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expanded by Jozef Goetz, 2012
March 20, 2016
The McGraw-Hill Companies, Inc., 2007
Jozef Goetz, 2012
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
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LANs
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Wireless LANs are found on college campuses,
office buildings, and public areas.
At home, a wireless LAN can connect roaming
devices to the Internet.
In this chapter, we concentrate on two
promising wireless technologies for LANs:


IEEE 802.11 wireless LANs, sometimes called
wireless Ethernet, and
Bluetooth, a complex technology for small
wireless LANs.
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LANs


IEEE 802.11, which covers the physical
and data link layers.
The Industrial, Scientific and Medical
(ISM) radio bands were originally
reserved internationally for the use of
RF electromagnetic fields for Industrial,
Scientific and Medical purposes other
than communications.
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WIRELESS LANs
A set of wireless LAN standards has been
developed by the IEEE 802.11 committee.
Three transmission schemes are defined in the current
802.11 standard:
[1] Infrared: at 1 Mbps & 2 Mbps.
[2] Direct-sequence spread spectrum: 2.4GHz ISM band
[3] Frequency-hopping spread spectrum: 2.4GHz ISM band
802.11b - 11 Mbps
802.11g - 54 Mbps
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Basic Service Set BSS (Cell)



IEEE 802.11 defines the Basic Service Set (BSS) or
Cell as the building block of a wireless LAN.
The Basic Service Set (BSS) is made of stationary
or mobile wireless stations and a possible central
Base Station (BS), known as the Access Point (AP).
BSS without an AP is a stand-alone network and
cannot send data to other BSSs.
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Note
•a BSS without an AP is called an ad hoc
network;
•a BSS with an AP is called an
infrastructure network.
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Extended Service Set (ESS)
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An Extended Service Set
(ESS) is made up of two or
more BSSs with APs.
In this case, the BSSs are
connected through a
distribution system,
which is usually a wired
LAN.

The distribution
system connects the
APs in the BSSs.

IEEE 802.11 does not
restrict the distribution
system;

it can be any
IEEE LAN such
as an Ethernet.
Note that the extended
service set uses two types
of stations: mobile and
stationary.

The mobile stations
are normal stations
inside a BSS.

The stationary
stations are AP stations
that are part of a
wired LAN.
When BSSs are connected,
we have what is called an
infrastructure network.
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Extended Service Set (ESS)
In this infrastructure network, the
stations within reach of one another can
communicate without the use of an
AP.

However, communication between two
stations in two different BSSs usually occurs
via two APs.


The idea is similar to communication in a cellular
network
 if we consider each BSS to be a cell and each
AP to be a base station.
Note that a mobile station can belong to
more than one BSS at the same time.
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WIRELESS LANs
ESS
10
Wired Backbone LAN
functions as a bridge
BSS
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Wireless LAN

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A multicell 802.11 network (WiFi).
The connection between the 802.11 system and the outside world is called a portal
All the base stations are wired together using copper or fiber
Unlike cellular tel. systems, each cell has only one channel, covering the entire
available bandwidth ( 11 – 54 Mbps) and covering all the stations in its cell
Jozef Goetz, 2012
Station Types


IEEE 802.11 defines 3 types of stations based on their
mobility in a wireless LAN: no-transition, BSStransition, and ESS-transition.
No-Transition Mobility


BSS-Transition Mobility


A station with no-transition mobility is either stationary (not
moving) or moving only inside a BSS.
A station with BSS-transition mobility can move from one
BSS to another, but the movement is confined inside one
ESS.
ESS-Transition Mobility

A station with ESS-transition mobility can move from one ESS
to another.

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However, IEEE 802.11 does not guarantee that communication is
continuous during the move.
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The 802.11 Protocol Stack

ISM: Abbreviation for Industrial, Scientific, and Medical applications (of radio frequency energy).
range is 7 times greater
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Figure
14.14
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MAC layers in IEEE 802.11 standard
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Note: there is one LLC sublayer for all IEEE LANs
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Reminder: IEEE standard for wired LANs
Modulation


In analog transmission, the sending device
produces a high-frequency signal that acts as
a base for the information signal.
This base signal is called the carrier
signal or carrier frequency.



