Ad hoc network notes MAC Protocols for Ad-Hoc Wireless Networks
Ad Hoc Probe
Transcript of Ad Hoc Probe
AdHoc Probe: Path Capacity AdHoc Probe: Path Capacity Probing in Wireless Ad Hoc NetworksProbing in Wireless Ad Hoc Networks
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!