 When calculated, the Shannon capacity of a narrowband line is formula_1, with formula_2 the (linear) signal-to-noise ratio.Narrowband phone lines have a bandwidth of 3,000 Hz so using formula_3 (SNR = 30 dB), the capacity is approximately 30 kbit/s.In the late 1990s, technologies to achieve speeds above 33.6kbit/s began to be introduced, colloquially referred to as 56k.