Adhoc and Sensor Networks - Chapter 04

Copyright © 2006, Dr. Carlos Cordeiro and Prof. Dharma P. Agrawal, All rights reserved. 1
 Introduction
 Why Wireless LANs
 Transmission Techniques
 Wired
 Wireless
 Medium Access Control Protocol Issues
 Hidden Terminal Problem
 Reliability
 Collision Avoidance
 Congestion Avoidance
 Congestion Control
 Energy Efficiency
 Other MAC Issues
 The IEEE 802.11 Standard for Wireless LANs
 Network Architecture
 The Physical Layer
 The MAC Layer
 Security
 System Design Considerations
 An Overview of Past and Present IEEE 802.11 Efforts
 The IEEE 802.11e MAC Protocol
 Enhancement to IEEE 802.11 MAC
 Power Control
 Spatial Reusability
 QoS Provisioning
 The HIPERLAN/2 Standard for Wireless LANs
 Physical Layer
 MAC Layer
 Conclusions and Future Directions
Table of
Contents

Introduction
 New networking technologies and paradigms such as:
 Wireless LANs (Local Area Networks)
 Wireless PANs (Personal Area Networks)
 Wireless MANs (Metropolitan Area Networks)
 Wireless WANs (Wide Area Networks)
 Wireless RANs (Radio Access Networks)
 Dominant choice:
 Ad hoc and mesh (or infrastructureless) mode offered
by the WLANs and WPANs technologies
 For WLANs, IEEE 802.11 standard and their
variations: the most well known representatives:
discusssed in this chapter
 IEEE 802.15 Working Group for WPANs: discussed in
the next chapter

Why Wireless LANs?
 Ethernet Standard LAN protocol operates at a
fairly high speed using inexpensive connection
hardware
 LANs have been limited to the physical, hard-
wired infrastructure of the building
 Many mobile users in businesses, medical
profession, factories, and universities find many
benefits from the added capabilities of wireless
LANs
 Wireless LANs provide mobility and untethered
from conventional hardwired connections
 Practical use of wireless networks is limited by an
individual’s imagination
 It may even be economical to use a wireless LAN
in old buildings

Transmission Techniques
 We would like to transmit data with:
 Highest possible data rate
 With the minimum level of the signal power
 Minimum channel bandwidth
 Reduced transmitter and receiver complexity
 Wired
 Data received from upper layers are line coded and the voltages are
applied to the medium directly
 Transmissions are often referred to as baseband transmission schemes
 In voice-band modems, Digital Subscriber Line (DSL), and coaxial cable
model applications, the transmitted signal is modulated over a carrier
 Wireless: 3 categories
 Pulse transmission techniques employed mostly in Infrared (IR)
applications and, more recently, in the so-called impulse radio or ultra-
wideband (UWB) transmission
 Basic modulation techniques widely used in Time Division Multiple Access
(TDMA) cellular, as well as a number of mobile data networks
 Spread spectrum systems used in the Code Division Multiple Access
(CDMA) and in wireless LANs operating in ISM frequency bands

ISM Frequency Bands
Extremely
Low
Very
Low
Low Medium High Very
High
Ultra
High
Super
High
Infrared Visible
Light
Ultra-
violet
X-Rays
Audio
AM Broadcast
Short Wave Radio FM Broadcast
Television Infrared wireless LAN
Cellular (840MHz)
NPCS
(1.9GHz)
5 GHz
(IEEE 802.11a,
HiperLAN,
HiperLAN2)
 22 MHz channel bandwidth
 Must implement spread spectrum technology
 Must operate at 500 mill-watts or less
902 - 928 MHz
26 MHz
2.4 - 2.4835
GHz
83.5 MHz
(IEEE 802.11b,
Bluetooth)

Infrared (IR)
 Remote controls for TVs, VCRs, DVD and CD players use IR
technology
 A direct line of sight of the transmitter needed in order to
successfully establish communication link
 New diffused IR technologies can work without LOS inside a room
 Systems are simple to design and are inexpensive
 Use the same signal frequencies used on fiber optic links
 Systems detect only the amplitude of the signal, so interference is
greatly reduced
 IR transmission can be aimed and range extended to a couple of
kilometers and can be used outdoors
 It also offers the highest bandwidth and throughput
 The other way is to transmit omni-directionally and bounce the
signals off of everything in every direction, which reduces the
coverage to 30 - 60 feet
 IR signals cannot penetrate opaque objects
 IEEE 802.11 standard for high-speed diffused IR employs pulse-
position-modulation (PPM) with a wavelength of 850nm–950nm
and data rates of 1 and 2 Mbps

Microwave and Radio
Frequencies Microwave:
 Operate at less than 500 milliwatts of power (MW)
 Use narrow-band transmission with single frequency modulation mostly in
the 5.8 GHz band
 Radio Frequency:
 To link appliances that are distributed throughout the house
 Can be categorized as narrowband or spread spectrum
 Narrowband technology includes microwave transmissions which are
high-frequency radio waves that can be transmitted to distances up to 50
Km
 Not suitable for local networks, but could be used to connect networks in
separate buildings
 Transmitted signal in Subscriber Station occupies a much larger
bandwidth than the traditional radio modems
 Two basic techniques used: frequency hopping spread spectrum (FHSS)
and direct sequence spread spectrum (DSSS)

Spread Spectrum
 Frequency Hopping Spread Spectrum:
 Splits the band into many small subchannels (e.g., 1 MHz)
 Signal then hops from subchannel to subchannel transmitting
short bursts of data on each channel for a set period of time, called
dwell time
 Hopping sequence must be synchronized at the sender and the
receiver or else, the information is lost
 FCC requires that the band is split into at least 75 subchannels
 Extremely difficult to intercept and gives a high degree of security
 Direct Sequence Spread Spectrum:
 Transmission signal is spread over an allowed band
 Two-stage modulation technique: a random binary string, called
the spreading code, is used to modulate the transmitted signal, and
in the second stage, the chips are transmitted over a traditional
digital modulator

