AdHoc Probe: Path Capacity Probing in Wireless Ad Hoc Networks

1. AdHoc Probe: Path Capacity Probing in Wireless Ad Hoc Networks Ling-Jyh Chen, Tony Sun, Guang Yang, M.Y. Sanadidi, Mario Gerla Computer Science Department, UCLA

2. Why Path Capacity? • Why do we want to measure path cap? • To adjust video rates; adapt end to end encoding • To select TCP parameters, etc

3. Example Scenario • Server is streaming video to user roaming in ad hoc net • a shopping mall with “opportunistic” ad hoc extensions • Assume 802.11g; path capacity varies from 2-54Mbps • If user walks outside of 802.11 area, capacity drops to GPRS (< 100Kbps) • Server must know capacity to adjust video rate and avoid network overflow!!

4. CapProbe (Rohit et al, SIGCOMM’04) Capacity • Key insight: a packet pair that gets through with zero queueing delay on both packets yields the exact estimate • Equivalently: zero queues -> Delay Sum Min -> exact CAP • CapProbe uses “Minimum Delay Sum” filter

5. Ad hoc path capacity • The capacity of an ad hoc path is related to link speeds in a different way than on a wired path • This is because consecutive transmissions interfere with each other • Ad Hoc Path capacity definition • the data rate achieved by a UDP stream on the unloaded path (no other traffic) - this is the general definition • In the ad hoc path, path capacity = “narrow neighborhood” capacity (different from “narrow link” capacity in wired net)

6. Ad hoc path capacity (cont) • Ad Hoc Neighborhood • The minimal set of nodes that must be inactive (no tx nor receive) while a transmission takes place • Equivalently, the region affected by the transmission • Only one pkt can be in the neighborhood at a time • PP measures N-hood capacity • N-hood capacity = (link speed)/(# of N-hood hops) • The N-hood Capacity trivially reverts to link capacity for the wired section of the path. • Key result: • The PP measurement yields the correct “path capacity” regardless of wired, wireless or mixed configuration

7. Neighborhood Capacity • N-hood Cap in an ad hoc net can vary with: • MAC protocol and link scheduling • Link interference • S/N ratio; • Tx power • Encoding/modulation scheme

8. Previous Work (Li et al, Mobicom 01) • Used UDP flow stream to probe the maximum achievable throughput (brute force method) • UDP file transfer may introduce excessive O/H in some applications

9. back to back packets dispersion 1 dispersion 2 wired Internet wireless multihop AP 1 hop 2 hop 3 hop 4 hop 5 hop 6 hop 7 hop sender Multihop path simulation 1500 byte pkts

10. Simulation of mobile hosts • Probing capacity of path (n1 -> n6) • n2~5 move clockwise • 200 samples/run, 20 runs

11. 1800 0 2200 3000 2800 2600 1200 600 Simulation of mobile end hosts • Probing the capacity of path (0 ->25) • Mobility: 1 m/sec; Cross Traffic: 1kbps/flow • 200 samples/estimation; 4 samples/second

12. Testbed Measurements (WiTMeMo’05) • Testbed configurations • 802.11b fixed rate (2Mbps mode); chain topology • 802.11b auto rate; varying distance between two nodes • 802.11b auto rate; w/ Bluetooth interference • 802.11b fixed rate (2Mbps mode); remote probing from the Internet

13. Experiment Results (1) • Fixed rate, variable hop length

14. Experiment Results (2) • Auto Rate, variable distance

15. Experiment Results (4) • Probing from the Internet

16. Summary • Wireless ad hoc Capacity estimation critical for • Battlefield networks • Emerging commercial ad hoc nets (eg car2car) • Ad Hoc Probe estimates correct e2e path capacity of wired, wireless ad hoc and mixed nets . • User need not specify the environment or the MAC parameters • Simulation and measurements validate the findings • Next step: design the “adaptive” server