Error Detection And Correction Methods-Cont

POLL o    Used by the primary device to receive transmissions from the secondary devices o    The secondaries are not allowed to TX data until asked o    By keeping control with the primary, the multipoint system guarantees that only one TX can occur at a time o    When the primary is ready to receive, data , it must ask (POLL) each device in turn if it has anything to send o    When the first secondary is approached, it responds either with a NAK frame if it has nothing to send or with data if it does o    If the response is negative, primary then polls the next secondary o    When the primary has received data, it acknowledges by sending an ACK ŠTwo possibilities for termination: -A secondary sends all its data and finishes with EOT frame -Primary says " Time is Up" ™  Flow Control -Definition Š  2nd aspect of data link control is Flow Control ŠIn most protocols, flow control is a set of procedures that tells  the  sender  how much data it can transmit before it must wait for an ACK from the receiver ŠThe flow of data must not be allowed to overwhelm the receiver