To avoid any confusion, it is worth stating explicitly that in this book, as in computer science in general, metric units are used instead of traditional English units (the furlong-stone-fortnight system). The principal metric prefixes are listed in Fig. 1-31. The prefixes are typically abbreviated by their first letters, with the units greater than 1 capitalized. Thus a 1-TB database occupies ${10}^{12}$ bytes of storage and a 100-psec (or 100-ps) clock ticks every ${10}^{-10}$ seconds. Since milli and micro both begin with the letter ‘‘m,’’ a choice had to be made. Normally, ‘‘m’’ is for milli and ‘‘$\mu$’’ (the Greek letter mu) is for micro.

It is also worth pointing out that, in common industry practice, the units for measuring memory sizes have slightly different meanings. There kilo means $2^{10}$ (1024) rather than ${10}^3$ (1000) because memories are always a power of two. Thus a 1-KB memory contains 1024 bytes, not 1000 bytes. Similarly, a 1-MB memory contains $2^{20}$ (1,048,576) bytes and a 1-GB memory contains $2^{30}$ (1,073,741,824) bytes. However, a 1-Kbps communication line transmits 1000 bits per second and a 1-Gbps LAN runs at 1,000,000,000 bits/sec because these speeds are not powers of two. Unfortunately, many people tend to mix up these two systems, especially for SSD or disk sizes. To avoid ambiguity, in this book, we will use the symbols KB, MB, and GB for $2^{10},2^{20},$ and $2^{30}$ bytes, respectively, and the symbols Kbps, Mbps, and Gbps for ${10}^3,{10}^6,$ and ${10}^9$ bits/sec, respectively.