What is the mathematical queueing theory, which states that long-term average number of customers in a stationary system equals long-term average effective arrival rate multiplied by the average time a customer spends in the system?

Result applies to any system, particularly to systems within systems. This law is widely used in processing to predict lead time (latency between initiation and completion) and similarly can be used to determine flow in software development. Alternatively, software testers can use the law to ensure that observed performance results are not due to bottlenecks imposed by the testing apparatus.

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Answer to yesterday's question: The unit is "baud" (Bd), named after Jean-Maurice-Émile Baudot, a French telegraph engineer. He is known for inventing the first digital communication means, the Baudot code.

A common confusion is that baud equals bit rate — however the two coincide only when each symbol carries a single bit. Once a symbol carries several bits, the bit rate (bits per second) climbs well above the baud rate (symbols per second). This is why "my modem is 56,000 baud" is usually wrong: that's a bit rate.