IEEE 802.11 Ad Hoc Mode: Measurement studies

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IEEE 802.11 Ad Hoc Mode Measurement studies
Marco Conti Computer Networks Dept., IIT
CNR Marco.conti_at_iit.cnr.it http//cnd.iit.cnr.it/m
obileMAN
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MobileMAN Enabling Technologies

• Reference technology IEEE 802.11b (Wi-Fi)
• Develop an enhancement wireless multiple access
  layer starting from existing wireless
  technologies
• Design and prototype a new MAC card
• minimal change modification only to MAC (not to
  physical layer)
• compatibility with original 802.11

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IEEE 802.11 Simulative Studies
Simulative studies are highly dependent on the
802.11 channel model
Measurements studies are required

• Measurements of IEEE 802.11 in Ad Hoc
  configurations
• Understanding of important phenomena in ad hoc
  configuration
• Channel model
• Tuning of simulative experiments

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IEEE 802.11 behavior
The Transmission Range (TX_Range) represents the
range (with respect to the transmitting station)
within which a transmitted packet can be
successfully received. The Physical Carrier
Sensing Range (PCS_Range) is the range (with
respect to the transmitting station) within which
the other stations detect a busy
channel. Interference Range (IF_Range) is the
range within which stations in receive mode will
be interfered with by a transmitter, and thus
suffer a loss.
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IEEE 802.11 behavior Simulative Studies
The following relationship exists between the
ranges TX_Range lt IF_Range ltPCS_Range
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Experimental Environment

• Hardware
• Wireless D-LinkAir DWL-650 card
• ( IEEE 802.11b )
• Laptops (41)

• Software
• Operative System Linux Mandrake 8.2
• Software for the traffic generation DBS
• Software to trace MAC PDU Snuffle

• Physical Environment
• Open-space areas near CNR in Pisa.

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802.11b Throughput
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Measurements of the Transmission Range
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Transmission Range TXDATA and TXCONTROL

• 11 Mbps
• 5.5 Mbps
• 2 Mbps

• DATA Frame

IEEE 802.11b

• Control Frame 2 Mbps

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Impact of Ground on TX
The transmission ranges depend on the devices
height from the ground
The experiments were performed with the Wi-Fi
card set at two different transmission rates 2
and 11 Mbps. In each set of experiments the
distance among the two devices was set close to
guarantee that the receiver is always inside the
sender transmission range. Specifically, the
sender-receiver distance was equal to 30 and 70
meters when the cards operated at 11 and 2 Mbps,
respectively.
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Impact of Ground on TX
The Fresnel Zone Effect
Most of the radio-wave energy is within the First
Fresnel Zone, i.e., the inner 60 of the Fresnel
zone. Hence, if this inner part contacts the
ground (or other objects) the energy loss is
significant R1 is highly dependent on the nodes
distance. For example, when the sender and the
receiver are at an height of 1 meter from the
ground, the First Fresnel Zone has a contact with
the ground only if D gt 33 meters
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Physical Carrier Sensing
d(1,2)d(3,4) 25m d(2,3)80m Rate11 Mbps
Throughput in isolation UDP 3 Mbps TCP
1,3 Mbps
Hypothesis
interdependencies among the stations extends
beyond the transmission range and the physical
carrier sensing range, including all the four
stations, produces a correlation between active
connections
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Physical Carrier Sensing Range
HYPOTHESIS The large physical carrier sensing
range, including all the four stations, produces
a dependency between active connections
Indirect measurement increase d(2,3) until no
correlation is measured among the two sessions.
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Physical Carrier Sensing Range
The hypothesis is that dependencies are due to a
large physical carrier sensing that includes all
the stations
The idea is to increase d(2,3)x (while
d(1,2)d(3,4)10 meteres) until no correlation
(i.e., D1(x)0) is measured among the two
sessions.
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Physical Carrier Sensing Range
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802.11 Channel Model

• Some reference values
• Tx 110 m
• PCS_range 200 m
• Radiated area 300-350 m

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802.11 Channel Model
S

1. Nodes at a distance d gt PCS_Range do not measure
   any significant energy on the channel when S is
   transmitting, therefore they can start
   transmitting contemporarily to S in this case
   some interference phenomena may occur if
   d lt PCS_Range TX_Range(x).

