data communication network

1. BECE3, BEEE 3E, BEEE3 & BETE3
Data Communication Networks
ELE-DCN-321
Lesson 7 : Data Elements Switching
E. Chiwaya, Staff Associate in Computer Engineering

2. Last Lesson Review
In the last lesson, we:
• explained the transmission modes and channel configuration;
• discussed the data terminal and circuit terminating equipment as
applied in data computer networks; and
• explained the process of transforming data signals for ease of
transmission.
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3. Intended Learning Outcomes
• On completion of this lesson, the student should be able to:
• explain how switching is utilised in a data communication network
• discuss the types of data switching methods
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4. References
• Chadza, T. (2013). Data Communication Networks Lecturer Notes,
Blantyre: MUBAS
• Namaona, T. (2017). Data Communication Networks Lecturer Notes,
Blantyre: MUBAS
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5. Outline
• Introduction
• Data switching methods
• Circuit switching
• Packet switching
• Message switching
• Fast packet switching
• Lesson summary
• Next lesson
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6. Introduction
• A network is a set of connected devices.
• Problem with having multiple devices:
• how to connect them to make one-to-one communication possible
• Solution: make a point-to-point connection between each pair of devices (a
mesh topology) or between a central device and every other device (a
star topology)
• Shortfall: impractical and wasteful when applied to very large networks
• Better solution: switching
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7. • A switched network consists of a series of interlinked nodes, called
switches.
• Switches are devices capable of creating temporary connections between
two or more devices linked to it.
• Some of the nodes are connected to the end systems, and some not.
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8. 8/1/2023
Fig 1: Switching Network
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9. Data switching methods
• There is also fast packet switching!!
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Fig 2: Switching Methods

10. Circuit Switching
• There is a dedicated communication path between two stations.
• That path is a connected sequence of links between network nodes.
• Each link is normally divided into n channels by using FDM or TDM.
• On each physical link, a logical channel is dedicated to the
connection.
• Common example is the telephone network.
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11. • Three circuit switching phases:
• Circuit establishment:
• Dedicated connection must be established before signal transmission of end-to-end
(station-to-station).
• Data transfer:
• Transmission may be analog or digital, depending on the nature of the network.
• No addressing involved.
• Switches route the data based on their occupied band (FDM) or time slot (TDM).
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12. •Three circuit switching phases cont’d:
• Circuit disconnection:
• After some period of data transfer, the connection is
terminated, usually by the action of one of the two stations.
• Signals must be propagated to nodes to de-allocate the
dedicated resources.
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13. Advantages:
• Ideal for smooth network traffic e.g. telephone.
• Delay at node is negligible as data is never stored at any point.
• No overhead bits after call setup.
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14. Disadvantages:
• Channel capacity is dedicated for the duration of a connection, even
if no data are being transferred.
• Delay prior to signal transfer for call establishment.
• Fixed bandwidth.
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15. Packet Switching
• Data is divided into smaller segments, called packets, prior to
transmission through the network.
• Also known as hold-and-forward network, as a packet can be held in
memory at a switch for a short period of time.
• Each packet need to have control information as well source and
destination address for it to be recognized by the receiver as
belonging to a particular stream.
• There is no resource reservation; resources are allocated on
demand.
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16. Advantages over circuit switching
• Great Line efficiency
• A link can be dynamically shared by many packets over time.
• Packets are queued up and transmitted as rapidly as possible over the link.
• Perform data-rate conversion
• Two stations of different data rates can exchange packets because each
connects to its node at its proper data rate.
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17. Advantages over circuit switching cnt’d
• No blocking
• When traffic becomes heavy on a circuit-switching network, some calls are
blocked.
• Packets are still accepted, but delivery delay increases.
• Priorities used
• A node can transmit the higher-priority packets first.
• These packets will therefore experience less delay than lower-priority packets
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18. Disadvantages:
• Requires complex and expensive switching arrangements and
complicated protocols.
• Has overhead bits in each packet.
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19. •Packet switching approaches
• Datagram
• Virtual circuit network
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20. Datagram Network
• Packets are referred to as datagrams.
• All packets:
• treated independently, with no reference to packets that have gone before.
• can take different path through the network.
• may not arrive at the receive end at the same time or in the order of transmission.
• routed using routing tables to their destinations.
• No setup or teardown phases.
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21. 8/1/2023
Fig 3: Packet switching :
Datagram Approach
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22. Advantages
• Call setup phase is avoided
• if a station wishes to send only one or a few packets, datagram delivery will
be quicker.
• More flexible
• If congestion develops in one part of the network, incoming datagrams can
be routed away from the congestion.
• More reliable
• If a node fails, subsequent packets may find an alternate route that bypasses
that node.
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23. Virtual Circuit Networks
• Preplanned route is established before any packets are sent.
• All the packets between a pair of communicating parties follow this
same route through the network.
• A cross between a circuit-switched network and a datagram
network.
- Has some characteristics of both.
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24. • As in a circuit-switched network, there are setup and teardown
phases in addition to the data transfer phase.
• Resources can be allocated during the setup phase, as in a circuit-
switched network, or on demand, as in a datagram network.
• As in a datagram network, data are packetized and each packet
carries an address in the header.
• However, the address in the header has local jurisdiction (it defines what
should be the next switch and the channel on which the packet is being
carried).
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25. • As in a circuit-switched network, all packets follow the same path
established during the connection.
• Normally implemented in the data link layer, while a circuit-switched
network is implemented in the physical layer and a datagram
network in the network layer
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26. 8/1/2023
Fig 4: Packet switching : Virtual
circuit approach
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27. Addressing
• Global
• An address that can be unique in the scope of the network or
internationally.
• Local (virtual-circuit identifier (VCI))
• A small number that has only switch scope; it is used by a frame between
two switches.
• When a frame arrives at a switch, it has a VCI; when it leaves, it has a
different VCl.
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Fig 5: Virtual circuit identifier

