transmission line, especially if he is an unskilled one-finger operator. In this case, there is no point in worrying too much about the line. Accuracy controls can be de- vised for human input. On many systems a tight network of controls is necessary to stop abuse or embezzlement. It is also important to ensure that nothing is lost or double-entered when hardware failures occur on the system or when switchover takes place. In designing a system, it is important to know what error rate is expected. Calculations should then be done to estimate the effect of this error rate on the system as a whole. On some systems the effect of infrequently occurring error is cumulative, and it is in situations PROTOCOLS AND CODES 49 such as these that special care is needed in eliminating errors. For example, if messages cause the updating of files, and an error in the message causes an error to be recorded on the file, then it is possible that as the months pass the file will accumulate a greater and greater number of inaccuracies. 3.13 FRONT-END PROCESSORS The most important use of programmable communications proces- sors today is front-end processing, in which the processor handles all message control activities, and performs enough preprocessing to relieve the call of the communications housekeeping activities. The concept of front-end processing essentially involves off-load- ing or removing the data communications control function from the central processing unit and setting it up as an external, largely self- contained system. This decentralized approach permits both the communications and central processors to perform their primary functions in parallel and with little interference. Data is passed be- tween the processors only when necessary and with a high degree of efficiency. A typical front-end processor might control a hundred or more communications lines of varying speeds and types attached to a large number of diverse remote terminals. The front-end processor would ideally assume all terminal, line, buffering, and m