The receiving device is tuned to the
frequency of the carrier signal that it expects
from the sender.
This kind of modification is called modulation
(shift keying).
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Modulation

Modulation - is the process of encoding
source data onto a carrier signal with
frequency carrier fc
 i.e. process of modulating the carrier
signal by modifying one or more of
parameters:
amplitude,
 frequency, &
 phase
of a sine wave.

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Modulation of Digital Data
•Basic Digital-to-Analog conversion or
modulation or encoding
•Amplitude Shift Keying (ASK)
•Frequency Shift Keying (FSK)
•Phase
Shift Keying (PSK)
•Quadrature Amplitude Modulation
(QAM)
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Figure
Digital-to-analog modulation
The digital data must be modulated on an analog signal
•Modulation of binary data or (digital-to-analog modulation)
is the process of changing one of the characteristics
of an analog signal
•a sine wave is defined by 3 characteristics:
•amplitude,
•frequency, and
•phase
based on the information in a digital signal (0s and 1 s).
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Figure
Jozef Goetz, 2012
Digital-to-analog conversion
Figure
Types of digital-to-analog modulation
Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase
Shift Keying (PSK)
Quadrature Amplitude Modulation (QAM)
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Note:
Bit rate is the number of bits per
second [bps]
Baud rate is the number of signal
elements (or impulses or symbols or
signal units) per second [baud]
Baud rate <= Bit rate.
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Note
Bit rate N is the # of bits (passengers by
analogy) per second.
Baud rate S is the # of signal
elements (vehicles by analogy) per
second.
In the analog and digital transmission of
digital data,
S <= N
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In data transmission:


a signal element as the smallest
unit of a signal that is constant
The baud rate determines the
bandwidth required to send the
signal.
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In analog transmission:

We can define the data rate (bit rate) and the S signal
rate (baud rate)
S = N / r [baud]


where N is the data rate (bps) and
r is the # of data elements (bits) carried in one signal element.
S = N / r =>
N=Sr
The value of r in analog transmission is
r = log2 L
where L is the # of signal elements (symbols), not the
level
 e.g. for FSK is # of different frequencies

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Example
An analog signal carries 4 bits in each signal element
(unit).
If 1000 signal elements (units or pulses) are sent per
second, find the baud rate and the bit rate
Solution
In this case, r = 4, S = 1000, and N is unknown. We can
find the value of N from
S = N / r => N = S r
S = Signal rate =Baud rate = 1000 pulse/sec
N = Data rate = Bit rate = S r = 1000 [pulse/sec] 4 [bit/pulse] =
4000 bps
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Example
The bit rate of a signal is 3000. If each signal element
carries 6 bits, what is the baud rate?
Solution
Baud rate = N / r = 3000 [bit/sec] / 6 [bit/pulse] =
500 [pulse/sec] = 500 baud
Jozef Goetz, 2012
DIGITAL DATA, ANALOG SIGNALS
AMPLITUDE-SHIFT KEYING (ASK)

In ASK, the two binary values are represented by two different
amplitudes of the carrier frequency.
 A1 cos( 2f ct ) binary 1
s(t )  
 A0 cos( 2f ct ) binary 0
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DIGITAL DATA, ANALOG SIGNALS
FREQUENCY-SHIFT KEYING (FSK)

In FSK, the two binary values are represented by two different
frequencies. The frequencies of the modulated signal is constant for
the duration of one signal element.
 A cos( 2f1t )
s(t )  
 A cos( 2f 2t )
4 FSK
binary 1
 A cos( 2f1t )
 A cos( 2f t )

2
s (t )  
 A cos( 2f 3t )

 A cos( 2f 4t )
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binary 0
binary 00
binary 01
binary 10
binary 11
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DIGITAL DATA, ANALOG SIGNALS
PHASE-SHIFT KEYING (PSK)

In PSK, the phase of the carrier signal is shifted to represent data.
BPSK
 A cos( 2f ct  180) binary 1
s(t )  
binary 0
 A cos( 2f ct )
QPSK
 A cos( 2f c t  00)
 A cos( 2f t  90)

c
s (t )  
 A cos( 2f c t  180)

