y acknowledgement 1. The receiver will later close down or reset links. be sent the retransmitted frame, still numbered 0 and can see 2. Supervisory frames control information flow, request retrans- immediately what has happened. missions and acknowledgement frames. They contain com- To avoid the sender having to wait for an acknowledgement mands such as Receive Ready from a DTE or DCE or a before transmitting the next frame, thereby limiting throughput due temporary busy condition, for example. to the time taken to transmit, check and process frames, X.25 level 3. Information frames are the only frames to transport data across 2 adopts a window size of seven. This means the sender can transmit the link. up to seven frames before an acknowledgement is required, allow- ing the receiver to be relatively slow and utilising the circuit more To establish the link under LAPB, either end of the DTE/DCE link efficiently. can begin link initialisation by sending out the relevant command, As data will normally flow in both directions on a link simulta- known as the Set Asynchronous Balanced Mode (SABM). Accept- neously (full duplex operation) different sequence numbers are ance of the SABM is confirmed by the other end when it issues an allotted for each direction. A device will therefore transmit and Unnumbered Acknowledgement response. Once this process is receive a mixture of information frames and acknowledgement complete, I-frames can begin to flow. frames. Some packet switch protocols, including X.25 level 2, can piggy- back an I-frame with an acknowledgement frame that acknowledges 54.4.2.2 Error correction and flow control a I-frame from the other direction. Although often difficult to implement, it reduces the total number of frames transmitted and As mentioned above, the Data Link Layer is responsible for error received. correction and flow control of I-frames. The most common tech- nique is known as positive acknowledgement. A recipient must The Modulo-N or window-N system of sequence numbering acknowledge that 