The definition of signal rate is number of signal elements (timewise shortest unit) sent in 1s. The book says that the signal rate of all 1s or all 0s may be different from a data pattern of alternating 0s and 1s. (And there can be three cases : best, worst and average) The strange thing is, it means that the signal rate changes in real time. Data rate and signal rate are interconnected by a formula : S = N/r How does the signal rate change dynamically even if N(data rate) and r(number of data elements per one signal element) are static?
Data Communications and networking (of Forouzan)