ght of Figure 6.13, the signal can lie on the wrong side of the threshold and in that case we have a bit error. A bit error can be either a 1 that should have been a 0 or a 0 that should have been a 1. Bit Error Rate Bitslot Bitslot Fig. 6.12 Three eye diagrams for NRZ transmission showing progressive deterioration of the bits because of noise. The diagrams become more complicated in the Bitslot presence of dispersion, jitter, and nonlinear effects. The bit error rate is defined as the mean number of errored bits divided by the total number received in a time interval that is sufficiently long to provide realistic averaging. Optical transmission systems are commonly designed to provide BER values that are better than 10 or 10-10. Indeed, recent standards documents recommend that performance be no worse than 10-12 (ITU-T, G.959.1), in some cases without the use of Forward Error Correction. Operation to achieve such values can be demanding. The point is illustrated by Chapter 6 Optical Fiber Transmission Decision Decision Instant Instant Fig. 6.13 Schematic of individual corrupted bits, showing the photocurrents at the logical 1, 0, and decision thresholds (/1, 10, and ID, respectively). (a) Slightly corupted bitstream: Most decisions are correct. (b) Badly corrupted bitstream: More decisions are wrong. comparing with transmission by wireless or (metallic) coaxial cable, where BER = 10-6 is considered to be good. Another perspective on optical systems is that when operating at 10 Gbit/s, a 10-12 BER implies that on average there will be one error every 1.7 minutes whereas systems that perform at BER = 10-15 have mean times between errors of 11.6 days. One consequence of such performance is that there is a practical limit to the BER values that can realistically be measured in experimental systems; indirect methods (Bergano, Kerfoot, and Davidson, 1993) or computer modeling often must be used to predict system performance limits. Formulating BER A straightforward formulation of BER can be provided if we are able to 