Introduction

  • A switch is used to connect the incoming link to the desired outgoing link and directs the
    incoming message to the appropriate outgoing link.
  • A communication switch is used to allow/stop flow of message through the path  connecting between the receiver and the transmitter. 
  • The switching is the manipulation of the ends of the communications channel and is used to make the connections.
  • The electrical signals/messages of senders needs to be switched only when the users need to communicate.
  • The role of a switch becomes more important when there are a large number of users and a particular user at one time may want to communicate with another user and wants to communicate still another user at a different time. Thus the same user has to be connected to two different users at two different times.This can be done by a controlled switch.

Definition

  • The process of making communication connection/link between sender and receiver using switch is known as switching.

Features/Characteristics

  • Switching plays a very important role in telecommunication networks.It enables any two users to communicate with each other.
  • In message and packet switching, the incoming message/packet to the node is stored temporarily in a memory location.Then, the stored message/packet is re-transferred to another desired memory location, from where the message/packet can be delivered/forwarded to the next node or the receiver. The transfer from the incoming bin to the outgoing bin is done with a control/command signal.

Advantages

  • Switching is used to connect two nodes that are not in direct connection to each other to transfer data between them successfully.

Types

  • Basically, there are three categories of switching – 
    (A) Message Switching
    (B) Circuit Switching
    (C) Packet Switching
(A) Message Switching (Outdated)
    • Message switching is one of the initial/earliest mode of switching, which helped a lot in the proliferation of the electrical communication.
    • Telegraph, arrived earlier than the Telephone, is one of the best example of Message Switching.
    • This method of switching works on the principle of ‘Store and Forward’.
    • This method of switching is now outdated i.e. not used now.
    • Unlike Circuit Switching, no circuits are switched in this mode of data transmission.
    • Working Mechanism of Telegraph –
      • Initially, the message is taken from the user and sorted first on the basis of the receiver’s address and clubbed with other messages moving in the same direction/destination, then they are bundled and are sent.
      • The operator at the next intermediate node receives all these messages, stores, sorts and forwards them.
      • Here, Information is transmitted through various nodes as discrete messages. Hence, this method of switching is known as Message Switching. Here, the storage & sorting of message was done by manually.
      • Later on the storage process was automated using paper tapes.The advantage of using paper tapes is that the incoming signal is punched onto it automatically and the same tape can be directly fed into the telegraph machine for further transmission.
      • As the teleprinters came, Morse code was replaced by machine telegraphy resulting in faster operations for sending messages.Later computers were introduced to do the function of message switching. Computer based message switching is still used in many organizations having many locations of working.
    • Mechanism of Message Switching –
      • In this technique, at each node (telegraph office) the message that arrives from the previous node in the path is stored for some time, sorted, and depending on the availability of the path from this node to the next in the path, the message is forwarded.
    • The Message Switching (telegraph) is less costly than the Circuit Switching (Telephone) due to –
      • Better utilization of transmission media.
      • The message switching is done over distributed time.
      • Hogging (Capturing the entire path) does not occur in message switching. Only
        one of the links in the entire path may be busy at a given time.
    • Demerits –
      • Message Switching requires storage and this may raise-up the cost of the systems.
      • This Switching method is not effective to send multimedia data.
(B) Circuit Switching

Definition :

      • Circuit switching is a switching technique in which a particular (dedicated) physical  communication link is selected/created statically between sender and receiver via various intermediate nodes, from available physical links to transmit data/messages successfully.

Features :

    • In this mode, the user is allocated the full use of the communication channel for the duration of the call.
    • This mode of switching is said to be as connection-oriented i.e. when a sender and a receiver wish to communicate with each other, the sender has to first dial the numbers of the receiver. Once those numbers are dialed, the originating/sender exchange will find a path to the terminating/destination exchange, which will in turn
      find the sender. Finally, the circuit or channel has been set up, then communication
      will take place, this mechanism is referred to as being connection-oriented.
    • In this mode of switching, the physical connections are made by the switching offices to connect intermediate switching elements between sender and receiver to complete the call of two users.
    • For each connection, physical switches are set/linked in the telephone network to create a physical circuit This is the job of the switching office that links switches and makes set up at the beginning of the connection and maintained throughout the transmission period. 
    • Telephone/Telecommunication (PSTN) is one of the best example of Circuit Switching.
    • Advantages :
      • Once the physical circuit has been established, then the dedicated path will remain exist until the connection is terminated and communication is fast and without error.
      • It is highly reliable.
    • Disadvantages :
      • A major drawback of circuit switching is the requirement of a dedicated path between the calling/sender and the called parties/receiver. This dedicated path reserves network resources like the chain of intermediate switches and connected transmission media over the entire path during the communication period of time which is obviously a costly proposition. This is not a very efficient strategy as a connection holds a physical line even during silence periods (when there is nothing to transmit/talk).
      • It involves a lot of overhead, during channel set up.
      • It wastes a lot of channel bandwidth, especially when a user is sometimes mainly listening, and not talking.
      • It may take longer time to channel set up between sender and receiver .
(C) Packet Switching
    • Definition :

