ieee 802.11 ad hoc mode: measurement studies marco conti computer networks dept., iit cnr...

IEEE 802.11 Ad Hoc Mode: IEEE 802.11 Ad Hoc Mode: Measurement studiesMeasurement studies

Marco ContiComputer Networks Dept., IIT CNR

[email protected]://cnd.iit.cnr.it/mobileMAN

MobileMAN meeting (Helsinki, 7 June 2004)

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

MobileMAN: Enabling Technologies


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

IEEE 802.11 Simulative Studies


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.

STx_Range

.

R S

IEEE 802.11 behavior


The following relationship exists between the ranges:

TX_Range <= IF_Range <=PCS_Range

Ranges used in simulatorsQualNet/Glomosim NS2

Pathloss Two-Ray Two-Ray

TX_Range 376m 250 m

CS_Range 670m (= IF_Rangemax) 550m

IF_Range 1.7*(sender-receiver)distance

550m

IEEE 802.11 behavior: Simulative Studies


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

HardwareHardware:: Wireless D-LinkAir DWL-650 card ( IEEE 802.11b ) Laptops (4+1)

Physical Environment:Physical Environment:

Open-space areas near CNR in Pisa.

2

1

3

4

M

Experimental Environment


m Bytes

TCP/UDP payload

IP payload

Hdr

Hdr

MAC payloadHdr+FCS

PSDUPHY Hdr

TDATA

TPayload

ApplicationLayer

TransportLayer

NetworkLayer

MACLayer

PhysicalLayer

ThnoRTS / CTS Tpayload

DIFS TDATA SIFS TACK CW min

2* Slot _ Time

ThRTS / CTS Tpayload

DIFS TRTS TCTS TDATA TACK 3* SIFS CW min

2* Slot_ Time

802.11b Throughput

Table. Maximum throughput at different data rates.m= 512 Bytes m=1024 Bytes

No RTS/CTS RTS/CTS No RTS/CTS RTS/CTS

11 Mbps 3.337 Mbps 2.739 Mbps 5.120 Mbps 4.386 Mbps

5.5 Mbps 2.490 Mbps 2.141 Mbps 3.428 Mbps 3.082 Mbps



11 Mbps UDP

0

0.5

1

1.5

2

2.5

3

3.5

no RTS/CTS RTS/CTS

Th

rou

gh

pu

t (M

bp

s)

ideal

real UDP


Measurements of the Transmission Range

2 1

TXDATA

Table 1. Estimates of the transmission ranges at different data rates.

11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

Data TX_range 30 meters 70 meters 90-100 meters 110-130 meters

Control TX_range 90 meters 120 meters

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100 120 140

11 Mbps5.5 Mbps2 Mbps1 Mbps

Distance (metres) 2 Mbps data rate

0

50

100

150

200

250

300

350

400

1TX

Ran

ge (m

)

NS2QualNetrealereal


IEEE 802.11bIEEE 802.11b DATA Frame:

Control Frame : 2 Mbps

• 11 Mbps• 5.5 Mbps• 2 Mbps

TXTXCONTROLCONTROL

TXTXDATADATA

TXTXCONTROLCONTROL

TXTXDATA DATA

(11 Mbps)(11 Mbps) TXTXDATA DATA

(5.5 Mbps)(5.5 Mbps) TXTXDATA DATA (2 Mbps)(2 Mbps) = =

Transmission Range: TXDATA and TXCONTROL


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.

Impact of Ground on TX 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 > 33 meters

R

R1

DISTANCE

Fresnel Zone

Ground Plane

Impact of Ground on TX Impact of Ground on TX


Physical Carrier Sensing

S1 S2 S3 S4

Session 1 Session 2

d(1,2) d(2,3) d(3,4)

d(1,2)=d(3,4)= 25m

d(2,3)=80m

Rate=11 Mbps

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

Hypothesis:

Throughput in isolation:

UDP = 3 Mbps

TCP = 1,3 Mbps

0

500

1000

1500

2000

2500

no RTS RTS/CTS

UDP

1->23->4

Th

rou

gh

pu

t (

Kb

ps

)

0

500

1000

1500

2000

2500

no RTS RTS/CTS

TCP

1->23->4

Th

rou

gh

pu

t (

Kb

ps

)


0

500

1000

1500

2000

2500

3000

100 150 200 250 300 350 400

2 Mbps

Ag

gre

ga

te T

hro

ug

hp

ut

(Kb

ps

)

d(2,3) (meters)

independent sessions

Physical Carrier Sensing Range

0

1000

2000

3000

4000

5000

6000

7000

100 150 200 250 300 350 400

11 Mbps

Ag

gre

ga

te T

hro

ug

hp

ut

(Kb

ps

)

d(2,3) (meters)

independent sessions

S1 S2 S3 S4

Session 1 Session 2

d(1,2) d(2,3) d(3,4)

Indirect measurement: increase d(2,3) until no correlation is measured among the two sessions.