Direct Sequence Spread
Spectrum
 Physical layer of the IEEE 802.11 standard employing
DSSS requires a spreading ratio of eleven
 If orthogonal spreading codes are used, then more than
one LAN can share the same band
 However, because DSSS systems use wide subchannels,
the number of co-located LANs is limited by the size of
those subchannels
 As the transmitted chips are much narrower than data
bits, the bandwidth of the transmitted DSSS signal is
much larger than systems without spreading
 The DSSS systems provide a robust signal with better
coverage area than FHSS
 The DSSS systems provide a robust signal with better
coverage area than FHSS

Medium Access Control Protocol
Issues
 Many issues need to be addressed in order to
design an efficient MAC protocol
 Hidden Terminal Problem
 Station A can hear B but not C, and station C
can hear station B but not A
 First, assume A is sending to B
 When C is ready to transmit, it does not detect
carrier and thus commences transmission; this
produces a collision at B
A B C

Hidden terminals
 A sends to B, C cannot detect A’s transmission
 C wants to send to B, C senses a “free” medium (CS fails)
 Collision at B, A cannot detect the collision (CD fails)
 A is “hidden” for C
Hidden Terminal Problem
B
CA

Issues
 Exposed Terminal Problem
 Station A can hear B but not C, and station C can hear
station B but not A
 If we assume that B is sending to A rather than A sending to
B
 Then, when C is ready to transmit, it does detect carrier and
therefore defers transmission
 However, there is no reason to defer transmission to a station
other than B since station A is out of C’s range
 Station C’s carrier sense did not provide the necessary
information since it was exposed to station B even though it
would not collide or interfere with B’s transmission
A B C
D

Exposed terminals
 B sends to A, C wants to send to D
 C senses carrier, finds medium in use and has to wait
 A is outside the radio range of C, therefore waiting is
not necessary
 C is “exposed” to B
Exposed Terminal Problem
BA C D

Multiple Access with Collision
Avoidance (MACA)
 MACA uses signaling packets for collision avoidance
 RTS (request to send)
-Sender request the right to send from a receiver with
a short RTS packet before it sends a data packet
 CTS (clear to send)
-Receiver grants the right to send as soon as it is ready to
receive
 Signaling packets contain
 Sender address
 Receiver address
 Duration
Variants of this method are used in IEEE 802.11

MACA avoids the problem of hidden terminals
A and C want to send to B
A sends RTS first
C waits after receiving
CTS from B
MACA avoids the problem of exposed terminals
B wants to send to A, C to another terminal
Now C does not have
to wait, as it cannot
receive CTS from A
MACA Solutions
CTSCTS
A B C
RTS
CTS
RTS
B CA
RTS

Issues Solution to the hidden terminal problem proposed in the
Medium Access with Collision Avoidance (MACA)
 Transmitting Request-to-Send (RTS) and Clear-to-Send (CTS)
packets
 Stations in the neighborhood overhear either the RTS or CTS,
keep quiet for the duration of the transfer
 Reliability
 Packet error rates of wireless mediums are much higher than that
of their wired counterparts
 E.g., execution of TCP congestion control mechanism, a packet loss
is erroneously assumed due to congestion and congestion control
mechanisms are fired
 This ultimately decreases the transmission rate, with the intention
to reduce the network congestion
 In wireless environment, packet loss occurs every now and then
due to effects such as multipath fading, interference, shadowing,
distance between transmitter and receiver, etc.
 A common approach to reduce packet loss rates experienced by
upper layers is to introduce acknowledgment (ACK) packets

Issues
 Collision Avoidance
 To minimize collisions, wireless CSMA/CA MAC
protocols often use collision avoidance techniques in
conjunction with a carrier sense
 Collision avoidance is implemented by making node
wait for a randomly chosen duration before attempting
to transmit after the channel is sensed idle
 Congestion Avoidance
 When a node detects the medium to be idle, it chooses a
backoff interval between [0, CW], where CW is called
contention window
 CW usually has a minimum (CW_min) and maximum
value (CW_max)

Congestion Avoidance
BO1 = 5
BO2 = 15
BO1 = 25
BO2 = 20
data
wait data
wait
BO2 = 10
BO1 and BO2 are backoff intervals
at nodes 1 and 2
CW =
31
 BO1 and BO2 are the backoff intervals of nodes 1 and 2
 We assume for this example that CW = 31
 Node 1 and node 2 have chosen a backoff interval of 25 and 20, respectively
 Node 2 will reach zero before five units of time earlier than node 1
 When this happens, node 1 will notice that the medium became busy and freezes its
backoff interval currently at 5
 As soon as the medium becomes idle again, node 1 resumes its backoff
countdown and transmits its data once the backoff interval reaches zero

Issues Other MAC Issues
 Many other issues need to be considered such as fairness
 Fairness has many meanings and one of them might say that
stations should receive equal bandwidth
 Unfairness will eventually occur when one node backs off
much more than some other node in the same neighborhood
 MACAW’s solution to this problem [Bharghavan1994] is to
append the contention window value (CW) to packets a node
transmits, so that all nodes hearing that CW, use it for their
future transmissions
 Since CW is an indication of the level of congestion in the
vicinity of a specific receiver node, MACAW proposes
maintaining a CW independently for each receiver
 All protocols discussed so far are sender-initiated protocols
 In other words, a sender always initiates a packet transfer to
a receiver
 The receiver might take a more active role in the process by
assisting the transmitter in certain issues such as collision
avoidance, and some sort of adaptive rate control

IEEE 802.11 Standard for
Wireless LANs
Logical Link Control
IEEE 802.11b
PHY Layer
Infrared
1 Mbps
2 Mbps
IEEE 802.11
Access Points
IEEE 802.11a
Ad hoc
2.4 GHzOFDM
1, 2, 5.5, 11
Mbps
6, 9, 12, 18, 24,
36, 48, 54 Mbps
2.4
GHz
FHSS
1 Mbps
2 Mbps
2.4 GHz
DSSS
1 Mbps
2 Mbps
5 GHz
OFDM
6, 9, 12,
18, 24, 36,
48, 54
Mbps
2.4 GHz
DSSS
1, 2, 5.5,
11 Mbps
IEEE 802.11g
Contention service
Contention-free service
MAC Layer
Point Coordination Function
(PCF)
Distributed Coordination Function (DCF)
Details of 802.11
Protocol Stack

