunications network
• Subscribers
• Subscriber line (or local loop)
• Exchanges
• Trunks
• Fig. 10.2 illustrates the public switched
  telephone network (PSTN).
• Fig. 10.3 illustrates two possible connections
  over the PSTN.

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10.3 Circuit-Switching Concepts

• Fig.10.4 Elements of a Circuit-Switch Node
• Digital Switch
• Provides a transparent signal path between any
  pair of attached devices.
• Control Unit
• Establishes connections.
• Maintains connections.
• Tears down connections.
• Network Interface
• Functions and hardware needed to connect digital
  and analog terminals and trunk lines.

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10.3 Circuit-Switching Concepts (p.2)

• Blocking vs. Nonblocking
• Relates to the capability of making connections.
• A blocking network is one in which blocking is
  possible.
• A nonblocking network permits all stations to be
  connected (in pairs) as long as the stations are
  not in use.

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10.3 Circuit-Switching Concepts (p.2)

• Space-Division Switching
• Defn A circuit-switching technique in which
  each connection through the switch takes a
  physically separate and dedicated path.
• Basic building block--a metallic crosspoint or
  semiconductor gate.
• "Crossbar" Matrix (Fig. 10.5)
• Multi-stage space-division switches reduces the
  total number of crosspoints required, but
  increases complexity and introduces the
  possibility of blocking.(Fig. 10.6)

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10.3 Circuit-Switching Concepts (p.3)

• Time-Division Switching
• Defn A circuit-switching technique in which
  time slots in a time-multiplexed stream of data
  are manipulated to pass data from an input to an
  output.
• All modern circuit switches use digital time
  division techniques or some combination of space
  division switching and time division switching.

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10.4 Softswitch Architecture

• Specialized software is run on a computer that
  turns it into a smart phone switch (Fig.10.10).
• Performs traditional circuit-switching functions.
• Can convert a stream of digitized voice into
  packets (VoIP).
• Media Gateway (MG) performs the physical
  switching function.
• Media Gateway Controller (MGC) performs call
  processing.
• RFC 3015--communications between the two.

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10.5 Packet-Switching Principles

• Definition A method of transmitting messages
  through a communication network, in which long
  messages are subdivided into short packets. The
  packets are then sent through the network to the
  destination node. (See Fig. 10-8)

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10.5 Packet-Switching Principles (p.2)

• Two Techniques
• Datagram (Fig. 10.9)
• Each packet contains addressing information and
  is routed separately.
• Virtual Circuits (Fig. 10.10)
• A logical connection is established before any
  packets are sent packets follow the same route.

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10.5 Packet-Switching Principles (p.3)

• Packet Size
• Each packet has overhead.
• With a larger packet size
• Fewer packets are required (less overhead.)
• But longer queuing delays exist at each packet
  switch.
• Figure 10.11 illustrates this issue.

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10.5 Packet-Switching Principles (p.4)

• Delay in Switching Networks
• Setup Time--connection oriented networks (removed
  from chapter but not problems)
• Transmission Time
• Propagation Delay
• Nodal Delay--processing time at nodes.
• Fig. 10.13 and Table 10.1 compare the
  performance of circuit switching, datagram packet
  switching, and virtual-circuit packet switching.

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10.6 Packet-Switching Principles (p.5)

• Delay in Circuit Switched Networks
• Call setup time.
• Message transmission time--occurs once at the
  source.
• Propagation delay--sum of all links.
• Very little node delay.

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10.6 Packet-Switching Principles (p.6)

• Delay in Packet Switching
• Connection Setup Time
• Required for virtual circuit.
• None for datagram.
• Packet transmission time and propagation delay
  occurs on each link.
• Processing delay occurs at every node.
• Datagram networks may require more than virtual
  circuit networks.

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Problem 10.4

• Consider the delay across a network.
• Let B data rate on every link.
• Let N the number of links.
• Let L the length of the source message.
• Let D the average delay on a link.
• Let S setup time (when required.)
• Let P packet size for packet switched
  networks--fixed length packets.
• Let Hthe number of bits of overhead in each
  packet header, for packet switched networks.

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Problem 10.4 (p.2)

• Circuit Switching Delay
• Let t0 be the time that the first bit is
  transmitted at the source node and t1 be the time
  that the last bit is received at the destination
  node.
• Then let T t1-t0 be the "end-to-end" delay.
• Follow the last bit across the network.
• No network layer overhead and little nodal delay.
• Ignore any data link protocol delay (U1).
• T S L/B N x D

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Problem 10.4 (p.3)

• Datagram Packet Switch Delay
• Let NoPa Number of Packets L/(P-H) rounded up
  (ceiling).
• Assume no link level related overhead (U1.)
• The last packet waits at the source and then is
  transmitted over every link in a store and
  forward fashion.
• T (NoPa-1)P/B N(P/B D)
• Virtual-Circuit Packet Switch Delay
• T S (NoPa-1)P/B N(P/B D)

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X.25 (no longer in text)

• First approved in 1976 and revised in 1980, 1984,
  1988, 1992, and 1993.
• Specifies an interface between a host system and
  a packet-switched networks.
• Almost universally used and is employed for
  packet-switching in ISDN.
• Virtual circuits are used in an X.25 network.

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X.25 (p.2)

• Three Layers are defined
• X.21 is the physical layer interface (often
  EIA-232 is substituted)
• LAP-B is the link-level logical interface--it is
  a subset of HDLC.
• Layer 3 has a multi-channel interface--sequence
  numbers are used to acknowledge packets on each
  virtual circuit.