Data Communications Networking Devices - 4th Ed.pdf

120 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTScorrupted back into its original bit form when echoed to the transmitting device. Inaddition,a character received correctly may be corrupted during its echo,resultingin the false impression that an error occurred.Echoplex was a popular method for detecting transmission errors that was usedwith teletype terminals. This method of error detection was also used in suchmessage switching systems as TWX and is currently used with many types ofasynchronous transmission,including personal computers. Concerning the latter,a PC communicating in an asynchronous full-duplex mode to another full-duplexcomputer will have each character transmitted echoed back. Since the detection oferroneous characters,however,depends upon the visual accuity of the operatormore modern methods of error detection and correction have replaced the use ofechoplex in applications where we cannot rely upon an operator to correct errors.Thus,a large number of ®le transfer protocols that group characters into datablocks and append a checking mechanism were developed to automatically detectand correct transmission errors.Today,the most commonly employed method to correct transmitted errors is toinform the transmitting device simply to retransmit a block. This procedurerequires coordination between the sending and receiving devices,with thereceiving device either continuously informing the sending device of the statusof each previously transmitted block or transmitting a negative acknowledgementonly when a block is received in error.If the protocol used requires a response to each block and the block previouslytransmitted contained no detected errors,the receiver will transmit a positiveacknowledgement and the sender will transmit the next block. If the receiverdetects an error,it will transmit a negative acknowledgement and discard the blockcontaining an error. The transmitting station will then retransmit the previouslysent block. Depending upon the protocol employed,a number of retransmissionsmay be attempted. However,if a default limit is reached owing to a bad circuitor other problems,then the computer or terminal device acting as the masterstation may terminate the session,and the operator will have to re-establish theconnection.If the protocol supports transmission of a negative acknowledgement only whena block is received in error,additional rules are required to govern transmission.As an example,the sending device could transmit several blocks and,in fact,couldbe transmitting block n ‡ 4 prior to receiving a negative acknowledgementconcerning block n. Depending upon the protocol's rules,the transmitting devicecould retransmit block n and all blocks after that block or ®nish transmitting blockn ‡ 4,then transmit block n and resume transmission with block n ‡ 5.Types of protocolsNow that we have examined protocol tasks,let us focus our attention upon thecharacteristics,operation and utilization of several types of protocols that provide aprede®ned agreement for the orderly exchange of information. To facilitate thisexamination we will start with an overview of one of the simplest protocols in useand structure our overview of protocols with respect to their complexity.