Mega in Computer Network
 |
There are
several pitfalls you need to be aware of when working with the common units of
networking— MB, Mbps, KB, and Kbps. The first is to distinguish carefully
between bits and bytes. Throughout this book, we always use a lowercase b
for bits and a capital B for
bytes. The second is to be sure you are using the appropriate definition of
mega (M) and kilo (K). Mega,
for example, can mean either 220 or 106. Similarly, kilo
can be either 210 or 103. What is worse, in networking we typically use both definitions. Here’s why.
Network
bandwidth, which is often specified in terms of Mbps, is typically governed by
the speed of the clock that paces the transmission of the bits. A clock that is
running at 10 MHz is used to transmit bits at 10 Mbps. Because the mega
in MHz means 106 hertz, Mbps is usually also defined as 106 bits
per second. (Similarly, Kbps is 103 bits per second.) On the other hand, when
we talk about a message that we want to transmit, we often give its size in
kilobytes. Because messages are stored in the computer’s memory, and memory is
typically measured in powers of two, the K in
KB is usually taken to mean 210. (Similarly, MB usually means 220.) When you
put the two together, it is not uncommon to talk about sending a 32-KB message
over a 10-Mbps channel, which should be interpreted to mean 32 × 210 × 8 bits are being transmitted at a
rate of 10×106 bits per second.
This is the interpretation we use throughout the book, unless explicitly stated
otherwise. The good news is that many times we are satisfied with a back-of-the-envelope
calculation, in which case it is perfectly reasonable to pretend that a byte
has 10 bits in it (making it easy to convert between bits and bytes) and that 106
is really equal to 220 (making it easy to convert between the two definitions
of mega). Notice that the first approximation introduces a 20% error, while the
latter introduces only a 5% error. To help you in your quickand- dirty
calculations, 100 ms is a reasonable number to use for a cross-country
round-trip time—at least when the country in questionis the United States—and 1
ms is a good approximation of an RTT across a local area network. In the case
of the former, we increase the 48-ms round-trip time implied by the speed of
light over a fiber to 100 ms because there are, as we have said, other sources
of delay, such as the processing time in the switches inside the network. You can
also be sure that the path taken by the fiber between two points will not be a
straight line.