802.11 Protocol Architecture
Extension of Wired InfrastructureReplace Wired LAN
Possible Network Topologies
Configuration:
• To Replace Wired LAN
• Extension of Wired LAN Infrastructure

Sublayers within Physical
Layer
PMD Sublayer
PLCP Sublayer
MAC Layer
The PHY layer
 PHY layer is the interface between the MAC and wireless media
 Provides three levels of functionality
 Frame exchange between the MAC and PHY
 Uses signal carrier and spread spectrum modulation to
transmit data frames over the media
 Provides a carrier sense indication back to the MAC

Limiting factors for high-
speed Network
Performance
 Limiting factor for speed is the fast fading
due to multipath propagation
 Fading between the transmitter and the
receiver caused by:
 Atmospheric scattering
 Reflection
 Refraction or diffraction of the signal
 Causes signal to arrive at the receiver with
different delays and interfere with itself causing
inter-symbol interference (ISI)

Diffracted
Signal
Multipath Propagation of Radio
Signal
Reflected
Signal
Transmitter Receiver
Direct Signal
Building
Scattered Signal

Frequency Selective Fading
 Frequency selective fading can be overcome by Spread Spectrum
(FHSS or DSSS) and OFDM:
 In Spread Spectrum, signal is processed in to occupy a
considerably greater bandwidth to lessen the impact of frequency
selective fading
 In OFDM, the data stream is split into a certain number of
substreams, each having a bandwidth smaller than the coherence
bandwidth of the channel
 GFSK is a modulation scheme in which the data are first filtered
by a Gaussian filter in the baseband, and then modulated with a
simple frequency modulation
 DBPSK is a phase modulation scheme using two distinct carrier
phases for data signaling, providing one bit per symbol
 DQPSK is a type of phase modulation using two pairs of distinct
carrier phases, in quadrature, to signal two bits per symbol
 A third physical layer alternative not widely used, is an infrared
system using near-visible light in the 850 nm to 950 nm ranges as the
transmission medium

IEEE 802.11 Standard
 Three supplements to the IEEE 802.11 standard:
802.11a, 802.11b and 802.11g, as well as an ETSI
standard HIPERLAN/2
 Both 802.11a and HIPERLAN/2 operate in the 5
GHz band and use the modulation scheme OFDM,
but the MAC layers are considerably different
 Compare physical layer characteristics of the
IEEE standards 802.11a and 802.11b
 HIPERLAN/2 shares several of the same physical
properties as 802.11a, and 802.11g

 DSSS uses the 2.4 GHz frequency band
 IEEE 802.11 implements DSSS to fight frequency-selective fading
 The IEEE 802.11b supports 5.5 and 11 Mb/s of higher payload data rates
in addition to the original 1 and 2 Mb/s rates of IEEE 802.11
 IEEE 802.11b also operates in the highly populated 2.4 GHz ISM band
(2.40 to 2.4835 GHz), which provides only 83 MHz of spectrum to
accommodate a variety of other products, including cordless phones,
microwave ovens, other WLANs, and WPANs such as Bluetooth
 Maximum allowable radiated emission limited to 100 mW
DSSS Channels

IEEE 802.11 and the
11-Chip Barker Sequence
Peaks when correlating the sequence
‘10’ with the 11-chip Barker sequence
Peaks when correlating a received sequence
with the 11-chip Barker sequence
 11-chip Barker sequence [1, 1, 1, -1, -1, -1, 1, -1, -1, 1, -1] is the PN chosen for
DSSS PHY layer
 Its autocorrelation shows some very sharp peaks when the transmitter and the
receiver are synchronized
 The first IEEE 802.11 standard used a symbol rate of 1 Mega-symbol per
second (Msps) yielding a 11 MHz chipping rate with the Barker sequence, and
data rates of 1 Mbps (using DBPSK) and 2 Mbps (using DQPSK)

IEEE 802.11b and the 8-Chip
Complementary Code Keying
 DSSS implemented to give data rates of 5.5 Mbps and 11 Mbps using
symbol rates of 1.375 Msps (Million symbols per second) with an 8-
chip Complementary Code Keying (CCK) modulation scheme
 CCK employs a nearly orthogonal complex code set called
complementary sequences
 The chip rate remains consistent with the original DSSS system at
1.375 Msps ⋅ 8 chips/s = 11 Mchip/s while data rate varies to match
channel conditions by changing the spreading factor and/or the
modulation scheme
 The spreading length is reduced from 11 to eight chips
 This increases the symbol rate from 1 Msps to 1.375 Msps
 For the 5.5 Mbps bit rate with a 1.375 MHz symbol rate, it is necessary
to transmit 4 bits/symbol (5.5 Mbps / 1.375 Mspss) and for 11 Mbps,
an 8 bits/symbol

FHSS
 FHSS PMD takes the binary bits of information from the whitened PLCP
service data unit (PSDU) and transforms them into RF signals
 PSDU Data Whitening
 Data whitening is applied to the PSDU before transmission to minimize
bias on the data if long strings of 1’s or 0’s appear in the PSDU
 The PHY stuffs a special symbol every 4 octets of the PSDU in a PPDU
frame
 A 127-bit sequence generator using the polynomial S(x) = x7
+ x4
+ 1 and
32/33 bias-suppression encoding algorithm are used to randomize and
whiten the data
 Modulation
 IEEE 802.11 version released in 1997 uses two-level GFSK in the FHSS
PMD to transmit the PSDU at the basic rate of 1 Mbps
 The PLCP preamble and PLCP header are always transmitted at 1 Mbps
 Four-level GFSK is an optional modulation method defined in the
standard that enables the whitened PSDU to be transmitted at a higher
rate
 GFSK is a modulation technique used by the FHSS PMD shifts the
frequency either side of the carrier hop frequency
 Four-level GFSK is similar to two-level GFSK and modulator combines
two binary bits into symbol pairs (10, 11, 01, 00)

