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,

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 IP-layer throughput that a flow can

get, without any cross traffic. Available BandwidthAvailable Bandwidth: maximum IP-layer throughput that

a flow can get, given (stationary) cross traffic.

Ad hoc path capacityAd 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

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: Delay Sum Min -> zero queues -> 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 want to measure?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.

Effective capacity of a multihop linkEffective capacity of a multihop link

If N nodes are within the same interference domain, C’=C/N

The solid-line circle: effective transmission range (Dr)

The dotted-line circle: interference range (Di)

Distance between nodes: 200m

1. Dr=Di=250m =>C’=C/3

2. Dr=250m, Di=500m =>C’=C/4

Effective Capacity of 802.11bEffective 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

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, multihops

Experiment Results (2)Experiment Results (2)

Auto Rate, w/ different distance

Experiment Results (3)Experiment Results (3)

Auto Rate, w/ Bluetooth interference

Experiment Results (4)Experiment Results (4)

Probing from the Internet

SummarySummary 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

Thanks!Thanks!

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