 A cos( 2f c t  270)
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binary 00
binary 01
binary 10
binary 11
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Figure
Analog-to-analog modulation
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needed when the medium has a band-pass nature or if only band-pass bandwidth is available
and e.g. a radio produced by each station is a low-pass signal.
So a low-pass signal need to be shifted.
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
ANALOG DATA, ANALOG SIGNALS
There are two principal reasons for analog modulation of
analog signals:

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A higher frequency may be needed for effective
transmission.
Modulation permits frequency-division multiplexing.
Three techniques:

Amplitude Modulation (AM)

Frequency Modulation (FM)

Phase Modulation (PM)
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Figure
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Amplitude modulation
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ANALOG DATA, ANALOG SIGNALS
AM - MODULATION


Amplitude modulation (AM) is the simplest form of
modulation.
Mathematically, the process can be expressed as
s(t )  1  na x(t ) cos( 2f ct )
Input signal
Carrier
Modulation index
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Figure
Jozef Goetz, 2012
Amplitude modulation
ANALOG DATA, ANALOG SIGNALS
FM - MODULATION


In frequency modulation (FM) we modulate the instantaneous
frequency fi(t), with the signal s(t).
Mathematically, the process can be expressed as

s(t )  Ac  cos 2 ( f c  k f  s(t ))
Input signal
Carrier
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
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ANALOG DATA, ANALOG SIGNALS
PM - MODULATION


For phase modulation (PM), the phase is proportional to the
modulating signal s(t).
Mathematically, the process can be expressed as
s(t )  Ac  cos2f ct   (t )
Input signal
Carrier
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Industrial, Scientific, and Medical (ISM) band
3 unlicensed bands in the 3 ranges
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Frequency-Hopping Spread Spectrum FHSS
IEEE 802.11

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
The band (from 2.4 GHz to 2.48 GHz) is divided into 79 subbands of 1 MHz.
FHSS is a method in which the sender sends on one carrier
frequency for a short amount of time, then hops to another
carrier frequency for the same amount of time, hops again to still
another for the same amount of time, and so on.

After N hoppings, the cycle is repeated.
If the bandwidth of the original signal is B, the allocated spread spectrum
bandwidth is N x B.
Spreading makes it difficult for unauthorized persons to make
sense of transmitted data.
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Frequency-Hopping Spread Spectrum FHSS
IEEE 802.11


The band (from 2.4 GHz to 2.48 GHz) is divided into 79 subbands of 1 MHz.
In FHSS the sender and receiver agree on the sequence of the
allocated bands.
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
In the figure, the first bit (or group of bits) is sent in subband 1, the
second bit (or group of bits) is sent in subband 2, and so on.
An intruder who tunes his or her receiver to frequencies for one
subband may receive the first group of bits, but receives nothing in
this subband during the second interval.
The amount of time spent at each subband, called the dwell time, is
400 ms or more.
Note that this is not a case of multiple access;

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all stations contend to use the same subbands to send their data.
 Contention is a function of the MAC sublayer.
Frequency-Hopping Spread Spectrum FHSS

Band FHSS uses a 2.4-GHz industrial,
scientific, and medical (ISM) band.

The band in North America is from 2.4 GHz to 2.48
GHz.
 The band is divided into 79 subbands of 1 MHz.



A pseudorandom number generator selects
the hopping sequence.
Modulation and Data Rate
The modulation technique in this specification is FSK
(Freq. Shift Keying) at 1 Mbaud/s.
 The system allows 1 or 2 bits baud (two-level FSK or
four-level FSK), which results in a data rate of 1 or 2
Jozef Goetz, 2012 Mbps.
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Direct Sequence Spread Spectrum DSSS
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IEEE 802.11


IEEE 802.11 DSSS describes the direct sequence spread spectrum
(DSSS) method for signal generation in a 2.4-GHz ISM band.
In DSSS, each bit sent by the sender is replaced by a sequence of bits
called a chip code.
 To avoid buffering, however, the time needed to send one chip
code must be the same as the time needed to send one original
bit.
 If N is the number of bits in each chip code, then the data rate for
sending chip codes is N x the data rate of the original bit stream.
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Direct Sequence Spread Spectrum DSSS

DSSS is implemented at the physical layer. It is not a
multiple-access method for the data link layer.
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