      • Packet switching is a switching technique in which a logical communication link (either virtual circuit or datagram circuit) is created dynamically between sender and receiver via intermediate nodes, from available physical links to transmit data/messages in the form of packets successfully.
    • Features/Characteristics :
      • Packet Switching is the backbone of the present day communication systems.
      • The packet switching is the most favored technique in the present day communication systems.
      • The packet switching works on the principle that the long messages are fragmented into small size units, known as packets. It is these packets that are transmitted instead of the single long message.These packets are now transmitted over the network in the same manner as the messages in message switching.The packets are stored and forwarded at every node.Every packet has the source and destination addresses. Like message switching, repeated transmission of addresses at every intermediate node in this mode consumes network bandwidth.
      • Packets :
        • A packet contains the address of the computer that sent it and the address of the computer to which it is sent.
        • In general, packets need not be of the same size.
        • The Internet Protocol(IP) specifies the maximum size of packets in the form of maximum  transmission unit (MTU) and does not give the  minimum size of packets.
        • Packets are generated by the network hardware.
    • Examples :
      • Internet is one of the best example of Packet Switching.
    • Advantages :
      • Packets of the same message are launched into the network in parallel way over different available forward links at a node. These packets would travel through different paths to arrive at the destination. This simultaneous transmission of packets over different paths results in further improvement of the link utilization compared to the message switching.
      • No link/channel is engaged for a long time since the packets size are smaller than the single message as in message switching.
      • This switching permits better sharing of the links/channel amongst multiple users.
      • Packet switching is cost effective.
    • Disadvantages :
      • In packet switching, the overhead/wastage is more because every packet is required to carry the addresses on their head/header which are decoded each time on every intermediate nodes.
      • The network bandwidth consumed is maximum in packet switching and
        minimum in circuit switching.
      • Firstly, the packets of the same message traveling through different paths may arrive at the destination at different times due to different delays encountered in different paths. Thus the packets may arrive out of order. In order to deliver them to the destination, they need to be ordered which requires extra processing and so more delay. They need to be given sequence numbers
        for reordering them. The sequence number increases the overhead and requires more network bandwidth.
      • Secondly, some of the paths may not be very good and some packets may get lost. This worsens the quality. To improve quality, they require re-transmission which in turn requires more processing time and more bandwidth.
    • Types of Packet Switching :

Packet Switching occurs through two ways : –

(a) Connection-Oriented Packet Switching and (b) Connection less Packet                    Switching

(a) Connection-Oriented Packet Switching
      • In this mode of transmission, packets from a source machine to a destination machine are moved as per the source destination pair basis, i.e. all packets from the same source going to the same destination are transmitted over the same routes/channel and through the same routers.
      • This transmission results almost a constant delay for the different packets and the different packets reaching their destination in order.
      • This process of transmission is also called Virtual – Circuit Packet Switching as it involves the establishment of a fixed logical path from the available physical paths for talking period of time is called Virtual Circuit or Virtual Connection between the source and destination prior to the transfer of packets.
      • The virtual circuit creation takes in the following steps :-
        Step 1: Connection Request
        Step 2: Connection Confirm/Establishment 
        Step 3: Data Transmission Start
        Step 4: Connection Release/Terminate
      • Examples : ATM Networks
(b) Connection less Packet Switching
        • In this mode of transmission, packets from a source machine to a destination machine are transmitted as per-packet basis, i.e. each packet is transmitted and routed independently from all other packets. So, even if the source and destination machines do not change, routers in the middle may decide to change the routes that different packets follow ,
          resulting in the different packets reaching their destination in a different order from the sender because of the different transmission routes/path length , difference in transmission rates, and the amount of congestion in the different paths.
        • The packets may arrive at the destination machine in an order different from the transmission order. Since the details of this routing table change with the movement of the packets, the routing of different packets often
          changes.
        • Examples –
          • POTS (Plane Old Telephone Systems), ATM (Asynchronous Transmission Mode), Frame Relay, • MPLS (Multi Protocol Label Switching) etc.
        • Advantages :
        • Disadvantages :
          • Extra processing power is required at the nodes for attaching source and destination addresses with every packet which also increases the required time of transmission.
          • Connection less Packet switching requires overhead bits for indexing/numbering the packets.
          • Packets may arrive at the destination in a random manner. This requires that all the arriving packets are stored and rearranged.
          • Some packets may be lost during transmission using this method in the network.

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