- Enquiry/acknowledgment (ENQ/ACK) is used in systems where there is no question of the wrong
- receiver getting the transmission, that is, when there is a dedicated link between two devices so that the only
- device capable of receiving the transmission is the intended one.
- In both half-duplex and full-duplex transmission, the initiating device establishes the session. In halfduplex, the initiator then sends its data while the responder waits. The responder may take over the link when
- the initiator is finished or has requested a response. In full-duplex, both devices can transmit simultaneously
- once the session has been established.
Working of ENQ/ACK
- The initiator first transmits a frame called an enquiry (ENQ) asking if the receiver is available to receive data.
- The receiver must answer either with an acknowledgement (ACK) frame if it is ready to receive or with a negative acknowledgement (NAK) frame if it is not.
- By requiring a response even if the answer is negative, the initiator knows that its enquiry was in face received even if the receivers currently unable to accept a transmission.
- If neither an ACK nor a NAK is received within a specified time limit, the initiator assumes that the ENQ frame was lost in transit, disconnects, and sends a replacement. An initiating system ordinarily makes three such attempts to establish a link before giving up.
- If the response to the ENQ is negative for the three attempts, the initiator disconnects and begins the process again at another time. If the response is positive, the initiator is free to send its data.
- Once all of its data have been transmitted, the sending system finishes with an end of transmission (EOT) frame.
