Data Link Control

1. Line discipline, flow control and error control

2. Line discipline oversees the establishment of links and the right of a particular device to transmit at a given time.
   • Enquiry/acknowledgment (ENQ/ACK) is used primarily in systems when there is no question of the wrong receiver getting the transmission. 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 acknowledgment (ACK) frame if it is ready to receive or with a negative acknowledgment (NAK) frame if it is not.

   • Poll/select works with topologies where one device is designated as the primary station and the other devices are the secondary stations. The primary is always the initiator of a session. Polling is when the primary asks a secondary if it has anything to send. Selecting is when the primary tells a secondary to get ready to receive.

3. Flow control is a set of procedures that tells the sender how much data it can transmit before it must wait for an acknowledgment from the receiver.
   • In stop-and-wait the sender waits for an acknowledgment after every frame it sends.

   • In the sliding window method, the sender can transmit several frames before needing an acknowledgment. Frames can be sent one right after another, meaning that the link can carry several frames at once and its capacity can be used efficiently. The receiver acknowledges only some of the frames, using a single ACK to confirm the receipt of multiple frames. 10. For a sliding window of buffer size n, there can be a maximum of n-1 frames sent but unacknowledged.

4. Error control refers to methods of error detection and retransmission.
   • Automatic repeat request (ARQ) is when frames are retransmitted when a NAK is is returned.

   • Stop-and-wait ARQ is a form of stop-and-wait flow control extended to include retransmission of data in case of lost or damaged frames.

   • Sliding window ARQ
     • In go-back-n ARQ, if one frame is lost or damaged, all frames sent since the last frame acknowledged are retransmitted.

     • In selective-reject ARQ, only the specific damaged or lost frame is retransmitted. The receiving device must be able to sort the frames it has and insert the retransmitted frame into its proper place in the sequence. This complexity requires a smaller window size than is needed by the go-back-n method if it is to work efficiently. This method is expensive and not often used.