• सुखार्थी त्यजते विद्यं विध्यार्थी त्यजते सुखम्सु sखर्थीन: कुतॊ विद्या कुतॊ विध्यार्थीन: सुखम् ||
  • “Luxury people leave knowledge, but a student leaves Luxury No knowledge for one who seeks Luxury, No luxury for student”
  • न चोर हार्यं न च राज हार्यं न भात्रू भाज्यं न च भारकारि |व्ययं कृते वर्धत एव नित्यं विद्याधनं सर्वधनप्रधानम ||
  • Cannot be snatched away by thief, cannot be snatched away by king, Cannot be divided among brothers, Not heavy either If spent daily, it always keeps growing. The wealth of knowledge is the precious of wealth of all”

Switching







Introduction

A computer network consists of a large number or computers and interconnecting devices. Passage of a message from a source to a destination involves many decisions. When a message reaches a connecting device, a decision needs to be made to select one of the output ports through which the packet needs to be sent out. We can say, the connecting device acts as a switch that connects one port to another port and data movement between the devices is called switching.

 

A switched network consists of a series of interlinked nodes called switches. Switched are hardware and software devices capable of creating temporary circuit between two or more devices.

 

Classification of switched networks:

  1. Circuit Switching
  2. Packet Switching
  3. Message Switching


CIRCUIT SWITCHING

    • In circuit switching a dedicated physical connection is established between sender and receiver.  A circuit-switched network is made of a set of switches connected by physical links. The dedicated path established between sender and receiver is maintained for entire duration of conversation. Circuit switching takes place at the physical layer.

      Entire process of circuit switching can be completed in three phases.

      Phase I Setup phase:

      Before starting communication, the stations must make a reservation for the resources to be used for communication. The resources, such as channels, switch, input output ports must be dedicatedly allotted during entire communication.

       

      For example  Sender sends request to Switch 1, which in turn sends the request to switch 2,

      Switch 2 informs senders intention to receiver.

       

      In the next step to making a connection, an acknowledgment from receiver in opposite direction towards sender. Once sender receives the acknowledgement the connection is established.

       

      Bandwidth between various links can be shared using FDM or TDM.



      Phase II Data transfer phase

      Once circuit is established between sender are receiver continuous data (not packets) flows between sender and receiver there may be no data in between.

      Phase III Circuit tear down phase

      When any one of the parties needs to disconnect, a signal is sent to each switch to releasethe resources.

      Advantage of Circuit Switching

      • Dedicated path or circuit between sender and receiver provides a guaranteed data rate.
      • Once circuit is established data can be transmitted without any delay.
      • Method is suitable for long continuous transmission.


      Disadvantages of Circuit Switching

      • As the connection is dedicated it cannot be used to transmit any other data even if channel is free.
      • It is inefficient in terms of utilization of system resources .
      • Dedicated channels consume more bandwidth.
      • Not efficient for long distance communication.

PACKET SWITCHING

Adopted in 1970 for long distance communication.  In packet switching message is broken up into packets of fixed or variable size. Each of the packets contain header with the information of sender, receiver and other control information. No resources are dedicatedly allocated before the transfer of packets. Resources are allocated on demand on first come first server bases.

These networks are also called as store and forward networks as at each node stores the packet briefly and route it according to the information stores in header. A packet network is implemented in the network layer.


Advantages of Packet Switching

  • Line efficiency increases as node to node link can be shared by many packets over time.
  • As the routing is done on packet basis, intermediate nodes do not have to wait for the entire message, hence transmission is fast.
  • Buffer requirement at the nodes is small as packets are small.
  • More effective for real time bursty data.
  • Packets are accepted even when network is busy.
  • Priorities can be used
  • Switching nodes can route the packets as and when required.

Disadvantages of Packet Switching

  • Packets may be lost while routing.
  • Sequence number increases the overhead.
  • Header information is repeated for all the packets.

 

Message Switching

A virtual-circuit network is a hybrid of a circuit-switched network and a packet network. It has some characteristics of both.

  1. As in a circuit-switched network, there are setup and tear down phases in addition to the data transfer phase.
  2. Resources can be allocated during the setup phase, as in a circuit-switched network, or on demand, as in a packet  network.
  3. Like a packet  network, data are packetized and each packet carries an address in the header. However, the address in the header should be the next switch it is not end-to-end.
  4. As in a circuit-switched network, all packets follow the same path established during the connection.
  5. A virtual-circuit network is normally implemented in the data link layer, while a circuit-switched network is implemented in the physical layer and a packet network in the network layer.

Question Bank

Question    Make a technical comparison between circuit switched  and Packet switched network?  [10 marks]


Question: Make a technical comparison between Message, Circuit and Packet Switching.  [10]