Computer Networks

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Computer Networks

• Chapter 8 Circuit Switching versus Packet
  Switching

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Switches

• What is a switch?
• A central device usually used with a star
  topology
• Can be built in hardware and/or software
• Used to provide temporary connections between any
  two devices connected to the switch
• A network of switches can be made if a very large
  number of devices spread in a large geographic
  space need to be connected

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Telephone network Local Switch

• The telephone network uses switches
• Every subscriber ( telephone jack in a house) has
  a twisted-pair wire connected to the closest
  telephone exchange. They are called local
  switches or local exchanges.

subscriber

• This cannot provide connection to subscribers
  connected to another local switch.

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A Circuit Switch

• Device with a number of inputs and outputs
• Creates temporary physical connection between an
  input and output link

• The local switch can connect each telephone with
  each other

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Telephone network Connecting Local Switches

• It is expensive to connect each local switch with
  every other in a fully meshed topology

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• Higher order switches are introduced to save on
  cables
• Redundancy is introduced to provide reliability

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2nd order switch
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The trafic between the switches is multiplexed
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Local switch (1st order switch)
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Subscribers Connected to Different Local Switches

• The connection goes through several switches.
• If any part of the connection cannot be reserved,
  the connection is not established

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Circuit Switching

• Three phases of the connection
• Circuit establishment
• Data transfer
• Circuit disconnect
• The bandwidth is guaranteed during the connection
• The bandwidth cannot be used by anyone else, even
  if it is not needed at certain moment (no
  flexibility)

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Circuit Switching cont.

• The resources (for example, the bandwith) are
  divided into pieces
• Pieces are allocated to calls.
• No sharing no two calls can share the same piece
  of bandwidth.
• A piece is regarded idle if not assigned to any
  calls.
• A call is rejected if no idle piece exists on
  any part of the connection.

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Telephone calls Typical Example for Circuit
Switching
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Types of Switches

• Space switches
• Crossbar
• Multistage A way to save on the cross points
• Blocking input point cannot be connected to the
  output due to all cross points being used by
  others
• Time-division switches
• Time-slot interchange
• TDM bus
• Combined switches

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Characteristics of the Switches

• Space switches
• The advantage is that if a cross point is
  available, the connection is almost instantaneous
• The disadvantage is the need for many cross
  points which is expensive
• Time switches
• Advantage is that it does not need cross points
• Disadvantage is the time necessary for processing
  each time slot.
• Combined switches combine the advantages of both
  types

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Hierarchy of the Telephone Network
International network
International gateway exchange
National tandem exchanges
regional tandem exchanges
trunk network
Tandem offices
local tandem exchanges
local network
local exchanges (toll offices)
subscriber lines (local loops)
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Computer Networks

• Telephone network has been developing since the
  end of the 19th century
• Computer networks started their development late
  in the 20th century (ARPANet has emerged in the
  late 60s and early 70s)
• Circuit switching was not convinient for computer
  traffic because
• Data traffic is bursty a lot of traffic might
  need to be transmitted at one moment and nothing
  in another (dedicated bandwidth will then be
  wasted)
• Data traffic is delicate if binary file is
  transmiited, every bite must be correct for the
  program to work (for voice this is not too
  important)

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Packets A New Paradigm

• The message is splitted in smaller chunks called
  packets.
• Introducing packets is important for the
  following reasons
• Errors can appear only in a few packets. Then
  these packets need to be retransmitted again.
  However, retransmitting several packets instead
  of the whole message saves bandwidth.
• A very long message means monopolizing bandwidth
  by a single user. Using smaller packets provides
  better sharing.

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Message versus Packets
Whole message
Data
Header
Padding if necessary
Each packet carries the destination address in
the header and is independent from the other
packets in the message - datagram
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Store-and-forward Switching

• Instead of establishing a circuit and making
  reservation on every hop of the path, packet
  switching is actually store-and-forward
  switching.
• In store-and-forward switching every packet is
  switched (forwarded) as it comes to the switch.
• If the switch is busy, the packet waits in the
  input queue to be processed
• Once the switch is idle it checks the header of
  the packet and based upon that information
  forwards it to the output line which is closer to
  the packets destination.
• If the output line is busy, the packet waits in
  the output queue until it is transmitted to the
  next store-and-forward switch.
• The switch that is directly connected to the
  desination will deliver the packet.

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Router A Store-and-Forward Switch
outgoing links
Link 1
incoming links
router
Link 2
Link 3
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Statistical multiplexing

• The physical link is shared over time (like in
  STDM)
• Data are transmitted on demand rather than during
  the predetermined time slot (unlike in STDM)

Buffering (queues)

• Defines a cost-effective way for multiple users
  to share network resources in fine-grained manner

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Packet Switching

• Two types of packet switching
• Datagram switching
• The store-and-forward switch forwards the packet
  to the output link by looking only at its
  destination address.
• Packets belonging to the same message can travel
  different paths and their ordering is done at the
  receiver
• Example technologies IP
• Virtual circuit switching

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Datagram Switching
Each router forwards the datagram solely based
upon the destination address. Packets can arrive
out of order.
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Delays with packet Switching

• Packet experiences delay on each hop
• transmission delay (length of message/through.)
• propagation delay (distance/speed of prop.)
• nodal processing
• quieuing

transmission
A
propagation
B
nodal processing
queueing