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
Architecture
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Provide two kinds of services
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The Basic Service Set
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14.3
The Basic Service Set (BSS)
The Extended Service Set (ESS)
Made up of stationary or mobile wireless
station and the access point (AP)
The BSS is categories by without AP and with
AP
Note
A BSS without an AP is called an ad hoc
network;
a BSS with an AP is called an
infrastructure network.
14.4
The BSS without AP
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14.5
This is a stand-alone network
Cannot send a data among BSSs
They can locate one another and agree to
be part of a BSS
Figure 14.1 Basic service sets (BSSs)
14.6
Extended Service Set
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Made up of two or more BSSs with APs
The BSS are connected through a
distribution system
The distribution system connect APs in the
BSSs
The stations can be
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14.7
Mobile – normal station in a BSS
Stationary – refers as AP stations where it is
part of wired LAN
Figure 14.2 Extended service sets (ESSs)
14.8
Station Types
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IEEE 802.11 defines three types based on
mobility in a wireless LAN :
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No transition
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BSS-transition
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Move from one BSS to another
BUT the movement is inside one ESS
ESS-transition
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14.9
Not moving
Moving only inside a BSS
Move from one ESS to another
MAC Sublayer
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IEEE 802.11 defines two MAC Sublayers
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Figure 14.3 shows the relationship
between two MAC sublayers
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14.10
DCF (Distributed Coordination Function)
PCF (Point Coordination Function)
The LLC sublayer
The physical layer
Figure 14.3 MAC layers in IEEE 802.11 standard
14.11
DCF (Distribution Coordination
Function)
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Uses CSMA/CA as access method
Why not CSMA/CD ?
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14.12
When collision is detected, a station must be
able to send and receive a collision signal at
the same time. This can mean costly station
and increased bandwidth requirements
If there are the hidden station problems, a
collision may not be detected
The distance between stations are great so
signal fading could prevent from hearing a
collision
Figure 14.4 CSMA/CA flowchart
14.13
CSMA/CA Flowchart
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Before sending a frame
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14.14
Uses Persistence strategy with back-off until
the channel idle
If the channel is idle, wait for a period of time
called the DIFS (Distributed Interframe
Space)
Sends the control frame called the RTS
(Request To Send)
Continue …
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After received the RTS
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14.15
Waiting a period of time called the SIFS
(Short Interframe Space)
The destination station sends a control frame
called the CTS (Clear To Send) to the source
station
This control frame indicates the destination
station is ready to receive a data
Continue …
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After waiting an amount of time equal to
SIFS, the source station starts send a data
At destination station
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14.16
After waiting an amount of time equal to
SIFS, send an acknowledgement** to show
the time frame is received
**Acknowledgement is the way to inform
about arrival packet of data
Figure 14.5 CSMA/CA and NAV
14.17
NAV (Network Allocation Vector)
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How do other stations defer sending their
data if one station acquires access? In
other words, how is the collision
avoidance aspect of this protocol
accomplished? The key is a feature called
NAV.
14.18
Continue ..
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When a station send the RTS frame
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Collision during handshaking
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14.19
It includes the duration of occupy for the
channel
This station create a timer called NAV that
shows how much time must pass before other
stations are allowed to check the channel for
idleness
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It is happened during when RTS and CTS are
in transition
Uses back-off strategy and sender tries again
PCF (Point Coordination
Function)
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14.20
Optional access method and can be
implemented in an infrastructure network
It is also implemented on top of DCF
Mostly used for time-sensitive
transmission
Continue …
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PCF has a centralised and contention-free
polling access
The AP performs polling for the stations and
the stations sending data to the AP
To set priority to PCF, interframe spaces have
been defined
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14.21
PIFS – shorter than the DIFS
SIFS – Same as that in DIFS
Example : at the same time, (1) a station
wants use only DCF, (2) AP wants use PCF,
so as a result the AP has priority
Continue …
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Due to priority of PCF over DCF, stations
only use DCF may not access to the
medium
To prevent this, a repetition interval has
been designed to cover
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14.22
Contention-free (PCF)
Contention-based (DCF)
Continue …
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The repetition interval which is repeated
continuously as shown in Figure 14.6
It is start with a special control frame
(beacon frame)
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14.23
When the stations hear a beacon frame
It starts the NAV during contention-free
period
Figure 14.6 Example of repetition interval
14.24
Continue …
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During the repetition interval
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14.25
The PC sends a pool frame
The PC receives data
The PC sends ACK or receives ACK or do any
combination of these
At the end, the PC sends CF end (contentionfree end) frame to allow the contention-based
stations use the medium
Fragmentation
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14.26
A corrupt frame has been retransmitted
due to noisy in the wireless environment
Therefore, recommends fragmentation of
large frame
It is more efficient to resend a small frame
than a large one
Frame Format
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The MAC layer consists of nine fields as
shown in Figure 14.7
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14.27
FC (Frame Control) – defines the type and
control information
D – defines the duration of the transmission
Addresses – defines there are 4 addresses
SC (sequence control) – defines sequence
number of the frame
Frame body – defines the types and subtypes of
FC field
FCS – defines error detection sequence using
CRC-32 scheme
Figure 14.7 Frame format
14.28
