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 Capacity: maximum IP-layer throughput that a flow can get, without any cross traffic. Available Bandwidth Available Bandwidth: maximum IP-layer throughput that a flow can get, given (stationary) cross traffic.

3. Ad hoc path capacity  Definition: Path capacity: the data rate achieved on the idle path (no other traffic) Related to “narrow link” capacity  Path Cap in an ad hoc net can vary with: # of hops Link interference; S/N ratio; Tx power Encoding scheme Number of antennas (eg MIMO)  Why do we want to measure path cap? To adjust video rates; adapt end to end encoding To select TCP parameters, etc

4. 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!!

5. 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 Receiver Sender Bottleneck PP measure PP measure PP

6. CapProbe Background: Packet Pair Dispersion T3T3 T2T2 T3T3 T3T3 T1T1 T3T3 Narrowest Link 20Mbps 10Mbps5Mbps10Mbps20Mbps8Mbps Capacity = (Packet Size) / (Dispersion)

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

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

9. 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)

10. What do we want to measure? The effective end-to-end capacity is defined as the maximum achievable data rate in the absence of any cross traffic connection. It is smaller than the raw data rate on each physical link due to packet O/H and channel access coordination to handle pipelined packet transmissions on a path.

11. Effective capacity of a multihop link If N nodes are within the same interference domain, C’=C/N The solid-line circle: effective transmission range ( D r ) The dotted-line circle: interference range ( D i ) Distance between nodes: 200m 1. D r = D i =250m => C’=C/3 2. D r =250m, D i =500m => C’=C/4

12. Effective Capacity of 802.11b In 802.11b, a 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 of a one-hop link is: 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

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

14. 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-end path capacity is different to bottleneck link capacity in wireless net.

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

16. 1 hop 2 hop 3 hop 4 hop 5 hop 6 hop 7 hop AP dispersion 2 sender back to back packets wired Internet wireless multihop dispersion 1 Multihop path simulation

17. Grid Topology Fixed probing packet size: 1500bytes Estimate capacity (a -> b) with different cross traffic rates (Poisson traffic) CT: horizontal direction CT: horizontal & vertical directions a b

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

19. 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 0 600 1200 1800 2200 2600 2800 3000

20. Testbed Measurements (WiTMeMo’05) 1. 802.11b fixed rate (2Mbps mode); chain topology 2. 802.11b auto rate; varying distance between two nodes 3. 802.11b auto rate; w/ Bluetooth interference 4. 802.11b fixed rate (2Mbps mode); remote probing from the Internet

21. Experiment Results (1) Fixed rate, multihops

22. Experiment Results (2) Auto Rate, w/ different distance

23. Experiment Results (3) Auto Rate, w/ Bluetooth interference

24. Experiment Results (4) Probing from the Internet

25. Summary Wireless Capacity estimation critical for  Battlefield nets  Emerging commercial ad hoc net (eg car2car) We proposed AdHoc Probe to estimate e2e path capacity in ad hoc net. We performed NS-2 simulation to verify AdHoc Probe. Recent ad hoc net measurements have confirmed the findings Future experiment with Internet -> wireless and in sensor nets

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