.2, the boundary curve C(bit/s/Hz) = f(SNRbit) defines two regions in the plane. If Rbit/s is the bit rate, these two regions correspond to Rbit/s < Cbit/s and Rbit/s > Cbit/s, respectively. The interpretation of the bandwidth-efficiency diagram is then: In the region Rbit/s < Cbit/s, there exist codes for which transmission can be achieved with arbitrary low error (BER); In the region Rbit/s > Cbit/s, there is no code making transmission possible at arbitrarily low error. To avoid any misinterpretation, it is always possible to transmit information with some finite or nonzero BER in any location of the diagram. However, only in the lower-right region Rbit/s < Cbit/s of the diagram can the BER be made arbitrarily small with a proper code choice code, leading to arbitrarily error-free transmission. The ratio SNRbit = Es/EN is often referred to in textbooks as Eb/No, where Eb = Es is the bit energy and No = EN is the noise energy or noise spectral density. Gaussian channel and Shannon-Hartley theorem We also observe from Fig. 14.2 that the boundary curve C(bit/s)/Hz = f(SNRbit) has an asymptotic limit at SNRbit = 1.6 dB, which is referred to as the Shannon limit. As the figure indicates, when the SNR approaches this lower bound, the ISD rapidly vanishes to become asymptotically zero as SNRbit 1.6 dB. On the right-hand side of the boundary curve, there exist some codes for which error-free transmission is possible. But as observed from the figure, the closer one approaches the Shannon-limit SNR, the lower the ISD and the smaller the number of bits that can be effectively transmitted per unit time and unit bandwidth. The limiting condition SNRbit = -1.6 dB corresponds to a useless channel with an ISD of 0 (bit/s)/Hz. The bandwidth-efficiency diagram suggests two strategies for optimizing the trans- mission of information: Horizontally, or at fixed ISD, by increasing the bit-SNR to reduce the BER; Vertically, or at fixed SNR, to increase the ISD and, hence, to increase the bit rate. At this point, we, thus, need to ad