of a 10-6 bit error rate (BER). BER is the defining parameter of digital link performance; two digital links with comparable BERs are equally good. BER is simply the number of wrong bits divided by the total bit count over time. BER is expressed as a number between 0 and 1, so a channel with one bit wrong in a thou- sand has a BER of 0.001, or 10-3 in scientific notation. In casual usage, one hears of a BER of "3 percent" or "one in a million," but these refer to the fractional value of errors divided by the total. BER can vary widely from several percent on a noisy radio channel to some very small values in computer networks. A cellular telephone call voice channel has differ- ent digital accuracy requirements than a bank's computer network sending accounting data. Most digital systems send data grouped into frames, and one may hear the term frame error rate (FER), which is the fraction of frames with errors. Different architectures have different BER requirements. Robust error-correction al- gorithms may use more radio bits for the same signal data but tolerate higher error rates. And different modulation and error-correction schemes may be less tolerant bit errors that are close together. Since mobile telephones tend to produce their bit er- rors close together in time, in clumps, we can use a technique known as interleaving to spread the critical error-correcting bits over a longer period of time. One of the pleas- ant side effects of error-correction systems is that they give a good estimate of the BER because they know how many corrections they made. Error correction is discussed in more detail in Sec. 17.2. Another issue with highly processed digital signals is the time delay created by pro- cessing. Callers may not notice a half-second delay from speaker to listener, but they al- most certainly will notice 1.5 seconds of delay. It may take them a little while of "What? What?" to figure out that a round-trip radio link to the moon is faster than their cellu- lar telephone call. Wireless telephone users wil