We need a contention method at the data link layer, and
that will be discussed shortly.
Band DSSS uses a 2.4-GHz ISM band. The bit
sequence uses the entire band.
Modulation and Data Rate The modulation technique
in this specification is PSK (Phase Shift Keying) at 1
Mbaud/s.
The system allows 1 or 2 bits baud (BPSK or QPSK),
which results in a data rate of 1 or 2 Mbps.
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The 802.11 Protocol Stack

ISM: Abbreviation for Industrial, Scientific, and Medical applications (of radio frequency energy).
range is 7 times greater
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Orthogonal Frequency-Division Multiplexing (OFDM)
IEEE 802.11a

OFDM is the same as FDM, with one major difference:

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All the subbands are used by one source at a given time.
Sources contend with one another at the data link layer for access.
The specification uses a 5-GHz ISM band.
 The band is divided into 52 subbands, with 48 subbands for
sending 48 groups of bits at a time and 4 subbands for control
information.
 Dividing the band into subbands diminishes the effects of
interference.
 If the subbands are used randomly, security can also be
increased.
Modulation and Data Rate OFDM uses PSK and QAM for modulation.
The common data rates are
 18 Mbps (PSK) and
 54 Mbps (QAM) – quadrature amplitude modulation.
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High-Rate DSSS (HR-DSSS)
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IEEE 802.11b

method for signal generation in a 2.4-GHz ISM band.



HR-DSSS is similar to DSSS except for the encoding method, which
is called Complementary Code Keying (CCK).
CCK encodes 4 or 8 bits to one CCK symbol.
Modulation and Data Rate

To be backward-compatible with DSSS, HR-DSSS defines four data
rates: 1, 2, 5.5, and 11 Mbps.

The first two 1, 2 Mbps use the same modulation techniques as DSSS.

The 5.5-Mbps version uses BPSK and transmits at 1.375 Mbaud/s
with 4-bit CCK encoding.

The 11-Mbps version uses QPSK and transmits at 1.375 Mbps with 8bit CCK encoding.

Jozef Goetz, 2012
Note that the 11-Mbps version has a data rate close to 10-Mbps Ethernet.
Orthogonal Frequency-Division Multiplexing (OFDM)
IEEE 802.11g


This relatively new specification uses OFDM with
a 2.4-GHz ISM band.
The complex modulation technique achieves
a 54-Mbps data rate.
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802.11n
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Uses 5 GHz and 2.4 GHz frequency ranges
Theoretical throughput is 300 to 600 mbps;
realistically between 100 and 200 mbps
Maximum indoor distance is ~229 feet or 70
meters
Maximum outdoor distance is 820 feet or
250 meters
Uses channel bonding, where two or more
adjacent channels are linked together
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Jozef Goetz, 2012
MAC layers in IEEE 802.11 standard


2 MAC sublayers: the Distributed Coordination Function (DCF) and
Point Coordination Function (PCF).
PCF is an optional and complex access method that can be
implemented in an infrastructure network (not in an ad hoc network).

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We do not discuss this here; for more information refer to Forouzan, Local Area
Networks, McGraw-Hill.
DCF is similar to CSMA/CA, Carrier sense multiple access (CSMA)
Collision Avoidance (CA)
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base station polls asking them if
they have any frames to send. Since
transmission is controlled by the base
station, no collision ever occurs
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WIRELESS LANs - MAC
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Distributed Coordination Function (DCF)
•If the medium is idle, the station may transmit;
otherwise the station must wait until the current
transmission is complete before transmitting.
Point Coordination Function (PCF)
•The operation consists of polling with the
centralized polling mater (point coordinator).
•The point coordinator could issue polls in a
round-robin fashion to all stations configured.
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Wireless LAN Protocols
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

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• Wireless LANs are inherently different than
conventional LANs and require special MAC layer protocols.
With a wire, all signals propagate to all stations so only 1
transmission take place at once anywhere in the system
in a system on short-range radio waves, multiple
transmission can occur simultaneously
• CSMA (Carrier Sense Multiple Access Protocols) doesn’t work

because there is no way to tell if interference (collision) is happening at
the receiver – the sender may not hear it – see a hidden station problem.
most radios are half duplex - cannot transmit and listen for noise burst the
same time
• CDMA (Code Division Multiple Access) doesn’t work

because when a receiver is within range of two transmitters the
transmission is usually garbled
interference is happening at the receiver
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• (a) The hidden station problem results when one station is transmitting data but a
second station cannot “hear” the transmission and starts transmitting.
• (b) The exposed station (inverse) problem results when one station refrains from
transmitting data due to another transmission that would not have affected the data
transfer.
Also most radios are half duplex – cannot transmit and listen for noise burst the
same time
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Wireless LAN
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<=


