ed number location on power up, typically by raising the DSR signal As speeds of operation increase, modem links become more prone on the V.24 modem interface. Varying with available memory, to errors, because a line disturbance that corrupts one bit at 300bit/s storage is typically between 10 to 20 fourteen digit numbers, al- corrupts thirty two at 9.6kbit/s. At higher speeds, errors may less though up to 100 is available. For dialling behind a PABX, a pause obviously be generated by communications software or lower function in the dialling sequence is often still required, to allow the quality cables. public line to be accessed. Both the pulse dialling of the older Error correction need not necessarily take place in the modem. In Strowger telephone exchanges and the dual tone multifrequency fact, most dial up modems have an operating mode where error signalling of the new generation are standard features. Software correction is disabled. For example, if a workstation or computer programming can limit the modem to originating calls only. Stand- uses a built in synchronous error control protocol like that used by ards for automatic dialling include the proprietary Hayes 'AT' an X.25 Packet Assembler/Disassembler (PAD), data is presented to protocol, specifically designed for asynchronous personal com- the modem and just transmitted. Further error correction from the puters communications, or the CCITT's V.25bis command set modem would simply increase the overhead and degrade through- covering both asynchronous and synchronous environments. Ven- dors are finding it necessary to include both these, to satisfy user Where error correction does occur in the modem, the most basic demands on connectivity. form is the parity bit where an extra bit, either 1 or (), is added to each character SO that the sum of the bits is always odd or even. When bits are corrupted during transmission, the receiving modem 39.5.2.8 Auto answer can detect it and ask for a retransmission. This is known as Auto- matic Retransmission Request (