Error Detection And Correction Methods-Cont

SLIDING WINDOW ARQ Among several popular mechanisms for error control two protocols are important:    Go-back-n ARQ    Selective Reject ARQ Three features are added to sliding window flow control to allow for the retransmission of the lost or the damaged frames: o    The sending device keeps copies of the transmitted frames until all of them have been acknowledged o    In  addition  to  ACK  frames,  receiver  also  has  the  option  of  NAK  frames,  if  data  has been received damaged o    Because sliding window is a continuous TX mechanism, both ACK and NAK frames must be numbered for identification ƒ     ACK frames carry the number of the next frame expected •     ACK 5 tells sender that all frames up to frame 5 are received ƒ     NAK frames carry the number of the damaged frame itself •     If data frames 4 and 5 are damaged, NAK 4 and NAK 5 must be sent o    Like sender in stop-and wait ARQ, the sliding window ARQ is also equipped with a timer in the sender to deal with lost ACKs ™  Go Back n ARQ In  Go  Back  n  ARQ,  if  one  frame  is  lost  or  damaged,  all  frames  sent  since  last  frame acknowledged are retransmitted Go Back n- Damaged Frame    Go Back n- Lost Data Frame o    Sliding window requires that data frames be transmitted sequentially o    If one or more frames are so noise corrupted that they become lost in transit, the next frame to arrive at the receiver will be out of sequence ¾   Go Back n- Lost ACK o    When  the  window  capacity  is  reached  and  all  frames  allowed  have been sent, the sender starts a Timer o    If an ACK is not received before that Timer expires, sender retransmits all the frames since the last ACK