Information Technology Reference
In-Depth Information
Because both S1 and S2 are connected to the Internet, these ma
chines are called
access points
. (We will consider wireless networks
without access points later in the chapter.)
Let's suppose that we have just switched on both S1 and S2; L1
and L2 remain off. During initialization, both S1 and S2 announce
their presence to the nearby world by generating signals on the 2.4
GHz radio frequency band. Each signal is short, giving the identifi
cation number and other characteristics of these access points.
These signals are repeated at frequent, but random, intervals, and
this repeating of identifying information is called a
beacon
. By re
peating the signals frequently, other computers in the neighborhood
can learn of these access points in a timely manner. Although bea
cons are allowed on several different frequencies, each access point
must choose a frequency, and it might happen that two access
points could choose the same frequency. (As in Figure 8.9, these ma
chines may be out of radio range of each other, so they may not
know whether they choose the same frequency.) However, because
beacon signals are short and repeated at random intervals, other
computers in the area will be able to hear separate beacons even if
their accesspoint machines happen to choose common frequencies.
To summarize so far, when access points are turned on, they start
up beacons, so others can know the access points are available for
communication.
Now, let's turn on laptop L1. L1 will scan the relevant radio fre
quencies for beacons and determine that S1 is nearby. L1 then sends
an
association
message to S1, indicating that it wants to make a con
nection and giving its station identity code. A connection also may
involve special encryption instructions for security. If encryption is
required for the connection, then L1 also would supply that informa
tion. (We will consider questions regarding security in Chapter 10,
so we'll ignore encryption and security matters for now.)
Following L1's association message, access point S1 responds by
authorizing communication and providing various details for subse
quent transmissions, such as information regarding frequencies to
be used for messages and transmission rates. With this information,
L1 and S1 can begin exchanging data over this wireless connection.
At this stage, radio waves take the place of the cables in an Ethernet,
and material can flow smoothly between these two computers. For
example, if the laptop user wants to access a Web page on the
Internet, the user types in an appropriate address, L1 sends a re