A and B are within each other’s range and can potentially
interfere with one another
(a) A transmitting to B (while B and C are talking).
(b) B transmitting (D refrains from transmitting to C)
Can we use CSMA/CD for wireless LANs? NO

Even with CD (Collision Detection), “no sensed collision” does not
mean “no collision”, because a collision could still occur at the receiver
(hidden station problem) and the sender may not hear it.


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Again, due to hidden station, A can create collision by sending a frame
to B,
because, before transmission, A was not hearing C sending a frame to
B.
CSMA/CA (Collision Avoidance) flowchart
Frame Exchange Time Line




1.
Before sending a frame, the source
station senses the medium by checking
the energy level at the carrier frequency.
a. The channel uses a persistence strategy
with backoff until the channel is idle.
b. After the station is found idle, the station
waits for a period of time, called the
distributed interframe space (DIFS);

then the station sends a control frame
called the Request To Send (RTS).
2. After receiving the RTS and waiting a
short period of time, called the short
interframe space (SIFS), the destination
station sends a control frame, called the
Clear To Send (CTS), to the source station.



Jozef Goetz, 2012
This control frame indicates that the destination
station is ready to receive data.
3. The source station sends data after
waiting an amount of time equal to SIFS.
4. The destination station, after waiting for
an amount of time equal to SIFS, sends an
acknowledgment to show that the frame has
been received.

Acknowledgment is needed in this
protocol b/c the station does not have
any means to check for the successful
arrival of its data at the destination.
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CSMA/CA (Collision Avoidance) flowchart
Frame Exchange Time Line
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Collision During Handshaking

What happens if there is
collision during the time when
RTS or CTS are in transition,
often called the handshaking
period?



Jozef Goetz, 2012
Two or more stations may try to
send RTS frames at the same
time.
These control frames may collide.
However, because there is no
mechanism for collision
detection, the sender assumes
there has been a collision if it has
not received a CTS frame from
the receiver.
 The backoff strategy is
employed, and the sender
tries again.
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Note
•The CTS frame in CSMA/CA handshake
can prevent collision from
a hidden station.
Jozef Goetz, 2012
Collisions in Wireless LAN








The MACA protocol.
(a) A sending an RTS to B. C hears it but D doesn’t
(b) B responding with a CTS to A. D hears it but C doesn’t
E hears both RTS and CTS, so must be silent until during the CTS and data frame
transmission
Idea: sender stimulate the receiver by sending RTS and receiving
CTS, so stations nearby will avoid transmission for the duration of
the upcoming data frame sent from the sender
Collisions are still possible!
Consider B and C sending an RTS to A:
both RTS are lost and retransmitted after a random interval
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Collision Avoidance



How is the collision avoidance aspect of this
protocol accomplished?
When a station sends an RTS frame, it includes
the duration of the time that it needs to
occupy the channel.
The stations that are affected by this transmission
create a timer called a network allocation
vector (NAV) that shows how much time
must pass before these stations are allowed
to check the channel for idleness.

Jozef Goetz, 2012
Each time a station accesses the system and
sends an RTS frame, other stations start their
NAV.
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CSMA/CA and NAV
•each station, before sensing the physical medium to see if it is idle,
first checks its NAV to see if it has expired.
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Hidden stations can reduce the capacity of the network
because of the possibility of collision.
Use of handshaking to prevent hidden station problem
When a station sends an CTS frame, it
includes the duration of the time that it
needs to occupy the channel and C will
refrain from transmitting until that
duration is over.
Jozef Goetz, 2012
Use of handshaking in exposed station problem
In other words:
C is two
conservative
problem and wastes
the capacity of the
channel.
C cannot hear the
CTS from D b/c of
the collision.
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MAC Frame format
•The wireless environment is very noisy; 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 replace a small frame than a large one.
•Frame control (FC) is 2 bytes long and defines the type of the frame and
some control information
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Table 14.7 Subfields in FC field
Field
Explanation
Version
The current version is 0.
Type
Type of information: management (00), control (01), or data (10).
Subtype
Defines the subtype of each type (RTS, CTS and ACK).
To DS
Defined later.
From DS
Defined later.
More flag
When set to 1, means more fragments.
Retry
When set to 1, means retransmitted frame.
Pwr mgt
When set to 1, means station is in power management mode.
More data
When set to 1, means station has more data to send.
WEP
Wired equivalent privacy. When set to 1, means encryption
implemented.
Rsvd
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Reserved.
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Frame format
•


