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A MAC for WiFi Mesh Networks
Bhaskaran RamanDepartment of CSE, IIT Kanpur
Kameswari ChebroluDepartment of EE, IIT Kanpur
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IEEE 802.11 (WiFi)
Originally designed as a replacement (extension) for wired LANs
Multi-hop mesh networks are very popular
Wireless community networks in citiesShare wired Internet broadband connections
Replacement for last-hop telephony?
Wireless Internet in rural areasDeveloped as well as developing countries
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A WiFi Network in Djurslands, Denmark
www.DjurslandS.net
Remote area of Denmark
No broadband
Operators do not see returns to investment
802.11 mesh network used for broadband connectivity
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A WiFi Network (planned) for Bhimavaram, A.P., India
Requirements:Need 384 Kbps video for telemedicine and educational apps
This is the requirement per-village
Cost should be as low as possible
So that the network model is replicable
WiFi is the only serious option
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Network Model
Point-to-point links
Multiple interfaces (radios) per node
One directional antenna per link
To link-1
To link-2 To link-3
Ethernet hub
Link-1
Link-2Link-3
Point-to-point
802.11 links
Radio1
Radio2
Radio3
Landline node
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Single Channel Operation
802.11b has only three independent channels
802.11a has twelve independent channels
Four are meant for outdoor use
Why only a single channel for the mesh?
Mitigation of “RF pollution”
The mesh may not be 3-edge-colourable
If the frequency is licensed, more channels could imply more cost
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SynRx, SynTx, and Mix-Rx-Tx
Exposed interface problem within a node:CSMA/CA (802.11 DCF) inherently allows only one link operation per nodeProblems: (a) Immediate ACK, (2) CS back-off
N
A
B
T1
T2
R1
R2
(a) Syn-Rx
N
A
B
R1
R2
T1
T2
(b) Syn-Tx
N
A
B
R1
T2
T1
R2
(c) Mix-Rx-Tx
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SynOp: SynRx + SynTxLinks at a node operating simultaneously, synchronously (on the same channel)
Is this feasible? Yes, under certain conditions
0o
90o
180o
270o
0dB-3dB-10dB-20dB-30dB
Side-loberejectionlevels
Main direction
Side-lobes
N
A
B
T1
T2
R1
R2
Syn-Rx
∣PR1−PR2
∣SL−SIRreqd
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SynOp: Experimental Verification
=30o
T1
T2
R2
R1
N
A
B
8km
0.9km
MandhanaTower ht. = 40m
MS3Tower ht. = 15m
BithoorTower ht. = 40m
Used broadcast packets on both links (SynRx, SynTx)6.5 Mbps with and without simultaneous operationSynTx also verified – using antenna diversity for the setupExperiments along with: A. R. Harish & Sreekanth Garigala
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SynOp: Another Experiment
=110o
A
B
N
8km0.9kmMandhana
Tower ht. = 40mMSSS (MS3)Tower ht. = 30m
BithoorTower ht. = 40m
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The 2P MAC ProtocolTwo phases: each node switches between SynRx and SynTx
Topology has to be bipartite
A
BC
D
SynTxSynRx
SynTxSynRx
a) Links: A-->B, A-->D, C-->B, C-->D
b) Links: B-->A, B-->C, D-->A, D-->CNote: diagram ignores system and propogation delays
How to achieve 2P on off-the-shelf hardware?
Can 2P work without tight time synchronization?
Relation between 2P and network topology
2P performance versus CSMA/CA
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Achieving SynOpGoal: bypass DCF to achieve SynOp
Two offending factors: immediate ACKs, CSMA backoff
Avoiding immediate ACKs:
Use IBSS mode
IP unicast to/from MAC broadcast
Avoiding CSMA backoff:
Make use of diversityantenna
Change antsel_rx to theunconnected antennabefore transmitting
PCMCIA/PCI card
RIGHT
LEFT
To external antenna
Unconnected SynTx: antsel_rx =RIGHTSynRx: antsel_rx =LEFT
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2P on a Single P2P LinkB bytes in each phase
SynTx+SynRx = one round
Marker packet acts as a “token”
The two ends of the link are in loose-synchrony
How do we handle:Temporary loss of synchrony?
Link recovery or initialization?