FHSS
 Channel Hopping
 For use in the 2.4 GHz frequency band
 The channels are evenly spaced across the band over a span 83.5 MHz
 Hop channels differs from country to country
 Channel hopping is controlled by the FHSS PMD
 IR
 IR PHY is one of the three PHY layers supported
 Differs from DSSS and FHSS because IR uses near-visible light as the
transmission media
 IR communication relies on the light energy, which is by line-of-sight
or reflected off objects
 Operation is restricted to indoor environments and cannot pass
through walls
 Modulation
 Transmits binary data at 1 and 2 Mbps
 Uses PPM modulation to reduce the optical power required
 Specific data rate is dependent upon the type of PPM
 Modulation for 1 Mbps operation is 16-PPM, while it is 4-PPM for 2
Mbps
 PPM is a modulation technique that keeps the amplitude, pulse width
constant, and varies the position of the pulse in time

FHSS
Data bits 4-PPM symbol
00 0001
01 0010
11 0100
10 1000
4-PPM symbol
map for 2 Mbps

OFDM
 Standard employs 300 MHz bandwidth in the 5 GHz unlicensed national information
infrastructure (UNII) band
 The spectrum is divided into three “domains,” each having restrictions on the maximum
allowed output power
 The first 100 MHz in the lower frequency portion is restricted to a maximum power
output of 50 mW
 The second 100 MHz has a higher 250 mW maximum
 The third 100 MHz intended for outdoor applications, has a maximum of 1.0 W power
output
 OFDM in 802.11a and 802.11g combines multicarrier, multisymbol, and multirate
techniques
 Divides transmitted data into multiple parallel bit streams, each with relatively lower bit
rates and modulating separate narrowband carriers, referred to as sub-carriers
 The sub-carriers are orthogonal, so each can be received without interference from
another
 802.11a specifies eight non-overlapping 20 MHz channels in lower two bands; each of
these are divided into 52 sub-carriers (four of which carry pilot data) of 300-kHz
bandwidth each
 Four non-overlapping 20 MHz channels are specified in the upper band
 The receiver processes the 52 individual bit streams
 This multicarrier technique has some important properties such as reducing multipath
and allowing individual sub-carriers to be coded accordingly
 This is also known as Coded OFDM (COFDM)
 Four modulation methods are BPSK, QPSK, 16-QAM, and 64-QAM
 16-QAM has 16 symbols, each representing four data bits. 64-QAM has 64 symbols, each
representing six data bits
 The symbol rate for a constellation is 250 Kilo symbols per second (Ksps) or the data
rate for a 16-QAM is (4 bits/symbol ⋅ 250 Ksps) = 1 Mbps

IEEE 802.11a Data Rate
Description
Data
Rate
(Mbit/s)
Modulation
Type
Coding Rate
(Convolution
Encoding &
Puncturing)
Coded bits
per sub-
carrier
symbol
Coded bits
per OFDM
symbols
Data bits
per OFDM
symbol
6* BPSK ½ 1 48 24
9 BPSK ¾ 1 48 36
12* QPSK ½ 2 96 48
18 QPSK ¾ 2 96 72
24* 16-QAM ½ 4 192 96
36 16-QAM ¾ 4 192 144
48 64-QAM 2/3 6 288 192
54 64-QAM ¾ 6 288 216
* Support for these data rates is required by the IEEE 802.11a standard

IEEE 802.11a Data Rate
Description
Data
Rate
(Mbit/s)
Modulation
Type
Coding Rate
(Convolution
Encoding &
Puncturing)
Coded bits
per sub-
carrier
symbol
Coded bits
per OFDM
symbols
Data bits
per OFDM
symbol
6* BPSK ½ 1 48 24
9 BPSK ¾ 1 48 36
12* QPSK ½ 2 96 48
18 QPSK ¾ 2 96 72
24* 16-QAM ½ 4 192 96
36 16-QAM ¾ 4 192 144
48 64-QAM 2/3 6 288 192
54 64-QAM ¾ 6 288 216
* Support for these data rates is required by the IEEE 802.11a standard
 OFDM can eliminate inter-symbol interference at no bandwidth cost
 However, OFDM is very sensitive to frequency offsets and timing jitter

Comparison Among IEEE
802.11a/b/g

IEEE 802.11a/b/g
 A shorter wavelength is its main drawback
 Higher-frequency signals will have more trouble propagating
through physical obstructions in an office than those at 2.4
GHz
 An advantage of 802.11a is its intrinsic ability to handle delay
spread or multipath reflection effects
 The slower symbol rate and placement of significant guard
time around each symbol
 Greater number of channels available to 802.11a (thirteen in
US and up to nineteen in Europe) as compared to 802.11b/g
 Both 802.11b and 802.11g operate in the crowded 2.4 GHz
band used by several others equipment such as Bluetooth
devices, microwaves, cordless phones, garage door openers,
etc.
 Drawback of backward interoperability requirement with
802.11b devices

Maximum Transport Level
Throughput in 802.11a/b/g
 Absence of 802.11b devices (802.11g-only environment) 802.11g throughput is
equivalent to 802.11a
 However in a mixed mode 802.11b/g environment, 802.11g devices have to adjust some
properties
 Worst case 802.11g performance may be as low as the slowest 802.11b device
 All 802.11a/b/g use dynamic rate shifting by automatically adjusting data rate based on
the condition of the radio channel

