signals. A bit is the minimum unit of information of a digital signal. The bit can be repre- sented by a 1, if the continuous level is high, or by a 0, if the continuous level is low. If an alternating sequence of 1s and Os is considered as a digital signal, "1010 ," to be transmit- ted through the system, a datarate will characterize the signal. In this case, the duration of each bit is half the period of repetition. From the frequency analysis, the difference between the highest frequency and the lowest frequency will represent the transmission bandwidth of the signal. Figure 2.6 illus- trates the spectrum of data signal for a square wave at 2.5 GHz. In this case, the frequencies f, 3f, 5f, and 7f were used, where f is 2.5 Gbit/s. Therefore, the bandwidth of the transmitted signal would be 7f - f = 6f = 15 GHz. In general terms, to transmit a higher datarate through a system, a higher transmission bandwidth is required. The next section gives a more detailed relationship between datarate, channel capacity, bandwidth, and signal-to-noise ratio. Data Transmission Basics Channel Capacity Channel capacity is the upper bound on the amount of information that can be transmitted over a transmission channel. According to the Nyquist (1928) theory, a noise-free channel will be constrained only by the bandwidth of the signal. Therefore, the highest signal rate that can be transported is 2B for a given bandwidth of B. Signals at higher rates will deterio- rate because of delay distortion and ISI. If multilevel signaling is used to represent the data, the Nyquist formula will be C = 2Blog21 (2.3) where M is the number of discrete signal or voltage levels. The datarate can be increased for a given bandwidth by increasing M. However, the receiver becomes more complex because of the requirement of distinguishing one of M pos- sible signal levels. Furthermore, the addition of noise will limit the practical value of M for the transmission. Shannon's capacity formula states that the capacity, in bits per second, of a channel is 