Chapter 14
Wireless LANs
14.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
14-1 IEEE 802.11
IEEE has defined the specifications for a wireless
LAN, called IEEE 802.11, which covers the physical
and data link layers.
Topics discussed in this section:
Architecture
MAC Sublayer
Physical Layer
14.2
Note
A BSS without an AP is called an ad hoc
network;
a BSS with an AP is called an
infrastructure network.
14.3
Figure 14.1 Basic service sets (BSSs)
14.4
Figure 14.2 Extended service sets (ESSs)
14.5
Figure 14.3 MAC layers in IEEE 802.11 standard
14.6
Distributed Coordination Function
DCF uses CSMA/CA as the access method
14.7
Figure 14.4 CSMA/CA flowchart
14.8
Figure 14.5 CSMA/CA and NAV
14.9
Point Coordination Function (PCF)
The point coordination function (PCF) is an optional
access method that can be implemented in an
infrastructure network (not in an ad hoc network).
PCF has a centralized, contention-free polling access
method, which we discussed. The AP performs polling
for stations that are capable of being polled.
14.10
Figure 14.6 Example of repetition interval
14.11
Fragmentation
The wireless environment is very noisy, so
frames are often corrupted. A corrupt frame
has to be retransmitted. The protocol, therefore,
recommends fragmentation—the division
of a large frame into smaller ones. It is more
efficient to resend a small frame than
a large one.
14.12
Figure 14.7 Frame format
FC: Frame control
D: The duration of the transmission
SC: Sequence control
14.13
Table 14.1 Subfields in FC field
14.14
Frame Types
Three categories of frames: management
frames, control frames, and data frames.
Figure 14.8 Control frames
Table 14.2 Values of subfields in control frames
14.15
Addressing Mechanism
The IEEE 802.11 addressing mechanism
specifies four cases, defined by the value of the
two flags in the FC field, To DS and From DS
Table 14.3 Addresses
14.16
Figure 14.9 Addressing mechanisms
14.17
Hidden station problem
Figure 14.10 Hidden station problem
14.18
Note
The CTS frame in CSMA/CA handshake
can prevent collision from
a hidden station.
14.19
Figure 14.11 Use of handshaking to prevent hidden station problem
14.20
Exposed station problem
It happens when a wireless node cannot transfer data because
another node that is outside its communication range is sending
data to another node that is inside it. Throughput and network
performance may suffer as a consequence.
14.21
Figure 14.12 Exposed station problem
14.22
Figure 14.13 Use of handshaking in exposed station problem
14.23
Table 14.4 Physical layers
14.24
Figure 14.14 Industrial, scientific, and medical (ISM) band
14.25
Figure 14.15 Physical layer of IEEE 802.11 FHSS
14.26
Figure 14.16 Physical layer of IEEE 802.11 DSSS
14.27
Figure 14.17 Physical layer of IEEE 802.11 infrared
14.28
Figure 14.18 Physical layer of IEEE 802.11b
14.29
