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Wednesday, September 30, 2020

Line Discipline - ENQ/Acknowledgement - Poll/select

Line Discipline is a functionality of the Data link layer that provides the coordination among the link systems. It determines which device can send, and when it can send the data.  Line discipline method confirms existence and capability of receiver before data is sent by sender.

Line discipline function looks after establishment of link between sender and receiver and takes care of right of particular device to transmit data at a given time.

Line Discipline can be achieved in two ways:

  • ENQ/ACK (Enquiry/Acknowledgement )
  • Poll/select

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 coordinates which device may start transmission and whether receiver is ready to accept data or not.

Working of ENQ/ACK

The transmitter transmits the frame called an Enquiry (ENQ) asking whether the receiver is available to receive the data or not.

The receiver responses either with the positive acknowledgement(ACK) or with the negative acknowledgement(NACK) where positive acknowledgement means that the receiver is ready to receive the transmission and negative acknowledgement means that the receiver is unable to accept the transmission.

Following are the responses of the receiver:

  • If the response to the ENQ is positive, the sender will transmit its data, and once all of its data has been transmitted, the device finishes its transmission with an EOT (END-of-Transmission) frame.
  • If the response to the ENQ is negative, then the sender disconnects and restarts the transmission at another time.
  • If the response is neither negative nor positive, the sender assumes that the ENQ frame was lost during the transmission and makes three attempts to establish a link before giving up.

2. Poll/Select

The Poll/Select method of line discipline works with those topologies where one device is designated as a primary station, and other devices are secondary stations.

Working of Poll/Select

In this method, a primary device and multiple secondary devices consist of a single transmission line, and all the exchanges are made through the primary device even though the destination is a secondary device.

The primary device has control over the communication link, and the secondary device follows the instructions of the primary device.

The primary device determines which device is allowed to use the communication channel. It is an initiator of the session.

Poll

If the primary device wants to receive the data from the secondary device, it asks the secondary device to send data; this process is known as polling.

Firstly, the primary asks (poll) the first secondary device, if it responds with the NACK (Negative Acknowledgement) means that it has nothing to send. Now, it approaches the second secondary device, it responds with the ACK means that it has the data to send. The secondary device can send more than one frame one after another or sometimes it may be required to send ACK before sending each one, depending on the type of the protocol being used.




Select

If the primary device wants to send some data to the secondary device, then it tells the target secondary to get ready to receive the data, this process is known as selecting.

When the primary device wants to send some data, then it alerts the secondary device for the upcoming transmission by transmitting a Select (SEL) frame, one field of the frame includes the address of the intended secondary device.

The secondary device receives the SEL frame, it sends an acknowledgement that indicates the secondary ready status. If the secondary device is ready to accept the data, then the primary device sends two or more data frames to the intended secondary device.

Once the data has been transmitted, the secondary sends an acknowledgement specifies that the data has been received.




Functions of Data Link Layer

Ø  Data Link Control

Data Link Control is the service provided by the Data Link Layer to provide reliable data transfer over the physical medium. 

For example, In the half-duplex transmission mode, one device can only transmit the data at a time. If both the devices at the end of the links transmit the data simultaneously, they will collide and leads to the loss of the information. 

The Data link layer provides the coordination among the devices so that no collision occurs.

The Data link layer provides three functions:

  • Line discipline
  • Flow Control
  • Error Control