tself permits automatic error correction as well as detection. This is referred to as forward-error correction. To do this effectively re- quires a large proportion of redundancy bits. Codes that give safe forward-error correction are, therefore, inefficient in their use of communication line capacity. If the communication line permitted the transfer of information in one direction only, then they would be extremely valuable. However, most systems use half-duplex or full-duplex links. Forward-error correction becomes advantageous when the number of errors is SO high that the retransmission of data 48 TELECOMMUNICATIONS necessary would substantially degrade the throughput. On higher- speed links, the argument for forward-error correction becomes stronger because the time taken for reversing the direction of trans- mission is equivalent to many bits of transmission. This time is rel- atively high on wideband links and on half-duplex, voice-grade links with high-speed modems. 3.12.3 Loop Check One method of detecting errors does not use a code at all. Instead, all the bits received are retransmitted back to their sender, and the sending machine checks that they are still intact. If not, then the item in error is retransmitted. Sometimes referred to as a loop check or echo check, this scheme is normally used on a full-duplex line or on a continuous loop line. A loop check is most commonly found on short lines and in-plant lines where the wastage of channel capacity is less costly. It gives degree of protection that is more certain than most other methods. 3.12.4 Effect of Errors On many data transmission systems, the control of errors is of vital importance. On some, it is not of great significance. Some systems transmit vital information, like accounting data, financial figures, military orders, encoded medical data or programs. These must be word perfect (or bit perfect). Other systems transmit the information that an operator is keying in at a terminal. The operator is likely to make far more errors than the data 