Nyquist’s theorem: D = 2BLog2K Provides an absolute maximum that cannot be achieved in practice. Engineers have observed that a real communication system is subject to small amount of background interference called noise, and that such noise makes it impossible to achieve the theoretical maximum rate. B = bandwidth (2B for the RS-232 that uses two values of voltage data) K = Possible values of voltages D = Maximum date rate in bits per second Shannon’s theorem: C = BLog2(1 +S/N) Shannon extended Nyquist’s work to specify the maximum data rate that can be achieved over a transmission system that introduces noise. C = Is the effective limit on the channel capacity in bits per second S = Average signal power N = Average noise power B = Bandwidth