MAC Layer
 MAC protocol is the arbitration of accesses to a shared medium among several
end systems
 IEEE 802.11 specifies two medium access control protocols, Point
Coordination Function (PCF) and Distributed Coordination Function (DCF)
 DCF is a fully distributed scheme which enables the ad hoc networking
capabilities, whereas PCF is an optional centralized scheme built on top of the
basic access method
Logical Link Control
Contention service
Contention-free service
MAC Layer
(PCF)
Distributed Coordination Function (DCF)
IEEE 802.11b
PHY Layer
Infrared
1 Mbps
2 Mbps
IEEE 802.11
Access Point
IEEE 802.11a
Ad hoc
2.4 GHzOFDM
1, 2, 5.5, 11
Mbps
6, 9, 12, 18, 24,
36, 48, 54 Mbps
2.4
GHz
FHSS
1 Mbps
2 Mbps
2.4 GHz
DSSS
1 Mbps
2 Mbps
5 GHz
OFDM
6, 9, 12,
18, 24, 36,
48, 54
Mbps
2.4 GHz
DSSS
1, 2, 5.5,
11 Mbps
IEEE 802.11g

Area
cleared
by CTS
C
Hidden Terminal Problem
BA
Area
cleared
by RTS
RTS
 RTS/CTS
handshake
 Failure causes the
RTS frame to be
retransmitted
 This is treated as
a collision
 Rules for
scheduling the
retransmission is
considered later
 RTS/CTS
handshake can be
disabled in some
situations
 Setting Network
Allocation Vector
(NAV)
CTS

The Retry Counters
 Two retry counters associated with MAC: A short retry
counter and a long retry counter
 Former is associated with short frames (i.e., frames with
size less than dot11RTSThreshold), while the latter controls
long frames
 In addition to the counters, a lifetime timer is associated
with every transmitted MAC frame
 Upon an unsuccessful transmission, the corresponding
counters are incremented
 When they reach the threshold defined in the MIB (i.e.,
dot11ShortRetryLimit and dot11LongRetryLimit), the frame
is discarded

Time Intervals
 Five time intervals
 The slot time, defined in the PHY layer
 The short interframe space (SIFS) defined by the PHY layer
 The priority interframe space (PIFS)
 The distributed interframe space (DIFS)
 The extended interframe space (EIFS)
 Use of five time intervals
 IFSs provide priority levels for channel access
 SIFS is the shortest interval (equal to 10µsec), followed by the slot
time which is slightly longer (equal to 20µsec)
 PIFS is equal to SIFS plus one slot time
 DIFS is equal to the SIFS plus two slot times
 EIFS is much larger than any of the other intervals
 Used by a station to set its NAV when it receives a frame
containing errors, allowing the possibility for the ongoing MAC
frame exchange to complete before another transmission attempt
 These values may change from standard to standard
 For instance, in 802.11a the slot time value has been decreased
(now equal to 9µsec)

Ranges and Zones
Y Z
 Transmission Range
 Carrier-sensing Range
 Interfering Range
X
C-Zone
Carrier-sensing range
C-Zone
Transmission range

Distributed Coordination
Function (DCF)  Two forms of carrier sensing:
physical (by listening to the wireless
shared medium) and virtual
 Virtual carrier sensing uses the
duration field to set a station’s NAV
which is included in the header of
RTS and CTS frames
 If medium found idle for more than a
DIFS period, then the frame can be
transmitted
 Otherwise, the transmission is
deferred and the station uses an
Exponential Random Backoff
Mechanism by choosing a random
backoff interval from [0, CW]
 If collision occurs, the station doubles
its CW
 At the first transmission attempt, CW
= CWmin and is doubled at each
retransmission up to CWmax

802.11 with RTS/CTS
 Basic access mechanism of IEEE 802.11 can be extended by the RTS/CTS frame
exchange to reserve channel before data transmission
 When a station wants to send a frame larger than above a specified threshold
(dot11RTSThreshold), it first sends a short control frame RTS to the destination station
 The destination then sends, after a SIFS, another short control frame CTS back to the
source
 The source then transmits its DATA frame after SIFS period as the channel is reserved
for itself during all the frame duration
 Both RTS and CTS frames carry the duration needed
 After the destination receives the DATA, it sends an ACK back to the source after SIFS
period

Collisions not completely
avoided in IEEE 802.11
DB
HGD EBA
Transmission
Range for
RTS
Carrier
sensing zone
for RTS
Carrier
sensing zone
for CTS
Transmission
Range for
CTS
HEA
G
 Nodes in transmission range
correctly set their NAVs
when receiving RTS or CTS
 However, since nodes in the
C-zone cannot decode the
packet, they do not know
duration
 To prevent a collision with
the ACK reception, nodes
within the C-zone set their
NAVs for the EIFS duration
 IEEE 802.11 does not
completely prevent collisions
due to a hidden terminal
 Nodes in the receiver’s C-
zone but not in the sender’s
C-zone or transmission
range, can cause a collision
with the reception of a
DATA packet at the receiver

(PCF)
The MAC control logic
 PCF employs a poll and response
protocol so as to eliminate the
possibility of contention for the
medium
 A point coordinator (PC) controls
the medium access and is often co-
located with the AP
 PC maintains a polling list, and
regularly polls the stations for
traffic while also delivering traffic
to the stations
 PCF is built over the DCF, and
both of them operate
simultaneously
 PC begins a period of operation
called the contention-free period
(CFP)
 CFP occurs periodically to provide
a near-isochronous service to the
station
 CFP begins when the PC gains
access to the medium, by using the
normal DCF procedures
 During the CFP, the PC ensures
that the interval between
transmissions to be no longer than
PIFS

Framing
 MAC layer accepts MAC Service Data Units
(MSDUs) from higher layers and adds appropriate
headers and trailers to create MAC Protocol Data
Units (MPDU)
 MAC may fragment MSDUs into several frames,
hence attempting to increase the probability of
each individual frame being delivered successfully
 Header + MSDU + Trailer contain the following
information:
 Addressing information
 IEEE 802.11-specific protocol information
 Information for setting the NAV
 Frame check sequence for integrity verification

