response signaling as the line cards 12 presently in use are capable of higher sampling rates but not higher resolutions this is a minimal cost solution to higher data rates than 56 kbps for example if for each symbol a k we transmit the symbol a k u2212 1 1 2732a k a k 1 is received then the received signal has significantly more levels compared to the transmitted signal but that is okay because only the number of transmit levels is constrained in the line card 12 the user modem 18 can sample with a much higher resolution the advantage of receiving a signal with controlled isi intersymbol interference can be seen as follows in fig1 a standard sinc pulse is plotted for a given bandwidth a sinc pulse gives the maximum symbol rate for no intersymbol interference in fig1 the pulse is sampled at twice the zero isi symbol rate so that the isi caused by sending data at twice the usual rate can be seen the power in the isi all coefficients of the pulse except the center coefficient in this case is equal to the power in the center coefficient of the pulse and is spread over many coefficients on either side of the center pulse it would be difficult to get a good signal to interference ratio from such a pulse the pulse shape in fig2 generates the controlled isi a k.