SynTx
SynRx
InitSend pkts until
B bytes sent
B bytes sent,
send marker pktMarker pkt recd.,
or timeout
Wait for
marker pkt
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The 2P Timeout Mechanism
Timer started on entering SynRx
Put on hold on starting to hear
Link-resync takes only one round
CRC errors of non-marker pkts immaterial
SynTx
SynRx
Note: diagram ignores system and propagation delays
timeout
Time
N1
N2
t1t0
SynTx
SynRx
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Bumping to Avoid Repeated Timeouts
If SynTx phases coincide, repeated timeouts occur
Use random delay bumping to avoid this
SynTx
SynRx
timeout
Time
N1
N2
SynTx
SynRx
timeout
t2
b1
b2
Note: diagram ignores system and propagation delays
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Parameters in 2P
Phase duration: B bytes
Large B implies lower % overhead, but higher latency
For B=10KB, 6% overhead, 13ms latency
For B=4.5KB, 11% overhead, 6ms latency
Timeout:
Lower bound: one phase duration
Simulation: 1.25 times the phase duration
Implementation: 25ms (kernel jitter ~ 10ms)
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Communication Across Interface-Neighbours
NOTIF msgs to indicate end of SynRx
Wait for NOTIF msgs from all ifa-nbrs before SynTx
UP/DOWN state w.r.t. each ifa-nbr
Communication through shared-memory, or ethernet
SynTx
SynRx
InitSend pkts until
B bytes sent
B bytes sent,
send marker pktMarker pkt recd.,
or timeout
Wait for
marker pkt
Waitingto switch
Send NOTIF to all
ifa-nbrs
Wait for NOTIFs
Recv. NOTIFs from
all UP ifa-nbrs, or timeout
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Some Remarks on 2P
Dummy bytes sent when no IP data
Power consumption not a major concern
Embedded platform ~ 4-6W at least802.11 radio ~ 0.1-0.2W only
Unequal phase durations possible
But not really useful for more than a single hop network
RF leakages: not too many interfaces can be placed close to each other
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Topology Constraints
2P has two main constraints:
Topology should be bipartite
Power constraints
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Power ConstraintsDenote by P
i, the txpower at antenna A
i
Each transmission acts as interference to all other transmissions
Write a set of linearequations withvariables Pi
SIR >= SIRreqd
Probably should havesome head-room too
Feasibility of a solutionto this implies that thetopology is 2P-compatible
a j
ai
Overall gain from a i to a j=
(Gain of ai ' s Tx in a j ' s dirn)× (Gain of a j ' s Rx in ai ' s dirn)=Gain at angle×Gain at angle
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Topology FormationTree topology:
Trivially bipartite
Only one landline ==> tree is naturalOnly a tree is active at any time
Heuristics:
H1: use short links
H2: avoid short angles between links
H3: minimize the number of hops
Mimic a natural deployment pattern
Nodes close to landline connected first, then the next level
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Evaluation of 2P
Topology formation
Simulation studies
Implementation
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Evaluation of Topology Creation
Aspects of interest:
How well does the algorithm scale?
How much head-room in SIRreqd
is possible?
Evaluation:
Using parts of the map of Durg district, Chattisgarh, U.P.
Using random topologies
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Topology Creation on Durg District
Four clusters of villages
Qi (i=1..4) 31, 32, 32, and 32 villages each
14 15 16 17 18 19 200
4
8
12
16
20
24
28
32
Q1 Q2 Q3 Q4
SIR_reqd (dB)
# lin
ks fo
rmed
SIRreqd
of
18-20dB easily possible
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The Topology
on Q1
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Topology Creation on Random Scenarios
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200
5
10
15
20
25
30
35
40
45
50
SIR_reqd=14dB SIR_reqd=16dB SIR_reqd=18dB
Topology number
# lin
ks fo
rmed
SIRreqd
of 16-18dB mostly possible
for up to 30-50 node topologies
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Simulation-based Evaluation
TeNs:
http://www.cse.iitk.ac.in/~bhaskar/tens/
Channel interference, grey regions, multiple interface support, directional antennas
Further extensions:
Populating the ARP table appropriately
24dBi directional antenna support
MAC modifications: air propagation delay, ACK timeout
LLC: sliding-window protocol
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Simulation Parameters
Q1's 31-node topology used
UDP or TCP traffic
Packet size: 1400 bytes
UDP: saturating CBR traffic (every 2ms)
TCP: NewReno used
Simulated time duration: 10sec
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Saturation Throughput (UDP)
The difference is due to SynOp
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TCP Performance
Very poor between CSMA and TCP
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Implementation-based Evaluation
Implementation using HostAP v0.2.4, Linux 2.4 (also works on Linux 2.6)
2P on a single link: 6.1Mbps
Less than the max. possible 6.5Mbps
Overhead in antsel_rx, marker pkt, CWmin
being 32
2P performance on a pair of links:
A <--> N1, N2 <--> B, UDP traffic
2P 2.70 (0.31) 2.06 (0.24) 2.81 (0.15) 2.81 (0.10)
CSMA 2.07 (0.13) 1.13 (0.22) 1.90 (0.15) 3.11 (0.14)
Avg (SD) thrpt at A (Mbps)
Avg (SD) thrpt at N1 (Mbps)
Avg (SD) thrpt at N2 (Mbps)
Avg (SD) thrpt at B (Mbps)
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Concluding Remarks
2P good for 802.11 mesh networks
Reuse of spectrum for max. throughput
Applicable in a wide-range of deployments
Can be extended to P2MP scenarios as well
Provided the antenna is suitable
Topology creation is an interesting aspect of study