e over analogue telephone lines are approached. Controlled by the transmission standard invoked on call set up, Data compression relies on encoding recurring sequences more typically both asynchronous and synchronous operation is provided efficiently. If more information can be represented by fewer bits, the within a single unit. Synchronous operation is usually protocol faster it can be transmitted. Although vendors data sheets always transparent, whereas asynchronous operation will be possible over quote the maximum compression rates, they vary with the type of a variety of character lengths, parity and stop bit combinations data being transmitted and the quality of line being used. The two under software programme control. standards battling it out in the market-place are again the V.42bis and the MNP standard, currently at level 10. 39.5.2.4 Equalisation 39.5.2.7 Call control Equalisation circuitry compensates for the distortion caused as different frequencies travel at different rates down a transmission Originally dial up operations could only be performed by a tele- line changing the shape of the signal waveform. Fixed equalisers phone attached directly into the modem, the use of internal relays used in lower speed modems are designed to compensate for aver- preventing the phone from disturbing the data call, once it was in age line conditions, which include frequency response and envelope progress, as well as allowing normal phone operation at all other delay. For higher speed operation, automatic adaptive equalisers are times. It is now common to find manual call control possible by used which continually re-adjust for changes in line quality, espe- using front panel buttons which initiate a call stored in internal cially useful in dial up operation. registers. Automatic calling can be programmed through the con- nected terminal, PC or workstation SO that a number, or sequence of numbers, is dialled on a typed command. 39.5.2.5 Error correction For even simpler usage, some can initiate dialling to a stor