HYPOTHESIS: The large physical carrier sensing range, including all the four stations, produces a dependency between active connections

Correlated sessions Correlated sessions


The hypothesis is that dependencies are due to a large physical carrier sensing that includes all the stations

S1 S2 S3 S4

Session 1 Session 2

d(1,2) d(2,3) d(3,4)

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.

D1 x( )=1-Th1(x) +Th2 (x)

Thisolation (1) +Thisolation (2)

Physical Carrier Sensing RangePhysical Carrier Sensing Range


Table 1: Throughput values (Card rate =2 Mbps, payload size=512 Bytes)

Throughput Session 1 Throughput Session 2AccessMechanism

Distance

Th1() Th1(x) Th2() Th2(x)D1(x)

x=0 1279 577 1253 561 0.55x=150 1310 880 1310 780 0.37x=180 1310 930 1310 820 0.33x=200 1270 1030 1330 1130 0.17

x=250 1300 960 1330 960 0.27

x=300 1370 1360 1380 1050 0.12

No

RTS/CTS

x=350 1360 1110 1400 1390 0.09

Table 1: Throughput values (Card rate =11 Mbps, payload size=512 Bytes)

Throughput of Session 1 Throughput of Session 2Access

Mechanism

DistanceTh1()Kbps

Th1(x)Kbps

Th2()Kbps

Th2(x)Kbps

D1(x)

x=0 2780 1849 2981 1768 0.37x=150 1950 1500 2950 2250 0.23x=180 2920 2210 3040 1580 0.36x=200 2290 1930 3160 2660 0.16x=250 2820 1700 3170 2760 0.25x=300 2980 2800 3060 2750 0.08

No

RTS/CTS

x=350 2730 2590 3250 3230 0.03

Physical Carrier Sensing Range


802.11 Channel Model802.11 Channel Model

PCS_Range

Tx(11)Tx(1)

Radiated area

Some reference values

• Tx ≤ 110 m

• PCS_range ≤ 200 m

• Radiated area ≤ 300-350 m


PCS_Range

Tx(1)

Tx(2) Tx(11)S

Tx(5.5)

Rad

iate

d A

rea

S

i. Nodes at a distance d < TX_Range(x) are able to correctly receive data from S, if S is transmitting at a rate lower or equal to x;

ii. Nodes at a distance d, where TX_Range(x) < d < PCS_Range, are not able to correctly receive node S data but they are in the S physical carrier sensing range and therefore when S is transmitting they observe the channel busy, and thus they defer their transmissions;

iii. Nodes at a distance d > 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 < PCS_Range + TX_Range(x).



• 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


New New Hidden Hidden station phenomenumstation phenomenum

S

R

IF_Range

TX(1)

PCS_

Ran

ge

S1

R1

IF_Range

• 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.

PCS_Range < d < PCS_Range + TX_Range(x)

S and S1 may generate a new HIDDEN node phenomenon

Let d be the distance between S and S1

S1

R1

R

S


New EXPOSED node

PCS_rangePCS_range

PCS range - TX_range (1)PCS range - TX_range (1)

S1ER d

Nodes at distance d:

PCS_Range - TX_Range(1) < d < PCS_Range

are new EXPOSED nodes

• S1 is a station at a distance d1 from S: PCS_Range < d1 < PCS_Range+TX_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

S


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


Questions ?

Thank You !

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.


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 A new coordination mechanisms need to be designed to extend the coordination in the channel access beyond the PCS_Rangecoordination in the channel access beyond the PCS_Range


Fresnel Zone (2)

R1

R -

Fre

sne

l Zo

ne

Ground Plane

D

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. H=1 mt First Fresnel Zone touchs the ground D=33mt5. H=1.5 mt First Fresnel Zone touchs the ground D=73 mt6. H=2.0 mt First Fresnel Zone touchs the ground D=133 mt

ieee 802.11 ad hoc mode: measurement studies marco conti computer networks dept., iit cnr...

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