The sender allows one message to be transmitted, checked for errors and an appropriate ACK (Affirmative Acknowledgement) or NAK (Negative Acknowledgement) returned to the sending station. No other data messages can be transmitted until the receiving station sends back a reply, thus the name STOP and WAIT is derived from the originating station sending a message, stopping further transmission and waiting for a reply.
Its major drawback is the idle line time that results when the stations are in the waiting period. If the ACK is lost then the sending station retransmits the same message to the receiver side. The redundant transmission could possibly create a duplicate frame. A typical approach to solve this problem is the provision for a sequence number in the header of the message. The receiver can then check for the sequence number to determine if the message is a duplicate. The Stop and Wait mechanism requires a very small sequence Number, since only one message is outstanding at any time. The sending and receiving station only use a one bit alternating sequence of 0 and 1 to maintain the relationship of the transmitted message and its ACK/NAK status.
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