gan to allow multiple unacknowledged frames to be outstanding, a process called pipelining. In pipelined systems, a maximum window size is agreed upon that defines the maximum number of frames that can ever be allowed to be outstanding at any point in time. If the number is reached, the window closes, closing down the pipeline and prohibiting other frames from being transmitted. As soon as one or more frames clear the receiver, that many new frames are allowed into the pipeline by the sliding window. Obviously this protocol is reliant on the ability to number the frames so that the system knows when the maximum win- dow size has been reached. OK, back to error recovery. We mentioned earlier that there are two common techniques. The first of these is called selective retransmit. In selective retransmit environments, if an error occurs, the transmitter is directed to resend only the errored frame. This is a complex technique, but is quite efficient. The second technique is called Go Back N. In Go Back N environ- ments, if an error occurs, the transmitter is directed to go back to the errored frame and retransmit everything from that frame forward. This technique is less efficient but is far simpler from an implementation point of view. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Chapter 2 Let's look at an example. Assume that a transmitter generates five frames, which are numbered one of five, two of five, three of five, etc. Now let's assume that frame three arrives and is found to be errored. In a selective retransmit environment, the transmitter is directed to resend frame three, and only frame three. In a Go Back N environment, how- ever, the transmitter will be directed to resend everything from frame three going forward, which means that it will send frames three, four, and five. The receiver will simply discard frames four