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802.11 Channel Model

• The hidden station phenomenon, as it is usually
  defined in the literature, is almost impossible
  with the ranges measured in our experiments
• Indeed, the PCS_Range is more than twice
  TX_Range(1), i.e., the larger transmission range

The RTS / CTS mechanism is of little/no help
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New Hidden station phenomenum

• Two transmitting stations, S and S1 that are
  outside their respectively PCS_Range
• The receiver of station S (denoted by R in the
  figure) is inside the interference range
  (IF_Range) of station S1
• S and S1 can be simultaneously transmitting and,
  if this occurs, station R cannot receive data
  from S correctly.

R1
S1
R
S
Let d be the distance between S and S1
PCS_Range lt d lt PCS_Range TX_Range(x) S and
S1 may generate a new HIDDEN node phenomenon
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New EXPOSED node

• S1 is a station at a distance d1 from S
  PCS_Range lt d1 lt PCS_RangeTX_Range(x)
• E is inside the PCS_Range of S
• S1 can start transmitting, with a rate x, towards
  the station E
• E cannot reply because it observes a busy channel
  due to the ongoing station S transmissions

d
S1
E
R
S
PCS range - TX_range (1)
PCS_range
Nodes at distance d PCS_Range - TX_Range(1) lt
d lt PCS_Range are new EXPOSED
nodes
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Conclusions

• 802.11 channel model shows that hidden station
  phenomenon is impossible, but other new hidden
  station phenomenon can appear.
• There is also a never analyzed Exposed node
  phenomenon
• A new coordination mechanisms need to be designed
  to extend the coordination in the channel access
  beyond the PCS_Range

MAC alone cannot solve the problem CROSS
LAYERING MAC-Routing
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Questions ?

• References
• Deliverable D5
• Giuseppe Anastasi, Eleonora Borgia, Marco Conti,
  Enrico Gregori, Wi-Fi in Ad Hoc Mode A
  Measurement Study, Proc. IEEE PerCom 2004,
  Orlando, Florida, March 2004.
• G. Anastasi, M. Conti, E. Gregori, IEEE 802.11
  Ad Hoc Networks Protocols, Performance and Open
  Issues, Mobile Ad hoc networking, S. Basagni, M.
  Conti, S. Giordano, I. Stojmenovic (Editors),
  IEEE Press and John Wiley and Sons, Inc., New
  York, 2004.

Thank You !
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Conclusions

• The transmission ranges are
• much shorter than assumed in simulation analysis
• Not constant but highly variable in time, in
  space and height, even in the same session
• The carrier-sensing range is about twice
  TX_Range(1), i.e., the larger transmission range
  and it does not depend on data rate
• The dynamics of an IEEE 802.11b system are
  significantly complicated by the existence of
  different transmission (TXDATA and TXControl) and
  carrier-sensing ranges existing simultaneously on
  the channel

A new coordination mechanisms need to be designed
to extend the coordination in the channel access
beyond the PCS_Range
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Fresnel Zone (2)

1. The channel power loss depends on the contact
   between the Fresnel zone and the ground
2. The Fresnel zone for a radio beam is an
   elliptical area with foci located in the sender
   and the receiver
3. Objects in the Fresnel zone cause diffraction and
   hence reduce the signal energy (most of the
   radio-wave energy is within the First Fresnel
   Zone, i.e., the inner 60 of the Fresnel zone)
4. H1 mt First Fresnel Zone touchs the ground
   D33mt
5. H1.5 mt First Fresnel Zone touchs the ground
   D73 mt
6. H2.0 mt First Fresnel Zone touchs the ground
   D133 mt