decision will have to be made in the presence of any distortion and interference superimposed on the data sequence during transmission. The decision-making process will be more reliable if it is timed by a clock that distinguishes when each received symbol has reached an optimum value. The accuracy with which the decision threshold and sample times must be placed will depend on the severity of distortion suffered during transmission. Hence, another stage at output is used to extract the clock that produces a retimed baseband waveform. This clock signal at the receiver will sample the binary pulses at a specific time (see Figure 2.9). If at the sampling time the amplitude of the arriving signal is lower than the amplitude threshold, a 0 bit will be detected. On the other hand, if at the sampling time, the amplitude of the arriving signal is higher than the amplitude threshold, a 1 bit will be identified. The quality of the received signal will depend on the accuracy of the detection process, which will require the signal to be distinguished from the distortion and noise suffered during transmission. In general terms, channels with wide bandwidths will relax the timing and threshold requirements. However, a wider bandwidth allows more noise into the receiver; therefore, a compromise has to be considered. Modulation The processing of a signal to make it suitable for sending over a transmission medium is called modulation. The following are the reasons for using modulation: 1. Frequency translating (e.g., when an audio frequency baseband signal modulates a radio frequency carrier) Data Transmission Basics Fig. 2.9 (a) Binary signal after the channel (filter); (b) reconstructed signal after the receiver threshold. 2. Improving SNR by increasing the bandwidth (e.g., using frequency modulation) 3. Multiplexing (i.e., enabling many baseband channels to share the same wideband transmission path) Modulation schemes can be classified into two major categories, analogue and digital, based on the nature of the data. Analogue 