Methods of Line Discipline

Line Discipline :
Line discipline is the foundation of data link layer that determines which device can send data at a given time out of various connected devices in network and when it can send that data. Thus line discipline coordinate link system. It also used to make sure that data sent by sender is received by a particular receiver. It may be possible that when sender is sending data, receiver is not ready to accept that data or is busy. In such a case, sender will keep on sending data that will not be received by receiver, and data goes waste.

To overcome such a problem, line discipline method confirms existence and capability of receiver before data is sent by receiver before data is sent by sender. Line discipline function looks after establishment of link between sender and receiver and also take care of right of particular device to transmit data at a given time.

Methods of Line Discipline :
There are two ways to provide line discipline functions :

1. Enquiry/Acknowledgement (ENQ/ACK) :
   • This method is used when there is dedicated link between sender and receiver.
   • This method is used in peer-to-peer communication.
   • It coordinated which device may start transmission and whether receiver is ready to accept data or not.
   • If two communicating devices are in same rank, any of two device can start communication.
   

   Figure : ENQ/ACK Mode

2. Poll/ Select :
   • This method is used in client-server type networks having primary-secondary relationships.
   • In this, one device is called primary station that provides and control services to all other devices called secondary devices.
   • In such network, primary device controls link and secondary devices follows its instructions.
   • Primary device determines which device has control over link at a given time.
   • Whenever a communication is to be establish either between a primary and a secondary device or a secondary and a secondary device, primary device is always initiator of session.
   • In such networks two functions are possible : Poll, and Select. These are explained as following below.
     • (i). Polling :
       When primary device wants to receive details asks secondary devices if they have anything to send, this function is called polling.
       

       Figure : Poll Mode

     • (ii). Selecting :
       When primary device wants to send data to any secondary device, it tells that device to be ready to receive data, this function is called selecting.
       

       Figure : Select Mode