28. • Advantages
• Network may provide services related to the virtual circuit, including
sequencing and error control.
• Sequencing: all packets follow the same route therefore arrive in the original order.
• Error control: assures that not packets arrive correctly and in proper sequence.
• Packets transit the network more rapidly
• not necessary to make a routing decision for each packet at each node.
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29. Packet Size
• There is a significant relationship between packet size and transmission time.
• By breaking the message up into packets, each intermediate node can begin
transmission even sooner and the savings in time is greater.
• However, using more and smaller packets eventually results in increased, rather
than reduced delay.
- More packets mean more headers.
- Processing and queuing delays at each node increase.
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30. 8/1/2023
Fig 3: Effect of packet size on transmission time
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31. Message switching
• Also known as store-and-forward switching.
• Data, including source and destination identification codes, are
transmitted into the network and stored in a switch.
• Each switch within the network has message storage capabilities.
• Network transfers the data from switch to switch when it is
convenient to do so.
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32. Advantages:
• Traffic congestion can be reduced, because messages may be temporarily
stored.
• Message priorities can be established due to store-and-forward
technique.
• Has a dynamic bandwidth.
• Source and destination data terminal equipment do not need to be
compatible.
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33. Disadvantages
• Store-and-forward devices are expensive, because they must have
large disks to hold potentially long messages.
• Has overhead bits in each message.
• Slower than packet /circuit switching, as messages are stored for
variable times between nodes.
• Data is not transferred in real time; there can be a delay at each
switch.
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34. Fast packet switching
• A generic term for improved packet-switching technologies.
• Splits data into small sections called packets and sends them
separately by available channels until they reach destination but
eliminates overhead hence increasing throughput.
• Bypasses some error-checking protocols and flow control in the
network layer of an OSI model.
• A connection is established between two end points before the
actual data exchange
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35. Advantages
• Fast since it performs little or no error checking.
• No/less overhead bits in each packet.
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36. Disadvantages
• More complex.
• Connection establishment delay.
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37. 8/1/2023 37
Table: Switching types characteristics summary

38. Summary
• In this lesson, we have:
• explained how switching is utilised in a data communication network; and
• discussed the types of data switching methods.
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39. Next Lesson
• In the next lesson, we will:
• discuss the network classifications; and
• describe the topologies used in networks
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41. THANK YOU
CREDIT TO: T. CHADZA & T. NAMAONA
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