IEEE 802.11 Ad Hoc Mode: Measurement studies

1. IEEE 802.11 Ad Hoc Mode: Measurement studies Marco Conti Computer Networks Dept., IIT CNR Marco.conti@iit.cnr.it http://cnd.iit.cnr.it/mobileMAN

2. MobileMAN: Enabling Technologies • Reference technology: IEEE 802.11b (Wi-Fi) • Develop anenhancementwireless 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

3. 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

4. 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.

5. IEEE 802.11 behavior: Simulative Studies The following relationship exists between the ranges: TX_Range <= IF_Range <=PCS_Range

6. Experimental Environment M 1 2 3 4 Hardware: • Wireless D-LinkAir DWL-650 card ( IEEE 802.11b ) • Laptops (4+1) 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.

7. 802.11b Throughput

8. Measurements of the Transmission Range TXDATA 1 2 real

9. Transmission Range: TXDATA and TXCONTROL TXDATA TXCONTROL TXDATA (11 Mbps) TXDATA (5.5 Mbps) TXDATA (2 Mbps) = TXCONTROL • 11 Mbps • 5.5 Mbps • 2 Mbps • DATA Frame: IEEE 802.11b • Control Frame : 2 Mbps

10. 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.

11. 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

12. Physical Carrier Sensing TCP 2500 UDP 2500 1->2 3->4 2000 1->2 3->4 2000 1500 Throughput (Kbps) 1500 Throughput (Kbps) 1000 1000 500 500 0 no RTS RTS/CTS 0 no RTS RTS/CTS d(1,2)=d(3,4)= 25m d(2,3)=80m Rate=11 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

13. Physical Carrier Sensing Range Correlated sessions Correlated sessions 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.

14. 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.

15. Physical Carrier Sensing Range

16. 802.11 Channel Model Some reference values • Tx ≤ 110 m • PCS_range ≤ 200 m • Radiated area ≤ 300-350 m

17. 802.11 Channel Model 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; 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; S 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).

18. 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

19. 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 < d < PCS_Range + TX_Range(x) S and S1 may generate a new HIDDEN node phenomenon

20. New EXPOSED node • 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 d S1 R S E PCS range - TX_range (1) PCS_range Nodes at distanced: PCS_Range - TX_Range(1) < d < PCS_Range are new EXPOSEDnodes

21. 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

22. 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”, MobileAd hoc networking, S. Basagni, M. Conti, S. Giordano, I. Stojmenovic (Editors), IEEE Press and John Wiley and Sons, Inc., New York, 2004. Thank You !

23. 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

24. Fresnel Zone (2) The channel power loss depends on the contact between the Fresnel zone and the ground The Fresnel zone for a radio beam is an elliptical area with foci located in the sender and the receiver 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) H=1 mt First Fresnel Zone touchs the ground D=33mt H=1.5 mt First Fresnel Zone touchs the ground D=73 mt H=2.0 mt First Fresnel Zone touchs the ground D=133 mt