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Chapter 5. Data Link Layer Concepts and ... > 22. Hold it! What is a modulo system... - Pg. 189

Chapter 5: Data Link Layer Concepts and IEEE Standards 189 19. This is pretty straightforward. Is this what is used in most networks today? Yes, the stop-and-wait protocol is very straightforward and it is also very effective. However, it is not very practical for modern networking environments and hence is rarely (if at all) implemented. First, as described, stop-and-wait uses a simplex transmission; data frames flow in only one direction. In most data communication environments, data trans- mission is full-duplex. Second, the protocol is ideal when transmitting large frames. Unfortunately, large frames are generally partitioned into smaller data units to accommo- date a receiver's limited buffer size. Small frame sizes also facilitate faster error detection and reduce the amount of data that requires retransmission in the event that an error is detected. (Error-control concepts are discussed later in the chapter.) 20. Well, then, how about a more practical example? OK. An enhancement to the stop-and-wait protocol is the sliding window concept, which improves data flow by having the receiver inform the sender of its available buffer space. The sliding window concept improves data flow by having the receiver inform the sender of its available buffer space. Doing so enables the sender to transmit frames contin- uously without having to wait for acknowledgments to these frames as long as the number of frames sent does not overflow the receiver's buffers. The sliding window concept is implemented by requiring the sender to sequentially number each data frame it sends and by having the sender and receiver maintain information about the number of frames they can respectively send or receive. Flow-control protocols based on this concept are called sliding window protocols and are used in one form or another in many contemporary net- working applications.