AdHoc Probe: Path Capacity AdHoc Probe: Path Capacity Probing in Wireless Ad Hoc NetworksProbing in Wireless Ad Hoc Networks

Ling-Jyh Chen, Tony Sun, Guang Yang, Ling-Jyh Chen, Tony Sun, Guang Yang,

M.Y. Sanadidi, Mario GerlaM.Y. Sanadidi, Mario Gerla

Computer Science Department, UCLAComputer Science Department, UCLA

DefinitionDefinition CapacityCapacity: maximum throughput that a UDP flow can get,

without any cross traffic. Available BandwidthAvailable Bandwidth: maximum throughput that a UDP

flow can get, given (stationary) cross traffic.

Ad hoc path capacityAd 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

Neighborhood exampleNeighborhood 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 If Dr=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

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 ScenarioExample 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 toolAd 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

ReceiverReceiverSenderSender

BottleneckPP

PP

measure

PP

measure

PP

CapProbe Background: Packet Pair DispersionCapProbe Background: Packet Pair Dispersion

T3

T2 T3

T3

T1

T3

Narrowest LinkNarrowest Link

20Mbps 10Mbps 5Mbps 10Mbps 20Mbps 8Mbps

Capacity = (Packet Size) / (Dispersion)

Issues: Compression and ExpansionIssues: Compression and Expansion

• Queueing delay on the first packet => compression

• Queueing delay on the second packet => expansion

CapProbe CapProbe (Rohit et al, SIGCOMM’04)(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.

CapacityCapacity

Capacity Estimation in Ad Hoc Capacity Estimation in Ad Hoc Wireless NetworksWireless 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?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 802.11bEffective 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

PCS

SC ×

+++=

473940

Mbps8.124739401500

1500≈×

+++

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

throughput (brute force method)

AdHoc ProbeAdHoc 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 estimatesOne-way vs Round-trip estimates One-hop; 2Mbps mode

Immediate response packet of first probing packet will conflict with the second probing packet!Immediate response packet of first probing packet will conflict with the second probing packet!

1 hop1 hop 2 hop2 hop 3 hop3 hop 4 hop4 hop 5 hop5 hop 6 hop6 hop 7 hop7 hopAPAP

dispersion 2dispersion 2

sendersender

back to backback to back packets packets

wired Internet

wireless multihop

dispersion 1dispersion 1

Multihop path simulationMultihop path simulation

Simulation of mobile hosts Simulation of mobile hosts

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

Simulation of mobile end hostsSimulation 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

18002200

2600 2800

3000

Testbed MeasurementsTestbed Measurements (WiTMeMo’05)(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

Experiment Results (1)Experiment Results (1)

Fixed rate, variable hop length

Experiment Results (2)Experiment Results (2)

Auto Rate, variable distance

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

Probe receiver

Experiment Results (4)Experiment Results (4)

Probing from the Internet

SummarySummary 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!Thanks!

http://www.cs.ucla.edu/NRL/CapProbe