Error control refers  to mechanisms to detect and correct errors that occur in transmitting frames.The most common techniques for error control are based on some or all of the following -
 

• Error Detection : CRC-32 / Parity bits
• Positive acknowledgement : The receiver reutrns a positive acknowledgement to succesfully  received error free frames.
• Retransmission after time-out: The sender retransmits all the frames within the window if acknowledgement is not received after a pre-determined amount time span.
• Negetive ack and retransmission: Destination returns a negative acknowldgement to the source. The source then retransmits the frame.

 These mechanisms, collectively refered to as automatic repeat request (ARQ) exist in the following versions.

Stop-and-Wait ARQ
 
Stop-and-Wait ARQ is based on the stop-and-wait flow control technique. The source station transmits a frame, and then must wait for an acknowledgement. No other data frames can be sent until the destination station’s reply arrives at the source station. Two possible errors could occur. First, the frame that arrives at the destination could have been damaged. The receiver detects this by using error detection technique and simply discards the frame. To account for such errors, the source station is equipped with a timer. If no acknowldgement is received by the timer expires, then the same frame is sent again. This method requires that the transmitter maintain a copy of the transmitted frame until an acknowledgement is received for that frame. The second error is a damaged acknowledgement. If an ack is damaged in transit and is not recognisable by the source station, it will timeout and resend the same frame. Thus duplicate frames arrive at the destination. To avoid discrepencies, rettransmitted frames are alternately labelled with 0 or 1, positive acks are of the form ACK0 nad ACK1. If a duplicate frame is received at the destination, the destination simply discards it , and sends the positive ack corresponding to the discarded frame.
 The principle advantage of Stop-and-Wait ARQ is its simplicity. Its priciple disadvantage is that it is an inefficient mechanism because one can’t afford to send an ack for every data frame received where the medium is noisy and the data rates low.

Go-Back-N ARQ

The form of error control based on sliding window flow control that is most commonly used is called Go-Back-N ARQ. In this scheme, a station may send a series of frames sequentially numbered modulo some maximum value( eg. 7 means 0 to 6 ) determined by the window size. While no errors occur, the destination will acknowledge ( RR, receive ready ) incoming frames as usual. If the destination station detects an error in the frame, it sends a negative acknowldgement ( REJ = reject ) for that frame. The destination station will discard that frame and all future incoming frames until the frame in error is correctly received. Thus the source station, when it receives an REJ must retransmit the frame in error plus all succeeding frames that were transmitted in the interim.

Selective Reject ARQ

In selective reject ARQ, the only frames retransmitted are those which receive a negative acknowledgement ( SREJ ). This is more efficient than Go-BackN because it minimises the amount of retransmissions, but mandates the receiver to maintain a buffer large enough to save post-SREJ frames until the frame in error is retransmitted, and it must contain logic for re-inserting that frame in the proper sequence. The transmitter and the receiver also require more complex logic to be able to send/receive a frame out of sequence. Because of such complications, selective reject is much less used than Go-Back N arq.