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 below. The prefixes
are typically abbreviated by their first letters, with the units greater than 1
capitalized (KB, MB, etc.). One exception (for historical rea-sons) is kbps for
kilobits/sec. Thus, a 1-Mbps communication line transmits 10 6 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
used for milli and ‘‘μ’’ (the Greek letter mu) is used for
micro.
Exp. Explicit Prefix
10
−3 0.001 milli
10
−6 0.000001 micro
10−9 0.000000001 nano
10-12 0.000000000001 pico
10-15 0.000000000000001 femto
10-18 0.0000000000000000001 atto
10-21 0.0000000000000000000001 zepto
10-24 0.0000000000000000000000001 yocto
103 1,000 Kilo
10
6 1,000,000 Mega
10
9 1,000,000,000 Giga
10
12 1,000,000,000,000 Tera
10
15 1,000,000,000,000,000 Peta
10
18 1,000,000,000,000,000,000 Exa
10
21 1,000,000,000,000,000,000,000 Zetta
10
24 1,000,000,000,000,000,000,000,000 Yotta
It is also worth
pointing out that for measuring memory, disk, file, and data-base sizes, in
common industry practice, the units have slightly different mean-ings. 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. Note also the
capital ‘‘B’’ in that usage to mean ‘‘bytes’’ (units
of eight bits), instead of a
lowercase ‘‘b’’ that means ‘‘bits.’’ Similarly, a
1-MB memory contains 2 20
(1,048,576) bytes, a 1-GB memory contains 2 30
(1,073,741,824) bytes, and a 1-TB
database contains 2 40 (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, GB, and TB for 2 10
, 2 20 , 2 30 , and 2 40 bytes,
respectively, and the symbols kbps, Mbps, Gbps, and Tbps for 10
3 , 10 6 , 10 9 , and 10 12
bits/sec, respectively.
Taken from:- COMPUTER NETWORKS, FIFTH EDITION, ANDREW S. TANENBAUM
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