EE3900 Data Communications and LANs Packet Switching Slide 1

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Packet Switching
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Outline

• Packet-switching Principles
• Switching Techniques
• Comparison of Circuit Switching and Packet
  Switching
• Simple Routing Schemes
• Congestion Control

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Circuit Switching

• Circuit switching designed for voice or constant
  rate services
• Resources are dedicated to a particular call for
  the call duration
• In data traffic, some of the time, no data are
  sent
• Data rate is pre-arranged
• Both ends must operate at the same rate

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Packet Switching

• Data transmitted in small packets
• Typically 1k to 2k octets
• Longer messages are split into series of packets
• Each packet contains a data field plus some
  control info
• Control information
• Routing (addressing) info
• Packets are received, stored briefly (buffered)
  and passed on to the next node
• Store and forward

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Advantages

• Line efficiency
• Single node to node link can be shared by many
  packets on a on-demand basis, improve efficiency
  and reliability
• Packets are queued and transmitted as fast as
  possible
• Data rate conversion
• Each station connects to the local node at its
  own speed
• Nodes buffer data and send to the station
• When the network is busy, packet accepting and
  delivery rates may be reduced
• Priority services can be implemented

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Packets
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Use of Packets
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Example of Packet Routing
A Packet Switching Network
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Packet Size
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Outline

• Packet-switching Principles
• Switching Techniques
• Comparison of Circuit Switching and Packet
  Switching
• Simple Routing Schemes
• Congestion Control

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Switching Technique

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

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Datagram Mode

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

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Datagram Mode
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Virtual Circuit Mode

• Preplanned route decided at connection
  establishment
• Call request and call accept packets establish
  the connection (handshake)
• Each packet contains a virtual circuit identifier
  instead of destination address
• No routing decisions required for each packet
• Clear request to terminate circuit
• Not a dedicated path, links are shared by
  different packets

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Virtual Circuit Mode
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X.25 Use of Virtual Circuits
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Public Switched Data Networks

• An example of packet switching network
• A network for data communications over a wide
  area
• Also operated by telephone companies
• Based on X.25 standard from ITU-T, specifying an
  interface between a host system and a packet
  switching network
• low speed, up to 64kb/s

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Virtual Circuits vs Datagram

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

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Outline

• Packet-switching Principles
• Switching Techniques
• Comparison of Circuit Switching and Packet
  Switching
• Simple Routing Schemes
• Congestion Control

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Delays

• Propagation delay time for a signal to propagate
  from one node to the next
• Transmission time time for a transmitter to send
  out a block of data, transmission capacity
  dependent
• Node delay the time for a node to process and to
  switch the data

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Circuit Switching vs Packet Switching
Event Timing for Circuit Switching and Packet
Switching
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Circuit Switching vs Packet Switching (Contd)

• Circuit switching connection set-up is required
  before data transfer
• Virtual circuit packet switchingconnection
  set-up is needed before data transfer
• Datagram packet switching no set-up is required

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(No Transcript)
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Outline

• Packet-switching Principles
• Switching Techniques
• Comparison of Circuit Switching and Packet
  Switching
• Simple Routing Schemes
• Congestion Control

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• Introduction
• The need for routing e.g. Getting from A to F

D
B
F
A
E
C
Path 1 A-B-D-F Path 2 A-B-E-F Path 3
A-B-C-E-F Path 4 A-C-E-F Path 5
A-C-E-B-D-F Path 6 A-C-B-D-F Path 7
A-C-B-E-F
Which one?
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Routing
Routing

• Complex, crucial aspect of packet switched
  networks
• Performance used for selection of route Minimum
  hop, Least cost
• Characteristics required
• Correctness, Simplicity, Robustness, Stability
• Fairness, Optimality, Efficiency

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Routing Attributes

• Routing information gathering
• Information source local, adjacent nodes, all
  nodes
• Update time never (fixed routing), regular
  update (adaptive routing)
• Routing path selection/calculation
• Time per packet, connection set-up
• Place distributed nodes (distributed routing),
  centralized node (central routing), source node
  (source routing)