General Frame Format
The IEEE 802.11 MAC frame format
The Frame Control field

Frame Control (FC)
 Frame Control field is composed of a total of eleven sub-fields which
adds up to a total of 2 bytes and are:
 Protocol Version
 Frame Type and Sub Type
 To DS (Distribution System) and From DS
 More Fragments
 Retry
 Power Management
 More Data
 WEP (Wired Equivalent Privacy)
 Order
 Duration ID (D/ID)
 D/ID contains information for setting the NAV by setting 15th bit
to zero
 Address Fields: Four
 The IEEE 48-bit address comprises of three fields
 A single-bit Universal/Local bit
 BSS Identifier (BSSID)
 Transmitter Address (TA)
 Source Address (SA)
 Destination Address (DA)

Frame Control (FC)
 Sequence Control
 4-bit fragment number and a 12-bit sequence
number
 Fragment Number sub-field
 Sequence Number sub-field
 Frame Body
 A variable length field which contains the
information specific to the particular data or
management frame.
 It can go up to 2304 bytes, and 2312 bytes when
encrypted
 Frame Check Sequence (FCS)
 An IEEE 802 standard generated similar to IEEE
802.3, using the following CRC-32 polynomial:
1)( 245781011121622232632
++++++++++++++= xxxxxxxxxxxxxxxG

Control Frame Subtypes
 Request To Send (RTS)
 This is a 20 bytes frame which includes the FC field, the Duration field, the RA and
the TA fields, and the FCS
 Clear To Send (CTS)
 This is a 14 bytes frame which contains the FC field, the Duration field, the RA, and
the FCS field
 Acknowledgement (ACK)
 FC Field
 Duration/ID Field (ms)
 RA
 FCS
 Data Frame Subtypes
 Data frame is variable in length (from 29 to 2346 bytes)
 Address 2 field is used to identify the sender of the frame
 Address 3 field carries additional information for frame filtering or forwarding by
the DS
 Address 4 field is used only in a wireless DS as one AP forwards a frame to another
AP
 DA is the destination of the MSDU in the frame body field
 SA is the address of the MAC entity that initiated the MSDU in the frame body field
 RA is the address of the station contained in the AP in the wireless DS that is next
recipient
 TA is the address of the station contained in the AP in the wireless DS that is
transmitting the frame
 BSSID is the address currently in use by the station contained in the AP if the
station is the AP or is associated with an AP

WEP Security Mechanisms
 WEP security mechanisms include data encryption and integrity
 To prepare a protected frame, first an integrity check value (ICV) of the frame
payload is computed using a cyclic redundancy check (CRC) function
 The cleartext payload concatenated with the ICV is then encrypted using a
bit-wise Exclusive-OR operation with a pseudorandom keystream

WEP Security Flaws
 Suffers from many design flaws
 Data encryption based on an approximation of the “one-
time pad” algorithm
 Like WEP encryption, one-time pad encryption consists of
the bit-wise Exclusive-OR between a binary plaintext
message and a binary keystream
 The secrecy of the resulting cipher text is perfect, provided
that each new message is encrypted with a different secret
random keystream
 The secrecy is not guaranteed when the keystream is re-
used or its values can be predicted
 Hence, a first class of attacks on WEP exploits possible
weaknesses in WEP’s keystream generation process that
makes the secret keystream easily predictable or causes its
re-use

The IEEE 802.11i Amendment:
A New Security Scheme
 Amendment called 802.11i. IEEE 802.11i, also known as Wi-fi
Protected Access (WPA)
 Three major parts: Temporal Key Integrity Protocol (TKIP),
counter mode cipher block chaining with message authentication
codes (counter mode CBC-MAC) and IEEE 802.11x access control
 TKIP fixes its well-known problems, including small IV and
short encryption keys
 Counter mode CBC-MAC is designed to provide link layer data
confidentiality and integrity
 Centralized, server-based authentication occurs when a client
first joins a network and then, authentication periodically
recurs to verify the client has not been subverted or spoofed
 System Design Considerations
 The Medium
 Multipath
 Path Loss
Delay spread for various environments

Multipath Fading and
802.11’s
 Es/No vs. BER (Bit-Error-Rate) Performance
 Interference in the 2.4 GHz ISM Band
 An Overview of Past and Present IEEE 802.11 Efforts
 802.11a: in the 5 GHz radio band, with thirteen available radio channels, with
the maximum link rate per channel is of 54 Mbps
 802.11b: in the 2.4 GHz radio band with the maximum link rate per channel is
of 11 Mbps
 802.11d : this standard cannot legally operate in some countries, and the
purpose of 802.11d is to add features and restrictions to allow WLANs to
operate within the rules of those countries
 802.11e: is supplementary to the MAC layer to provide QoS support for 802.11
physical standards a, b and g
 802.11f: is a recommended practice document that enables interoperability
between a multi-vendor WLAN networks
 802.11g: physical layer standard in the 2.4 GHz and 5 GHz radio bands and
specifies three non-overlapping radio channels similar to 802.11b, with a
maximum link rate is 54 Mbps per channel using OFDM modulation but, for
backward compatibility with 802.11
 802.11h: supplementary to the MAC layer so as to comply with European
regulations for 5 GHz WLANs with transmission power control (TPC) and
dynamic frequency selection (DFS)
 802.11i: a part of a set of security features that should address and overcome
security issues and applies to 802.11 physical standards a, b and g
 802.11j: to enhance the 802.11 standard and amendments enabled addition of
channel selection for 4.9 GHz and 5 GHz in Japan

Multipath Fading
 An Overview of Past and Present IEEE 802.11 Efforts
 802.11k: to define Radio Resource Measurement enhancements that provide
mechanisms to higher layers for radio and network measurements
 802.11n: to develop enhancements for higher throughput that allows consumers
and businesses to transmit data at rates greater than 100 Mbps
 802.11p: expands wireless networking useful to automobiles
 802.11r: task group is working on speeding up the data signal transfer speed,
also known as handoff, between wireless access points
 802.11s: task group is working on a new standard to support extended set of
service for mesh networks which can be seen as an ad hoc network with massive
increment in bandwidth and reliability
 802.11t: to define a recommended practice for the evaluation of 802.11 wireless
performance, which is currently done on an ad hoc basis
 802.11u: 802.11 hotspot deployment has experienced tremendous growth
throughout the world, the task group is developing a standard specifying how
802.11 networks can work with other external networks
 802.11v: to develop an amendment to both the IEEE 802.11 PHY and MAC that
provides wireless network management of client stations
 802.11w: to develop enhancements to the IEEE 802.11 MAC that enable data
integrity, data origin authenticity, replay protection, and data confidentiality for
selected IEEE 802.11 management frames
 802.11x: for regulating access control for stations to a network via the use of
extensible authentication methods applicable to 802.11 physical standards a, b
and g

