Idle RQ

Idle RQ: (also known as send & wait or stop & wait)

Idle RQ technique is a stop and go protocol, a fundamental backward correction scheme used in many protocols. The data is transfered in packets by using error detection.

The receiver checks the incoming packets and sends an acknowledgement (ACK) to the sender if the packet was valid. If the sender receives an acknowledgement in the specified time it sends the next packet to the receiver. Otherwise, the sender must resend the packet. Idle RQ is very simple but it is often too inefficient. It can only send data to one direction at a time. In addition, the delay on the data transfer may result in situation where only a small fraction of the capapacity of the communications link is used.

Elements of Idle RQ

P (primary): source DTE

S (secondary): destination DTE

I-frame: data or information frame

ACK frame: acknowledges I-frame correctly received

NAK frame: acknowledges I-frame not received (corruption detected)

Idle RQ Strategy

-         Used to reliably transfer a frame or block ofdata.

-         The strategy will allow the data to betransferred without error or replication.

-         The data frames are referred to as I-Framesor information frames.

-         The Idle RQ operates in the half duplex modesince the source and the destination do nottransmit at the same time.

Type of Idle RQ

1.     Implicit Retransmission

-         The destination will only ACK correctly receivedframes.

-         The source interprets the absence of an ACK as

an indication that the previous frames was

corrupted.

2.     Explicit Request

-         If the destination detects that a frame is corrupteda negative acknowledgement is sent to explicitlyrequest a retransmission.