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Fixed Routing

• Single permanent route for each
  source-destination pair of nodes
• Determine routes using a least cost algorithm
• the link cost is based on expected traffic or
  capacity, but not on any dynamic variable such as
  instant traffic volume
• each node needs only to store the next forwarding
  address for each destination

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Cost of Routes in a Packet-Switched Network
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Fixed RoutingTables
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Least-Cost Path

• What is meant by least cost?
• It can be number of hops, physical distances,
  costs, delay, link capacity, current loading
• Or an arbitrary function of the above parameters,

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Flooding

• A packet is sent by the source node to every one
  of its neighbors
• At each intermediate node, an incoming packet is
  retransmitted on all outgoing links except for
  the incoming link
• Eventually a number of copies will arrive at
  destination
• Each packet is uniquely numbered so duplicates
  can be discarded
• Advantage no network information is required
• Disadvantage unlimited amount of traffic will be
  generated unless something is done

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Flooding Example
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Solution for Flooding

• Method 1 each node remembers the identity of
  those packets it has already transmitted. When
  duplicate copies of the packet return, they are
  discarded.
• Method 2 Each packet contains a hop count field.
  Each time a node passes on a packet, it
  decrements the count by one. When the count
  reaches zero, the packet is discarded.

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Properties of Flooding

• All possible routes between source and
  destination are tried. Therefore, a packet will
  always get through if there is a connection
  between source and destination
• Because all routes are tried, at least one copy
  of the packet will arrive at the destination
  using a minimum-hop route can be used to set up
  VC
• All nodes that are directly or indirectly
  connected to the source node are visited useful
  to distribute information (routing)

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Random Routing

• A node selects only one outgoing path for
  transmission of an incoming packet
• The outgoing link is chosen at random, round
  robin or probability based excluding the link on
  which the packet arrived
• Like flooding, no network information is required
• Random routing carries less traffic than
  flooding, but higher than optimum load
• Route is typically not least cost nor minimum hop

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Adaptive Routing

• The routing decisions change as conditions on the
  network change, such as node failure or link
  congestion
• Information about the state of the network are
  required and exchanged among the nodes
• The higher the amount and the more frequent
  information is exchanged (higher network
  overhead), the better decisions can be made
• React too quickly can cause oscillation, too
  slowly will not be adaptive

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Advantages of Adaptive Routing

• improve performance, as seen by the network user
• By-pass link congestions and node failures

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Drawbacks of Adaptive Routing

• The routing decision is more complex hence,
  increase node processing
• Adaptive strategies depend on status information,
  collected at one location but used in another
  therefore, the traffic loading on the network
  increases

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Isolated Adaptive Routing
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Outline

• Packet-switching Principles
• Switching Techniques
• Comparison of Circuit Switching and Packet
  Switching
• Simple Routing Schemes
• Congestion Control

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Congestion

• As packets arrive, they are stored in the input
  buffer of the corresponding link
• The node sends each incoming packet to the
  appropriate output buffer
• If packets arrive too fast for the node to
  process, or faster than the outgoing speed
  packets will arrive for which no memory is
  available and packets will be lost.

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Causes of Congestion

• Insufficient router memory, slow processors (at
  routers), low-bandwidth links, etc.
• Normal Traffic
• Congestion

Router
incoming links
outgoing links
Router
incoming links
outgoing links
Packets are dropped due to lack of buffers. Can
we solve congestion by using huge amount of
buffers?
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Effects of Congestion

• Throughput As the offered load increases,
  throughput first increases, then drop and finally
  approaches to zero (Why? Any solution?)
• Delay the average delay grows without bound as
  the load approaches the capacity of the system.

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Congestion

• Introduction
• The effect of network congestion

During congestion, the more you send, the less
the network can deliver.
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The effect of congestion.
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General Strategies for Congestion Control

• discard any incoming packets for which there is
  no available buffer space
• exercise flow control over its neighbors (ask
  them to reduce sending rate), so that the traffic
  flow remains manageable. In fact, the traffic on
  the entire network needs to be managed.

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Congestion Control Algorithms

• General Principles
• Open-loop congestion control algorithms
• Prevent congestion from occurring in the first
  place.
• Close-loop congestion control algorithms
• Monitor state of the network
• Feedback state information to the senders
• Adjust system operation to correct the problem
• Specific algorithms will be discussed in TCP