Computer Networks 4th Ed Andrew S. Tanenbaum [Electronic resources] نسخه متنی

1.7 Metric Units

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. 1-39. The prefixes are typically abbreviated by their first letters, with the units greater than 1 capitalized (KB, MB, etc.). One exception (for historical reasons) is kbps for kilobits/sec. Thus, a 1-Mbps communication line transmits 10⁶ bits/sec 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 ''µ'' (the Greek letter mu) is for micro.

Figure 1-39. The principal metric prefixes.

It is also worth pointing out that for measuring memory, disk, file, and database sizes, in common industry practice, the units have slightly different meanings. There, kilo means 2¹⁰ (1024) rather than 10³ (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 2²⁰ (1,048,576) bytes, a 1-GB memory contains 2³⁰ (1,073,741,824) bytes, and a 1-TB database contains 2⁴⁰ (1,099,511,627,776) bytes. However, a 1-kbps communication line transmits 1000 bits per second and a 10-Mbps LAN runs at 10,000,000 bits/sec because these speeds are not powers of two. Unfortunately, many people tend to mix up these two systems, especially for disk sizes. To avoid ambiguity, in this book, we will use the symbols KB, MB, and GB for 2¹⁰, 2²⁰, and 2³⁰ bytes, respectively, and the symbols kbps, Mbps, and Gbps for 10³, 10⁶, and 10⁹ bits/sec, respectively.