s can occur where At is very much greater than the mean; these events are the ones that cause concern. The bottom graph in Figure 6.28 tells us how likely it is for the DGD to exceed a given value, At. From the point of view of optical receivers what matters is the relationship between the pulse separation and the bit period. BER can become unacceptable when the two pulses Polarization Mode Dispersion Normalized Differential Group Delay 1.E+00 1.E-02 1.E-04 Fig. 6.28 (a) Maxwellian 1.E-06 probability distribution for DGD, AT; (b) cumulative sum from At to infinity, as required in 1.E-08 calculating outage probabilities. The horizontal coordinates are Normalized Differential Group Delay defined so that the mean value, <AT>, is unity. arrive at the detector with times that are a sizeable fraction of the bit period. It is difficult to generalize about permissible performance because it depends on the modulation format used and other more subtle factors, such as interactions with amplifier noise and nonlinear optical effects in the fiber. However, a common rule of thumb is that the DGD becomes problematic when it exceeds one-tenth of the bit duration. (The criterion of 0.1 bit periods is recommended in ITU-T standard G.691 [ITU-T G.691]). The consequence of PMD for systems is that it is not possible to design a link to guar- antee continually available operation. Designers have to accept system "outages" on those rare occasions when the DGD exceeds a predefined BER criterion. Network operators must therefore specify a tolerable outage probability, Poutage/ for which commonly used values range between 30 minutes (= 5.7 X 10-5) and 1 second per year (= 3.2 X 10-8). Other useful defined parameters are the mean outage rate, Poutage (seconds-1) and the mean outage dura- tion, Toutage (seconds). The three quantities are related by Toutage = Poutage/Routage- Chapter 6 Optical Fiber Transmission First- and Higher-Order PMD If the bitrate is relatively low, the launched pulses are long and therefore have narrow spectral bandw