Note that RTS/CTS is never used with broadcast or multicast traffic, nor for other
control packets (such as an ACK).
2.2.7 Physical Layer
The 802.11 WLAN standards specify the lowest layer of the OSI network model (physical)
and a part of the next higher layer (data link). A stated goal of the initial IEEE effort was to
create a set of standards which could use different approaches to the physical layer
(different frequencies, encoding methods, and so forth), and yet share the same higher
layers. They have succeeded, and the Media Access Control (MAC) layers of the 802.11a,
b, and g protocols are substantially identical. At the next higher layer still, all 802.11
WLAN protocols specify the use of the 802.2 protocol for the logical link control (LLC)
portion of the data link layer. In the OSI model of network stack functionality (Figure 2.4),
such protocols as TCP/IP, IPX, NetBEUI, and AppleTalk exist at still higher layers. Each
layer utilizes the services of the layers underneath. This section describes the nature of the
RF medium, some problems particular to it, and the solutions to those problems embodied
in the 802.11 standards.
Application
Presentation
Session
Transport
Network
Data Link
Layer
Physical
Network Protocols
(e.
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