Table 14.1 Subfields in FC field
14.29
Frame Types
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A wireless LAN defined by IEEE 802.11
has three categories of frames
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Management Frames
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Control Frames
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Used for accessing the channel and acknowledging
the frame as shown in Figure 14.8
Data Frames
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14.30
Used for initial communication between stations
and APs
Used for carrying data and control information
Figure 14.8 Control frames
14.31
Table 14.2 Values of subfields in control frames
14.32
Addressing Mechanism
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14.33
Specifies four cases by defining the value of the
two flags in the FC field such To DS and From DS
as shown in Table 14.3
The interpretation of the four addresses (address
1 to address 4) in the MAC frame depends on the
value of these flags
Note : Address 1 is always the address of the
next device; Address 2 is always the address of
the previous devices; Address 3 is the address of
the final station if it is not defined by address 1;
Address 4 is the address of original source
station if it is not same as address 2
Table 14.3 Addresses
14.34
Case 1 : 00
(refers to Table 14.3)
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14.35
To DS = 0 ; The frame is not going to to
a distribution system
From DS = 0 ; The frame is not coming
from a distribution system
The frame is going from one station in a
BSS to another without passing through
the distribution system as shown in Figure
14.9 (a)
Figure 14.9 Addressing mechanisms
14.36
Case 2 : 01
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14.37
To DS = 0 and From DS = 1; the frame
is coming from a distribution system (AP)
and going to the station (original)
The ACK frame should be sent to the AP
Note : the address 3 contains the original
sender of the frame (in another BSS) as
shown in Figure 14.9 (b)
Case 3 : 10
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14.38
To DS = 1 and From DS = 0; The frame
is going to the distribution system (AP)
from the station
The ACK frame is sent to the original
station
Note : Address 3 contains the final
destination of the frame (in another BSS)
as shown in Figure 14.9 (c)
Case 4 : 11
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14.39
To DS = 1 and From DS = 1; The frame is
going from one AP to another AP in a
wireless distribution system
Do not to define addresses if the distribution
system is a wired LAN because the frame in
these cases has the format of wired LAN
frame (Ethernet)
Note : need four addresses to define the
original sender, the final destination and two
intermediate APs as shown in Figure 14.9 (d)
Hidden and Exposed Station
Problems
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Based on Figure 14.10, Station A can hear a
signal transmitted by B or C
Assume;
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14.40
Station B is sending a data to station A
In the middle, station C also sends data to
station A
However, station C is out of B’s range and
transmission from B cannot reach C
Therefore, C thinks the medium is free
Station C sends data to A which results in a
collision at A because station A receiving data
from both B and C
Figure 14.10 Hidden station problem
1
14.41
2
Continue …
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14.42
In this case, station B and C are hidden
between each other with respect to A
Hidden stations can reduce the capacity of
the network because of the possibility of
collision
The solution is the handshake frames (RTS
and CTS) as shown in Figure 14.11 where
the RTS message from B reaches A but not C
Station C knows some hidden station is using
the channel and refrains from transmitting
until duration is over
Note
The CTS frame in CSMA/CA handshake
can prevent collision from
a hidden station.
14.43
Figure 14.11 Use of handshaking to prevent hidden station problem
14.44
Exposed Station Problem
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14.45
In this problem, a station is refrain from using a
channel when it is available
Figure 14.12 shows station A is transmitting to
station B
Station C has a data to send to station D which
can be sent without interfering with the
transmission from A to B
However, station C is exposed to transmission
from A; station C hears what A is sending and
refrains from sending
Station C is too conservative and wastes the
capacity of the channel
Figure 14.12 Exposed station problem
14.46
Handshaking
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14.47
Station C hears the RTS from A but does not
hear the CTS from B
Station C, after hearing the RTS from A, can
wait for a time so that the CTS from B
reaches A
It then sends the RTS to D to show that it
needs to communicate with D
Both station B and A may hear this RTS but
station A is in the sending state, not the
receiving state
Station B, however, responds with a CTS
Continue ….
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14.48
The problem begun, if station A has
started sending its data
Station C cannot hear the CTS from
station D because of the collision
Station C cannot send its data to D
Station C remains exposed until A finishes
sending its data as shown in Figure 14.13
Figure 14.13 Use of handshaking in exposed station problem
14.49
Physical Layer
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Except the infrared, all implementations
are operate in the industrial, scientific and
medical (ISM) band which defines three
unlicensed bands in the three ranges
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14.50
902-928 MHz
2.400 – 4.835 GHz
5.725 – 5.850 GHz
See Figure 14.14
Table 14.4 Physical layers
14.51
Figure 14.14 Industrial, scientific, and medical (ISM) band
14.52
14-2 BLUETOOTH
Bluetooth is a wireless LAN technology designed to
connect devices of different functions such as
telephones, notebooks, computers, cameras, printers,
coffee makers, and so on. A Bluetooth LAN is an ad
hoc network, which means that the network is formed
spontaneously.
Topics discussed in this section:
Architecture
Bluetooth Layers
Baseband Layer
L2CAP
14.53
Architecture
Defines two type of networks
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McGraw-Hill
Piconet
Scatternet
14.54
©The McGraw-Hill Companies, Inc., 2000
Piconet
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14.55
It is also called small net
Its can have up to 8 stations
One primary, the rest are secondary
Communication can be one-to-one or oneto-many
Continue ..
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14.56
An additional eight secondaries can be in the
“parked state”
A secondary in a “parked state” is
synchronised with the primary but cannot
take part in communication until it is moved
from the “parked state”
ONLY eight stations can be active
To activate a station from the “parked state”,
an active station must go to the “parked
state”
Figure 14.19 Piconet
14.57
Scatternet
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14.58
It has been formed by the combinations of
piconet
A secondary station in one piconet can be the
primary in another piconet
This station can receive messages from the
primary in the first piconet (as a secondary)
and acting as a primary, deliver them to
secondaries in the second piconet
A station can be a member of two piconets
Figure 14.20 Scatternet
14.59