Frame control (FC) is 2 bytes long and defines the type of the frame
and some control information
D. In all frame types except one, this field defines the duration of the
transmission that is used to set the value of NAV.

In one control frame, this field defines the ID of the frame.
Addresses. There are four address fields, each 6 bytes long.




The meaning of each address field depends on the value of the To DS and the
From DS subfields and will be discussed later.
Sequence Control SC defines the sequence number of the frame to be
used in flow control.
Frame body is between 0 and 2312 bytes, contains information based
on the type and the subtype defined in the FC field.
FCS is 4 bytes long and contains a CRC-32 error detection sequence
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Frame types

A wireless LAN defined by IEEE
802.11 has 3 categories of frames:
1. Management frames,
2. Control frames, and
3. Data frames.
1. Management Frames are used for the
initial communication between
stations and access points.
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2. Control frames
In FC the Type = 01 (control frames)
Values of Subtype in FC
Subtype
Meaning
1011
Request to send (RTS)
1100
Clear to send (CTS)
1101
Acknowledgment (ACK)
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Addressing Mechanism

There are 4 cases, defined by the value of
the two flags in the FC field, To DS and
From DS.


Each flag can be either 0 or 1, thus defining
four different situations.
The interpretation of the four
addresses (address 1 to address 4) in the
MAC frame depends on the value of
these flags
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70
Table 14.3 Subfields in FC field
Distribution System - DS




To
DS
From
DS
Address
1
Address
2
Address
3
Address
4
0
0
Destination
station
Source
station
BSS ID
N/A
0
1
Destination
station
Sending
AP
Source
station
N/A
1
0
Receiving
AP
Source
station
Destination
station
N/A
1
1
Receiving
AP
Sending
AP
Destination
station
Source
station
Note that address 1 is always the address of the next device.
Address 2 is always the address of the previous device.
Address 3 is the address of the final destination station if it is not
defined by address 1.
Address 4 is the address of the original source station if it is not the
same as address 2.
In general: the order is destination the first, source the second one

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Addressing mechanism: case 1




In this case, To DS = 0 and From DS = 0.
This means that the frame is not going to a
Distribution System (To DS = 0) and is not coming
from a distribution system (From DS = 0).
The frame is going from one station in a BSS to
another without passing through the distribution
system.
The ACK frame should be sent to the original sender.
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Addressing mechanism: case 2
In this case, To DS = 0 and From DS = 1. This means
that the frame is coming from a distribution system
(From DS = 1).
 The frame is coming from an AP and going to a station.
 The ACK should be sent to the AP.

Note that address 3 contains the original sender of
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2012 frame (in another BSS).

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Addressing mechanism: case 3





In this case, To DS = 1 and From DS = 0.
This means that the frame is going to a distribution system (To DS
= 1).
The frame is going from a station to an AP.
The ACK is sent to the original station.
Note that address 3 contains the final destination of the frame
(in another BSS).
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Addressing mechanism: case 4




To DS = 1 and From DS = 1.
This is the case in which the distribution system is also wireless.
The frame is going from one AP to another AP in a wireless
distribution system.
We do not need to define addresses if the distribution system
is a wired LAN because the frame in these cases has the format
of a wired LAN frame (Ethernet, for example).
Here, we need four addresses to define the original sender, the final
destination, and two intermediate APs.
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
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Table 14.3 Subfields in FC field
Distribution System - DS




