Error Detection And Correction Methods-Cont

•     ERROR CONTROL Refers primarily to error detection and correction ŠAUTOMATIC REPEAT REQUEST (ARQ) Error control in data link layer is implemented simply: -Anytime     an     error     is     detected     in     an     exchange,     a     negative acknowledgement   (NAK)   is   returned   and   the   specified   frames   are retransmitted

Stop and Wait ARQ

o    It    is    a    form    of    stop-and-wait    flow    control    extended    to    include retransmission of data in case of Lost or Damaged frames o    For retransmission to work, 4 features are added to the basic flow control mechanism o    Sending device keeps a copy of the last frame transmitted until it receives the ACK for that frame o    Both data and ACK frames are numbered 0 and 1 alternately o    A  data  0  frame  is  acknowledged  by  a  ACK  1  frame  indicating  that  the receiver has received data 0 and is now expecting data 1 o    Sending device keeps a copy of the last frame transmitted until it receives the ACK for that frame o    Both data and ACK frames are numbered 0 and 1 alternately -A  data  0  frame  is  acknowledged  by  a  ACK  1  frame  indicating  that  the receiver has received data 0 and is now expecting data 1 ƒ     If an error is discovered in a data frame an NAK frame is returned NAK frames which are not sent tell the sender to retransmit the last frame ƒ     The sending device is equipped with Timer. If an expected ACK is not received within an allotted time period, the sender assumes that the last frame sent is lost and resends the frame

•     Lost Frame Any of the 3 frame types can be lost in transit:    Lost Data Frame    Lost ACK Frame    Lost NAK Frame