To understand how distributing the gain influences noise performance, we can compare two strategies that provide the same end-to-end power budget. In the first we use a distrib- uted FRA of gain Gnet; in the second, there is a passive fiber of the same length, L, and signal loss, As, followed by a discrete amplifier. In order to achieve the same output power, the discrete amplifier must have a gain, Gon-off. The noise figure of the distributed FRA is given by equation 6.9. In contrast, by equation 6.6, the total noise figure when using the discrete amplifier must be Fdiscrete exp(+asL). We now pose the question: What must the noise figure of the discrete amplifier be to give the same value as the distributed Raman amplifier? The answer is provided by equa- tion 6.10, equating (6.11) When this applies, Fdiscrete is termed the "effective noise figure" of the distributed FRA and, by using equation 6.10, it is given by (6.12) which shows the importance of on-off gain. Feff is smaller than Fint and in realistic circum- stances it is commonly less than unity. (In other words, when expressed in decibels, it is often a negative number.) Bit Error Rate The possibility of a negative (decibel) value of an effective noise figure may be a sur- prise, but it is important to realize what it means. It does not imply that the Raman amplifier reduces the system's noise in any way, but it tells us how much better the distributed gain is than a passive lossy fiber followed by a discrete amplifier. This is consistent with our general observation from the Friis formula that the best overall noise performance can be obtained by bringing the gain element as close as possible to the transmitter end of the system. The argument presented here partly explains why, despite the presence of RBS, Raman amplifiers have received intense attention in optical systems research laboratories and have been extensively commercialized. Bit Error Rate The fundamental purpose of an optical transmission system is to carry data from one loca- tion to another