K (kilo) or M (mega) can be used in reference to communication or storage --
and there's a slight difference in the value that depends on how it's used.
When I'm talking about data-transmission rate, kilo means one thousand and
Mega means one million. If, however, I'm talking about a storage device --
say, your hard drive -- then kilo refers to 1024 and mega refers to 1,048,576.
A small b used in an abbreviation refers to a bit; a capital B represents a byte.
Comparing advertised and
actual wireless speeds
So you're thinking that the 11Mbps speed of 802.11b is pretty fast, right?
Yeah, me too -- but that's a theoretical maximum speed. 11 Mbps won't be
the actual speed at which data is sent. No, it isn't a matter of truth in adver-
tising. Not exactly, anyway. Here's why:
The data isn't all that's traveling. A whole bunch of those bits being
blasted through the air form a virtual envelope, a sort of logical con-
tainer known as a packet. Your data is broken down into chunks, placed
in these packets, and sent. Each packet includes a lot of information
about where the data came from, where it's going, how to reassemble it,
and so on. Believe it or not, this extra information -- known as protocol
overhead -- can take up 30 or 40 percent of the data stream.
Speed varies depending on the environment the signal is traveling
through. The actual speed may, for a number of reasons, drop to as little
as 1 Mbps. Due to distance or interference, the quality of the signal might
not be sufficient to support a bandwidth of 11 Mbps. In that case, the speci-
fications of the standard mandate the use of dynamic rate shifting -- which
cuts the transmission speed in half, to 5.5 Mbps. If the conditions still
aren't sufficient to support the 5.5Mbps speed, then the data gets slowed
down (by dynamic rate shifting) until it reaches a data-transmission rate
that can be supported.
When all is said and done, no matter which standard you end up utilizing,
you shouldn't count on achieving more than 50 percent of the theoretical
maximum data-transmission rate. In reality it'll probably be even less than
that. Okay, now that I've got you sufficiently bummed out, I've got some good
news (no, I didn't save a bundle on insurance): A real-world speed of 4, 3, 2
or even 1 Mbps is still faster than you'll probably need for most of the things
you're going to want to do with Wi-Fi -- such as (for example) connecting to
the Internet.
Chapter 2: Selecting a Standard
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