ARQ). Because its ability to detect When a modem requires an operator to receive a call and switch errors is limited (two compensating errors within a single character from voice to data it is called attended operation. Unattended would go undetected) the next stage is to incorporate a checksum. operation is when the modem turns on its associated equipment and Here a number of characters are grouped together into a frame and commences operation from a distance, especially useful in over- certain bits of it are combined together according to a mathematical night data collection by a central computer from geographically algorithm to generate a checksum, often called a Cyclical Redun- dispersed locations. dancy Check (CRC). This is then transmitted with the frame. The receiver does the same computation on the transmitted frame and compares its own checksum with that transmitted to it. Depending 39.5.2.9 Auto call back and line restoral on the result it sends an acknowledgement or a retransmission request back to the originating modem. For the leased line application, transferring large amounts of data Although a typical 16 bit checksum is far more efficient at detect- between two points, a PSTN link can be used as a back up in the ing errors it cannot detect all. As the move towards standards has event of the leased line failure. Dial up to one or more destinations been consolidated these techniques have been incorporated into can be initiated automatically by the modem keeping the network standards. The two major players are the international CCITT V.42 on line. Leased line restoral is either attempted after the expiry of a standard and the Microcom Networking Protocol. V.42 uses a 32 bit simple time out function when the back up line is inactive, or the CRC option. modem incorporates active circuitry that monitors and tests the 39/12 Modems leased line continuously and restores at the first available oppor- computer, management packages offer port traffic monitoring and tunity. statics coupled with local and remot