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. Definition • Capacity: maximum throughput that a UDP flow can get, without any cross traffic. • Available Bandwidth: maximum throughput that a UDP flow can get, given (stationary) cross traffic.

3. Ad hoc path capacity • Definition: Path capacity • the data rate achieved by a UDP stream on the unloaded path (no other traffic) • Path capacity = “narrow link” capacity in wired net • Path capacity = “narrow neighborhood” capacity in ad hoc net • 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 transmission per neighborhood • Neighborhood hops = # of hops to traverse the neighborhood • N-hood Capacity = avg link data rate/ n-hood hops

4. Neighborhood example • Assume 802.11 with RTS/CTS is used • If Dr=Di=250m , nodes {3,4,5, 6} are within the same n-hood; C’=C/3 • IfDr=250m, Di=500m, nodes {2,3,4,5. 6} are in n-hood, C’=C/4 solid-line circle: effective receive range (Dr) from node 4 dotted-line circle: interference range (Di) caused by node 4 Distance between nodes: 200m

5. 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 • Number antennas (eg MIMO) • Antenna directionality • etc

6. 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

7. Example Scenario • Internet Server is streaming traffic to user moving in ad hoc field • Assume autorate and smart antennas with dynamic config • Wireless path capacity may vary from 2Mbps to 25Mbps • Server must know capacity to avoid network flood!!

8. PP Bottleneck PP Receiver PP Sender PP measure measure Ad Hoc probe: end to end measurement tool • Statistics of packet pair (PP) at end points reveal much about path: capacity, load, buffering, and error rate

9. 20Mbps 10Mbps 5Mbps 10Mbps 20Mbps 8Mbps T1 Narrowest Link T2 T3 T3 T3 T3 CapProbe Background: Packet Pair Dispersion Capacity = (Packet Size) / (Dispersion)

10. Issues: Compression and Expansion • Queueing delay on the first packet => compression • Queueing delay on the second packet => expansion

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

12. Capacity Estimation in Ad Hoc Wireless Networks • Capacity estimation in wireless net is challenging. • Path capacity in wireless ad hoc net depends on bottleneck capacity, topology, interference, encoding, antennae, etc. • Data rate can be fixed or auto. • Note: Previous method (Li et al, MobiCom’01) was brute force (more later)

13. What do we actually measure? • The effective path capacity = maximum achievable E2E transfer rate when the channel is idle (no other users) • Path capacity smaller than channel raw data rate due to: • packet header O/H, and; • interference between multiple packets in the pipeline

14. Effective Capacity of 802.11b • In 802.11b, RTS packet is 40 bytes, CTS and ACK packets are 39 bytes, and the MAC header of a data packet is 47 bytes, • the effective capacity: • For instance, when the data packet size is 1500 bytes and the data rate of the wireless link is 2Mbps, the effective capacity is at most

15. Previous Work (Li et al) • Dr=250m, Di=500m • Used UDP flow stream to probe the maximum achievable throughput (brute force method)

16. AdHoc Probe • Adhoc Probe employs CapProbe concepts, and it is an active one-way technique. • Adhoc Probe measures end-to-end effective capacity in wireless ad hoc networks. • End-to-endpath capacity is different to bottleneck link capacity in wireless net.

17. One-way vs Round-trip estimates • One-hop; 2Mbps mode Immediate response packet of first probing packet will conflict with the second probing packet!

18. 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

19. Simulation of mobile hosts • Probing the capacity of path (1 -> 6) • N2~5 move clockwise • 200 samples/run, 20 runs

20. 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

21. Testbed Measurements(WiTMeMo’05) • 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

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

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

24. Experiment Results (3) • Auto Rate, w/ Bluetooth interference • Varying distance between Bluetooth nodes and AdHoc Probe receiver

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

26. Summary • Wireless Capacity estimation critical for • Battlefield networks • Emerging commercial ad hoc nets (eg car2car) • We have proposed AdHoc Probe to estimate e2e path capacity in ad hoc nets. • NS-2 simulation validates AdHoc Probe. • Recent measurements confirm the findings

27. Thanks! http://www.cs.ucla.edu/NRL/CapProbe