of Gaussian noise. Timing Decision Threshold Amplitude Decision Threshold Fig. 2.12 (a) Random binary pattern; (b) eye diagram. Transmission Performance In communication systems, performance measurements are required to evaluate the effi- ciency of the system. The main reasons for transmission errors are channel noise, echoes, signal fading, and errors due to loss of synchronization at the receiver. The following sub- sections introduce different performance measures commonly used in communication systems. Eye Diagram The eye diagram, which is also known as the eye pattern, is a time domain representation of a digital data signal from a receiver that is repetitively sampled and applied to the vertical input, while the datarate is used to trigger the horizontal sweep. Figure 2.12 shows a random binary sequence of data and the corresponding eye diagram. The binary pattern displayed in Figure 2.12(a) is noise free and without any jitter distor- tion. From Figure 2.12(b) it can be observed that the eye is completely "open" and correct Chapter 2 Data Transmission Fig. 2.13 (a) Random binary pattern with noise; (b) eye diagram with noise. Fig. 2.14 (a) Random binary pattern with jitter; (b) eye diagram with jitter. detection of binary sequences can be achieved if the amplitude decision threshold and the timing are located just in the middle of the opened eye. When noise is filtered in the trans- mission system, the eye becomes "closed." Figure 2.13 displays the binary pattern and its eye diagram with noise. From the eye diagram of Figure 2.13(b) it is clear that the noise effects are mainly in the superior and inferior part of the eye, closing the diagram and complicating the decision at the receiver. ISI can also be detected on these areas when the pulses are shaped after transmission over a lowpass channel. Another source of distortion that affects system trans- mission is jitter, which can be observed in the eye diagram as well. Figure 2.14 shows the effect of jittering in a binary pattern and its eye diagram. From 