Chapter 15
Wireless
LANs
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Chapter6: Outline
15.1 INTRODUCTION
15.2 IEEE 802.11 PROJECT
15.3 BLUETOOTH
15-1 INTRODUCTION
Wireless communication is one of the fastestgrowing technologies.
The demand for connecting devices without the
use of cables is increasing everywhere.
15.3
Figure 15.1: Isolated LANs: wired versus wireless
15.4
Figure 15.2: Connection of a wired LAN and a wireless LAN to other
networks
15.5
15.15.3 Access Control
An important issue concerning a wireless LAN is
access control—
Answering the question of how the wireless host can
get access to the shared medium.
The shared medium is free space for a wireless LAN.
15.6
15.15.3 Access Control
The CSMA/CD algorithm does not work in wireless
LANs for many reasons including these:
1.Wireless signal amplitude degrades proportional to
the inverse square of the distance. This makes relative
amplitudes somewhat meaningless with wireless
communication.
1.The hidden station problem prevents collision
detection.
15.7
Figure 15.3: Hidden station problem
15.8
15-2 IEEE 802.11 PROJECT
IEEE has defined the specifications for a
wireless LAN, called IEEE 802.11, which
covers the physical and data-link layers. It is
sometimes called wireless Ethernet.
15.9
15.2.1 Architecture
The standard defines two kinds of services:
BSS, the basic service set, and
• ESS, the extended service set (ESS).
•
15.10
Figure 15.4: Basic service sets (BSSs)
15.11
Figure 15.6: Extended service set (ESS)
15.12
15.2.2 MAC Sublayer
Like wired Ethernet, IEEE 802.11 defines two
sublayers within the data-link layer:
LLC (logical link control) handles framing, error
control, flow control.
MAC (media access control) physical addressing
15.13
15.2.2 MAC Sublayer
IEEE 802.11 defines two sublayers withing the MAC
sublayer:
DCF the distributed coordination function, and
PCF and point coordination function.
15.14
15.2.2 MAC Sublayer
DCF the distributed coordination function, and
•Uses CSMA/CA as the access method
•Implements a persistence strategy with expbackoff
•Implements a DIFS after the media is idle.
15.15
15.2.2 MAC Sublayer
DCF the distributed coordination function, and
PCF and point coordination function.
15.16
Figure 15.6: MAC layers in IEEE 802.11 standard
15.17
DIFS & SIFS


DIFS
SIFS
<= 50 microseconds
<= 10 microseconds
Figure 15.7: Distributed Coordination Funcion: DCF
NAV
15.19
PCF

Point Coordination Function, optional layer
to allow for transmissions needing higher
priority (quality of service)
Figure 15.8: Example of repetition interval
15.21
Figure 15.9: The frame format has 9 fields
15.22
Frame Format





FC = Frame control
D = duration
SC = sequence control used for
“fragmentation”
Frame body up to 2312 bytes.
FCS = CRC error detection
Table 15.1: Subfields in FC field
15.24
Figure 15.10: Control frames
15.25
Table 15.2: Values of subfields in control frames
15.26
15.2.3 Addressing Mechanism
The IEEE 802.11 addressing mechanism specifies
four cases, defined by the value of the two flags in the
FC field.
Each flag can be either 0 or 1, resulting in four
different situations.
15.27
Table 15.3: Addresses
15.28
Figure 15.11: Addressing mechanisms
15.29
Figure 15.12: Exposed station problem
15.30
802.11-Physical Layer
The unlicensed frequency bands in these three ranges
902–928 MHz,
2.400–4.835 GHz, and
5.725–5.850 GHz.
Are known as the ISM bands, Industrial, Scientific
and Medical.
15.31
Table 15.4: Specifications (DSSS = Direct Sequence...)
15.32
OFDM


Orthogonal Frequency Division
Multiplexing
OFDM uses QAM for modulation.
15-3 BLUETOOTH
Bluetooth is yet another wireless LAN
technology requiring short distances between
stations.
15.34
15-3 BLUETOOTH
A Bluetooth LAN is an ad hoc network.
The devices, (aka gadgets), find each other and
make a network called a piconet.
15.35
15.3.1 Architecture
Bluetooth defines two types of networks:
piconet and
● scatternet.
●
15.36
Bluetooth Piconet

Up to 8* stations


One station is designated the primary
The other stations are secondary
Bluetooth Piconet


The secondary stations synchronize their
clocks with the primary station.
The secondary stations receive their
hopping sequence from the primary
station.
Bluetooth Piconet


Communication is one-to-one or
One-to-many.
Bluetooth Piconet

While limited to seven “active” secondary
stations. It is possible to put a station in a
“parked state” to allow another station on
the piconet.
Figure 6.17: Piconet
15.41
Figure 15.18: Scatternet
15.42
Bluetooth V1





1 Mbps bandwidth
2.4-2.483GHz (overlaps with 802.11b & g)
FHSS – 1600 hops per second
79 frequency channels of 1MHz each
FSK modulation
Bluetooth V2

3 Mbps bandwidth
Bluetooth V3

24 Mbps bandwidth

Bluetooth establishes the link and uses 802.11
to achieve the 24Mbps data rate.
Bluetooth Range



Power Class 1
Power Class 2
Power Class 3
20db
4 db
~100 m
~10 m
~1 m
15.3.2 Bluetooth Layers
Bluetooth uses several layers that do not exactly
match those of the Internet model defined in the text
book. Figure 15.19 shows these layers.
15.47
Figure 15.19: Bluetooth layers
15.48
Bluetooth Access Method

Polling-select


The primary poles the secondary stations on
even clock cycles.
If a secondary is polled, it transmits on the
next odd clock cycle.
Figure 6.21: Single-secondary communication (diagram error!)
microseconds, not milliseconds
15.50
Figure 6.22: Multiple-secondary communication
15.51
Figure 6.23: Frame format types
15.52
Bluetooth V1 Throughput



1-slot frames, 192-Kbps
3-slot frames, 596-Kbps
5-slot frames, 730.484 Kbps