2.4GHz (802.11b)
The 802.11b standard was at one time the most widely deployed wireless standard, and it
operates in the 2.4GHz unlicensed radio band that delivers a maximum data rate of 11Mbps.
The 802.11b standard has been widely adopted by both vendors and customers who found
that its 11Mbps data rate worked pretty well for most applications. But now that 802.11b has
a big brother (802.11g), no one goes out and buys an 802.11b card or access point anymore,
because why would you buy a 10Mbps Ethernet card when you can score a 10/100 Ethernet
card for the same price?
An interesting feature of all Cisco 802.11 WLAN products is that they have the ability to
data-rate shift while moving. This allows the person operating at 11Mbps to shift to 5.5Mbps
and to 2Mbps and finally still communicate farthest from the access point at 1Mbps. And
furthermore, this rate shifting happens without losing connection and with no interaction
from the user. Rate shifting also occurs on a transmission-by-transmission basis. This is
important because it means the access point can support multiple clients at varying speeds
depending upon the location of each client.
The problem with 802.11b lies in how the Data Link layer is dealt with. To solve problems
in the RF spectrum, a type of Ethernet collision detection was created called
Carrier
Sense
Multiple Access with Collision Avoidance
(CSMA/CA), as shown in Figure 9.2.
FIGURE 9 . 2
802.11b CSMA/CA
CSMA/CA is also called a Request to Send, Clear to Send (RTS/CTS) because of the way
that hosts must communicate to the access point (AP). For every packet sent, an RTS/CTS and
acknowledgment must be received, and because of this rather cumbersome process, it’s kind
of hard to believe it all actually works!