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

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

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 Neighborhoo d hops = # of hops to traverse the neighborhood  N-hood Capacity = avg link data rate/ n-hood hops

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

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

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

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!!

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

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

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

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. Capacity Capacity

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)

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

Effective Capacity of b In b, 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

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

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.

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

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

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

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

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

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

Experiment Results (1) Fixed rate, variable hop length

Experiment Results (2) Auto Rate, variable distance

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

Experiment Results (4) Probing from the Internet

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

Thanks!