The IEEE 802.11e MAC
Protocol
Multiple parallel backoffs of MSDUs with different priorities
 To effectively support QoS, the 802.11e MAC defines the Hybrid Coordination
Function (HCF) that replaces DCF and PCF modes in IEEE 802.11 standard
 Two parts: the Extended Distributed Channel Access (EDCA) and the HCF
Controlled Channel Access (HCCA)
 MSDUs are now delivered through multiple backoff instances within one station,
wherein each backoff instance parameterized with Traffic Categories-specific
parameters

The IEEE 802.11e MAC
Protocol Each backoff sets a counter to a random number drawn from the interval
[1;CW+1]
 When the medium is determined busy before the counter reaches zero, the backoff
has to wait for the medium being idle for AIFS again, before resuming the count
down process
 When the medium is determined as being idle for the period of AIFS, the backoff
counter is reduced by one beginning the last slot interval of the AIFS period
 After any unsuccessful transmission attempt, a new CW is calculated with the help
of the persistence factor (PF), PF[TC], and another uniformly distributed backoff
counter out of this new, enlarged CW is drawn, so that the probability of a new
collision is reduced
 PF to increase the CW differently for each TC and is given by:
 CW never exceeds the parameter CWmax[TC]
 A single station may implement up to eight transmission queues realized as virtual
stations inside a station, with QoS parameters that determine their priorities
 If the counters of two or more parallel TCs in a single station reach zero at the
same time, a scheduler within the station avoids the virtual collision
 There is still a possibility that a transmitted frame could collide at the wireless
medium with a frame transmitted by other stations
 Another important part is the TXOP which is an interval of time when a station has
the right to initiate transmissions, defined by a starting time and a maximum
duration
1))1][((][ −⋅+≥ PFTColdCWTCnewCW

Virtual back of eight traffic
categories
sµ
sµ
,
Virtual back of eight traffic categories
(1) Left: legacy DCF close to EDCA with AIFS=34, C W min=15, PF=2;
(2) Right: EDCA with AIFS[TC] ≥ 34, C W min [TC]=0-255, PF[TC]=1-16

The HCCA
 The HCCA extends the EDCA access rules by allocating
TXOPs to itself in order to initiate MSDU Deliveries
whenever it desires, however, only after detecting the
channel as being idle for PIFS
 To give the HC priority over the EDCA, AIFS must be
longer than PIFS and can therefore not have a value
smaller than DIFS
 The QoS CF-Poll from the HC can be sent after a PIFS
idle period without any backoff
 An additional random access protocol that allows fast
collision resolution is defined
 The controlled contention mechanism allows stations to
request the allocation of polled TXOPs by sending
resource requests, without contending with other EDCA
traffic
 This control frame forces legacy stations to set their NAV
until the end of the controlled contention interval, thereby
remaining silent during the controlled contention interval

Enhancements to IEEE 802.11
MAC
 Power Control
 Power control can also increase effective capacity of the network by
enhancing spatial reuse of the wireless channel
 Current research on power control MAC protocols focus on suitably
varying transmit power in order to reduce energy consumption
 May be classified based upon the presence or absence of asymmetric links
between nodes
 The BASIC Protocol
 Power control can can also lead to asymmetry between nodes
 This asymmetry is addressed by transmitting the RTS and CTS packets at
maximum possible power level (pmax), while transmitting DATA and ACK
at lowest power level needed to communicate (pdesired)
 In the BASIC scheme, when nodes receive either a RTS or CTS packet
(always transmitted at pmax), they set their NAVs for the duration of the
DATA and ACK transmission
 In regular IEEE 802.11, the C-zone is the same for RTS-CTS and DATA-
ACK since all packets are sent using the same power level (pmax)
 In the BASIC scheme, the transmission range for DATA-ACK is smaller
than that of RTS-CTS whenever a source and destination pair decides to
reduce the transmit power for DATA-ACK
 Similarly, the C-zone for DATA-ACK is also smaller than that of RTS-
CTS


The BASIC protocol
Carrier sensing
zone for CTS
Carrier sensing
zone for RTS
Transmission
Range for CTS
Transmission
Range for RTS
Transmission
Range for DATA
Transmission
Range for ACK
C-Zone for
DATA
C-Zone for
ACK
A
CB
D E GF
H
The BASIC Protocol

The Power Control MAC
Protocol
 To address the deficiency of the BASIC protocol, the PCM protocol has been
proposed
 Similar to the BASIC protocol, PCM transmits the RTS and the CTS packets at
pmax and use the minimum power level (that is, pdesired) needed for communication
for DATA and ACK
 However, contrary to the BASIC scheme, eventual collisions with nodes in the C-
zone are avoided by making the source node in a transmission periodically
transmit the DATA packet at pmax so that nodes in the C-zone can sense the signal
and set their NAVs accordingly
 Transmit power level transitions in PCM during a regular sequence of RTS-CTS-
DATA-ACK transmission shown below

Power Control in PCM
 PCM overcomes the deficiency
of the BASIC scheme and can
achieve throughput comparable
to that of IEEE 802.11 with less
energy consumption
 However, note that PCM, just
like 802.11, does not prevent
collisions completely
 Despite of all this, PCM suffers
from a drawback, namely, the
inability to achieve spatial reuse
 Note that this shortcoming is
also present in other existing
power control MAC protocols
C
BA
H
F
D
E
G
A’s
Pdesired
C-Zone
Transmission Range
Blocking Region because
of A’s Transmission
Carrier Sensing Zone
(C-Zone) for A’s RTS
A’s Pdesired
Range

