Explain how enquiry/acknowledgment works as a form of line discipline
What will be an ideal response?
Whether half- or full-duplex, before communication can occur, a session must be established between the two devices. If half-duplex, the devices have an additional requirement in that they must take turns using the line. The form of line discipline used in this scenario is called enquiry/acknowledgment (ENQ/ACK). Let's consider how this might work. The sending device, device A, initiates a communication by transmitting an enquiry frame to the receiving device, device B. Device A then waits to hear from device B if it is available and accepting communications. Device B, if available and ready for communication, transmits an acknowledgment frame to the sender, device A. At that point a session is established and device A begins its communication with B. When finished, device A transmits an end of transmission (EOT) frame, terminating the session. If device B were not available for communication, it would transmit an NAK (negative acknowledgment) frame to device A. Device A will have to try again later. If device B, because it is disconnected or down, does not send an acknowledgment frame to A, after a specified time device A will issue another enquiry to device B. After three failed enquiry attempts, A assumes that B is completely unavailable and terminates communication entirely until a later time.
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