(Recall from the section Single-Channel Transmission that in NRZ signaling the transmitter laser is turned off for the full duration of the Os and on for the full duration of the 1s.) An eye diagram is formed by repeatedly sampling and super- posing all of the possible bits that could form a short sequence of the signal in the time domain. For example, for three bits the combinations are 000, 001, 010, 111. If the bits are uncorrupted (such as at the top of Figure 6.12) the pattern that is formed resembles an open eye (hence the name). However, if there is much noise, timing errors, pulse spreading due to dispersion, and SO forth, the bit degradation is clearly displayed through a closure of the eye. The characteristics of an eye diagram give a useful insight into the effects that are likely to cause bit errors. Although the precise details can differ in many ways, if we were to sample one of the channels at (1) the transmitter output, (2) around the midway point in the system, and (3) at the receiver end, we would often expect to obtain eye diagrams that resemble those in Figure 6.12. Definition of BER When a corrupted bitstream arrives at the decision circuit in the receiver it is in the form of a photocurrent, as shown schematically in Figure 6.13. The decision circuit sets a threshold value, ID, and measures the current at an appropriate instant within the bitslot, as indicated by the arrows. If the current is above ID, the symbol is interpreted as a 1; if below ID, as a 0. The threshold current could be arbitrarily chosen to be at the 50 percent level but, as will be argued, other values can improve the number of correct decisions. The decision instant is chosen to be at a point in the bitslot where there is the clearest differentiation between 1 and 0. For example, in NRZ signaling the most obvious choice is in the center and, in terms of Figure 6.12, this is when the eyes are wide open. The vast majority of the bits are correctly assigned to their proper values. However, once in a while, as shown on the ri