Spatial Reusability
 Spatial Reuse MAC (SRM) protocol
is introduced which uses power
control and employs a distributed
form of transmission sneaking to
accomplish appropriate spatial reuse
of the channel
 The SRM protocol is similar to the
BASIC scheme in that it transmits
RTS and CTS at Pmax, and DATA
and ACK at Pdesired
 However, SRM implements a fully
distributed transmission sneaking
technique so as to enable channel
spatial reuse
 Transmission sneaking is a spatial
reuse by which a pair of nodes can
communicate despite the ongoing
transmission in its radio range
(explained in the next slide)
Sneaking Zone
(S-Zone)
C-Zone for A’s
CTS
B A
Y
X
A’s Pdesired
C-Zone
Transmission Range
A’s Initial Interference Range for
CTS Reception
A’s Pdesired
Range
C-Zone for A’s
CTS

Channel Spatial Reusability
The size of the S-Zone may be larger
than the C-Zone
It may also include a part of RTS-CTS
transmission range, which becomes
free because of the low power DATA-
ACK transmission
This reduces the carrier-sensing range of
the communication between A and B
 SRM differentiates between two
types of transmission:
Dominating Transmission
Sneaking Transmission
C-Zone for A’s RTS
B
J
F
A
G
E
D
C
I
H
C-Zone for
A’s Data
Possible S-Zone
RTS Range
for A
A’s Data Range

Channel spatial reusability
in the SRM protocol
Mathematically speaking, node D, can sneak a packet at pdesired to node C
during the DT between nodes A and B if:
distanceD,C + distance(pdesired, CSThresh) < distanceD,A ; and
distanceD,C+ distance(pdesired, CSThresh) < distanceD,B

QoS Provisioning
 An Extension to the IEEE 802.11 DCF
 A scheme to extend the IEEE 802.11 DCF with the ability to support at least two
service classes: premium service (i.e., high priority) and best-effort
 Traffic of premium service class is given lower values for congestion window than
those of best-effort traffic
 If packets of both types collide, the packet with smaller congestion window value
is more likely to access the medium earlier
 The Black Burst Contention Scheme
 Avoids packet collision in a very distinctive way, while at the same time solving
the packet starvation problem
 Packets from two or more flows of the same service class are scheduled in a
distributed manner with fairness guarantees
 Nodes with best-effort traffic and nodes with real-time traffic use different
interframe space values to provide higher priority
 Each contending node is using a BB with different length, where the length of
each BB is an integral number of black slots
 The MACA/PR Protocol
 Multi-hop Access Collision Avoidance with Piggyback Reservation (MACA/PR)
protocol provides guaranteed bandwidth support (via reservation) for real-time
traffic
 The first data packet in the real-time stream makes reservations along the path
 A RTS/CTS handshake is used on each link for this first packet in order to make
sure that it is transmitted successfully
 When the sender transmits the data packet, it schedules the next transmission
time after the current data transmission and piggybacks the reservation in the
current data packet

The HIPERLAN/2
Standard for Wireless
LANs
 HIPERLAN/2 standard also operates at 5 GHz frequency band similarly to
802.11a
 HIPERLAN/2 and 802.11 standards primarily differ in the MAC layer,
however some minor differences are also present in the PHY layers
 HIPERLAN/2 radio network is defined in such a way that there are core-
independent PHY and data link control (DLC) layers as well as a set of
convergence layers (CLs) for internetworking
 The CLs include Ethernet, ATM, and IEEE 1394 infrastructure and technical
specifications for HIPERLAN/2–third generation (3G) internetworking have
also been completed
 Physical Layer
 OFDM is used to combat frequency selective fading and to randomize the
burst errors caused by a wideband fading channel
 Exact mechanism for different coding and modulation schemes is not
specified in the standards
 Data for transmission is supplied to the PHY layer in the form of an input
PDU train or PPDU frame
 This is then input to a scrambler that prevents long runs of 1s and 0s in
the input data being sent to the remainder of the modulation process
 Although both 802.11a and HIPERLAN/2 scramble the data with a length
127 pseudorandom sequence, the initialization of the scrambler is different
 The coded data is interleaved in order to prevent error bursts
 In order to prevent ISI and intercarrier interference (ICI) due to delay
spread, a guard interval is implemented by means of a cyclic extension

MAC Layer for
HIPERLAN/2 and 802.11
Standards In HIPERLAN/2, medium access is based on a TDMA/TDD
approach using a MAC frame with a period of 2ms
 This frame comprises of uplink (to the AP), downlink (from the
AP), and direct link (DiL, directly between two stations) phases
 These phases are scheduled centrally by the AP, which informs
stations at which point in time in the MAC frame they are
allowed to transmit their data
 Time slots are allocated dynamically depending on the need for
transmission resources
 The HIPERLAN/2 MAC is designed to provide QoS support,
essential to many multimedia and real-time applications
 Another significant difference between the two standards is the
length of the packets employed as HIPERLAN/2 employs fixed
length packets, while 802.11a supports variable length packets

MAC Layer for
HIPERLAN/2 and 802.11
Standard s
(a) – HIPERLAN/2 MAC frame
(b) – HIPERLAN/2 PDU format
(c) – 802.11a PDU format
(d) – DCF access mechanism

Conclusions and Future
Directions
 The low cost of wireless LANs has led to a tremendous growth of its
worldwide use
 Wireless LANs can be found in nearly all enterprise environments, in
many homes, hotspots, airport lounges, among others
 Today, there is a high demand for the efficient support of multimedia
applications over wireless LANs
 Needless to say that security is also a major concern
 IEEE is indeed addressing some of these aspects while the problem
space is much larger
 The efficient utilization of the scarce radio resource is also an existing
concern which needs more investigation, and cognitive and spectrum
agile radios are attempts to address this issue
 Finally, integration of wireless LANs into the future integrated next
generation heterogeneous networks beyond 3G is also a very hot topic
 Time to move to wireless PANs

Adhoc and Sensor Networks - Chapter 04

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Adhoc and Sensor Networks - Chapter 04