Firstly, the primary asks (poll) the first secondary device, if it responds with the NACK (Negative Acknowledgement) means that it has nothing to send.When a primary device wants to receive the data, then it asks each device whether it has anything to send.The Poll mode is used when the primary device wants to receive some data from the secondary device.Once the data has been transmitted, the secondary sends an acknowledgement specifies that the data has been received. 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.When the secondary device receives the SEL frame, it sends an acknowledgement that indicates the secondary ready status.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 select mode is used when the primary device has something to send.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.If the primary device wants to receive the data from the secondary device, it asks the secondary device that they anything to send, this process is known as polling.Therefore, we can say that it is an initiator of the session. The primary device determines which device is allowed to use the communication channel.The primary device has control over the communication link, and the secondary device follows the instructions of the primary device.In this, the 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 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. 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. If the response to the ENQ is negative, then the sender disconnects and restarts the transmission at another time.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.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.įollowing are the responses of the receiver: The transmitter transmits the frame called an Enquiry (ENQ) asking whether the receiver is available to receive the data or not. Line Discipline can be achieved in two ways:ĮND/ACK stands for Enquiry/Acknowledgement is used when there is no wrong receiver available on the link and having a dedicated path between the two devices so that the device capable of receiving the transmission is the intended one.ĮND/ACK coordinates which device will start the transmission and whether the recipient is ready or not. It determines which device can send, and when it can send the data. Line Discipline is a functionality of the Data link layer that provides the coordination among the link systems.The Data link layer provides three functions: The Data link layer provides the coordination among the devices so that no collision occurs. 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. For example, In the half-duplex transmission mode, one device can only transmit the data at a time. Data Link Control is the service provided by the Data Link Layer to provide reliable data transfer over the physical medium.
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