定义：
通信链路中错误传输的比特所占的比例。
即使在数字传输系统中也不是没有误码的。统计涨落相关的噪声会造成一小部分传输比特会发生损坏。这种平均的比特错误率被称为比特误码率。一个传输链路的最大传输容量并不是在比特误码率最低的情况下（近于避免任何错误）达到的，而是在比特误码率在可容忍的程度下（通常在10-12到10-9之间）提高传输速率得到的。通过校对并改正错误的码元是可以得到几乎完全无误码的传输。例如，检测到的误码可简单通过重新发送被影响的数据包来修正。
在实际情况下，由于噪声（特别是在接收器中，当然也包括在发射器和放大器中）、光损耗以及各种色散的影响会提高光学数据传输系统中的比特误码率。此外，非线性效应还会导致信号失真和信道串扰。可以通过提高传输光功率、减少光纤损耗、在链路中加入光纤放大器、加入色散补偿、采用改进的探测器（如通过电子色散补偿）、优化波分复用系统中的波长分布、减少数据传输速率等方式减少比特误码率。
比特误码率的测量设备比较复杂，其通常被称为比特误码率测试器（BERT），通过产生一个伪随机序列，并比较接受到的数据来计算比特误码率。
Acronym: BER
Definition: average fraction of wrongly transmitted bits in a communication link
Even a digital data transmission system is not totally error-free – statistical fluctuations related to noise influences cause a small percentage of the transmitted bits to be corrupted. The average fraction of incorrectly transmitted bits is called the bit error rate. The maximum capacity of a reliable data transmission system is not reached by keeping the bit error rate at an extremely low level (nearly avoiding any bit errors), but by pushing the data rate to a level where some tolerable bit error rate (typically between 10−12 and 10−9) can be maintained. A virtually error-free transmission can still be achieved by detecting and correcting most of the remaining bit errors, e.g. based on check sums. For example, detected bit errors may be corrected simply by resending the few affected data packages.
In practice, the bit error rate of a system for optical data transmission (e.g. a fiber-optic link) can be increased by noise influences (particularly in the receiver, but also in the transmitter and in amplifiers), by optical losses, and chromatic and other types of dispersion. Also, nonlinearities can cause signal distortion and channel cross-talk. Possible countermeasures include using a higher transmitted optical power, reducing fiber propagation losses, employing fiber amplifiers and/or dispersion compensation in the link, using improved photodetectors (possibly with electronic dispersion compensation), optimizing the arrangement of wavelength channels in a wavelength division multiplexing system, or simply by reducing the data transmission rate.
Bit error rates are typically measured with complex devices, called bit error rate testers (BERT), generating a pseudo-random bit sequence and comparing the sent and received data.