To
DS
From
DS
Address
1
Address
2
Address
3
Address
4
0
0
Destination
station
Source
station
BSS ID
N/A
0
1
Destination
station
Sending
AP
Source
station
N/A
1
0
Receiving
AP
Source
station
Destination
station
N/A
1
1
Receiving
AP
Sending
AP
Destination
station
Source
station
Note that address 1 is always the address of the next device.
Address 2 is always the address of the previous device.
Address 3 is the address of the final destination station if it is not
defined by address 1.
Address 4 is the address of the original source station if it is not the
same as address 2.
In general: the order is destination the first, source the second one

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WIRELESS LANs - Summary
Table 14.4 Physical layers
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WIRELESS LANs
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WLAN Standards (802.11)
Standard
Spectrum
Maximum
physical rate
Transmission
802.11
2.4 Ghz
2 Mbps
FHSS/DSSS
802.11a
802.11b
802.11g
5.0 Ghz
2.4 Ghz
2.4 Ghz
54 Mbps
11 Mbps
54 Mbps
Wireless Local Area Networks
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OFDM
DSSS
OFDM
Disadvantages
Advantages
Limited bit rate
Higher range
Smallest range of all
802.11 standards,
not back
compatible
Higher bit rate in
less-crowded
spectrum
Bit rate too low for
many emerging
applications,
overcrowded
Widely deployed;
higher range
Limited number of
collocated WLANs
higher range that
802.11a
Higher bit rate in 2.4
GHz spectrum
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WLAN Standards (802.11)
802.11b
• Runs
on 3 channels in 2.4GHz, unregulated spectrum
• Shares spectrum with cordless phones, microwave ovens and many Bluetooth products
• Transfers data at speeds of 11 megabits per second per channel, at distances of up to 300 feet
• Interference issues: In crowded 2.4GHz frequency, people may not be able to Web surf over a wireless network if they're
using the microwave oven or using a cordless phone at the same time.
802.11a
•
802.11e
Viewed as essential for voice-over WLAN in the enterprise, 802.11e is a proposed quality-of service standard that gives priority to
streaming media. A subset, WME (Wireless Multimedia Enhancements), is likely to emerge first.
802.11f
Also known as IAPP (Inter Access Point Protocol). Draft protocol specifies how APs (Access Points) should communicate on the layer 2
Runs on 12 channels in 5GHz spectrum, reducing interference issues
• Transfers data up to five times faster than 802.11b, improving quality of streaming media, extra bandwidth for big files
• not backward-compatible with 802.11b, businesses or homes must tear down the old networks to use 802.11a equipment
level in order to accommodate roaming users. Ratification is expected by the end of 2003.
802.11g
• Runs on 3 channels in 2.4GHz spectrum, the same as 802.11b
• Has the speed of 802.11a, up to 5 times faster than 802.11b
• Is more secure than 802.11b
• Is backward-compatible with 802.11b
802.11h
802.11i
802.11k
A standard to enable WLANs that operate in the 5GHz range to comply with European RF regulations. Ratification by the European Union
should be finalized by the end of the year (2003). It specifies spectrum and power control management.
The overarching specification for enterprise-class Wi-Fi security, built on the 802.1x authentication scheme. It replaces WEP (Wired
Equivalent Privacy) with AES (Advanced Encryption Standard), a much stronger encryption method. Ratification is expected by the end of
2003.
A proposed standard to increase the manageability of WLANs by defining and exposing radio and network information, which can be
used by network management applications.
802.11n
The next step up from the fastest current WLAN standards, 802.11 a and 802.11g, both of which top out at 54Mbps. The
proposed 802.11 n spec, which will use the same 5GHz frequency range as 802.11 a, will raise maximum throughput to
100Mbps or higher. The IEEE established an 802.11n working group in September; ratification is expected in 2005 or 2006.
802.1x
Provides for port-based authentication and authorization of wireless clients. Already implemented in marry devices as part of WPA,
802.1x incorporates EAP (Extensible Authentication Protocol), a framework that supports a variety of authentication servers, including
RADIUS and Kerberos.
Wireless Local Area Networks
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802.11 Idea Summary

• Ethernet idea, a station just waits until
the ether goes silent and start
transmitting.


if does not receive a noise burst back within
64 bytes, the frame has almost assuredly
been delivered correctly
with wireless this situation does not hold
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