Transcript Routing in Multi-Radio, Multi

Routing in Multi-Radio, Multi-Hop
Wireless Mesh Networks
Girish Nandagudi
Acknowledgements
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This presentation is based on the paper
“Routing in Multi-Radio, Multi-Hop Wireless
Mesh Networks” by Richard Draves, Jitendra
Padhye and Brian Zill
Introduction
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Routing in ad-hoc wireless
networks has been an active
area of research
Research is mainly
motivated by mobile
applications in battlefield
and other ad-hoc networks
It is important to provide
scalable routing in such
environments, where mobile
nodes dominate the network
Note: The image has been borrowed from an internet article in the website http://www.sensorsmag.com
Point of interest…
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The aim is to improve the network capacity or the performance
of individual transfers [by means of an efficient routing
algorithm]
Challenge
–
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To cope up with the problem of reduction in total capacity of the
network due to interference between multiple simultaneous
transmissions
Possible Solution
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Provide two radios per node, enabling the node to transmit and
receive simultaneously
Having two (or more) radios can improve robustness, connectivity
and performance
Advantage is that the nodes can utilize more of the radio spectrum
Other alternative solutions
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Using directional antennas
Improved MACs
Channel switching
Diagnosing the multiple radio scenario
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When the nodes in the network has multiple radios,
the shortest path algorithm does not perform
optimally
Given a choice between 802.11a and an 802.11b
radio, the shortest path algorithm chooses the slower
802.11b radio since it has longer range
A shortest path algorithm that selects the path
without ensuring that the hops are on different
channels will almost certainly, does not perform well
Why a new routing metric?
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Shortest-path routing has several drawbacks
when it comes to routing in multi-hop
wireless networks
ETX (expected transmission count) metric
performs well in single-radio environment,
but it does not perform well in environments
having different data rates and multiple
radios
ETX
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ETX uses the underlying packet loss probability, both forward
and reverse, denoted by pf and pr respectively to measure the
expected number of transmissions including re-transmissions
ETX is denoted by:
ETX =
•
Σ k * s(k) =
∞
K=1
1
1-p
The path metric is the sum of ETX values for each link in the
path. Thereafter, the routing protocol selects the path that has
the minimum path metric
Disadvantages of ETX
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When we have two radios per node, one radio with
an 802.11a and the other with 802.11b, ETX will
transmit the data over 802.11b
ETX only considers the loss rates over the links, but
not their bandwidths
ETX prefers to transmit over shorter paths, but not
on longer paths in order to minimize global resource
usage
ETX does not give preference to diverse-channel
paths. Hence, it does not perform well in a scenario
where two 802.11b radios are used
The MR-LQSR protocol
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New metric, WCETT (Weighted Cumulative
Expected Transmission Time) introduced
LQSR is a source-routed link-state protocol
derived from DSR
Differences between DSR and the MR-LQSR
protocol
DSR
MR-LQSR
DSR assigns equal weight to all the links in
the network. The path metric is simply the
sum of link weights along the path.
MR-LQSR assigns weight depending on
the transmission latency, bandwidth and
the channel diversity of the link.
DSR implements shortest path routing.
MR-LQSR uses the WCETT metric for
routing.
MR-LQSR: Assumptions
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All nodes in the network are stationary
Each node is equipped with one or more 802.11
radio. These can be among 802.11a, 802.11b and
802.11g radios or a mixture of them.
The number of radios per node may not always be
the same
If a node is equipped with one or more radios, they
are tuned to different, non-interfering channels
MR-LQSR: Design Goals
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The protocol should take both loss rate and
bandwidth of a link into account while considering it
for inclusion in the path
The path metric should be increasing. That is, if an
hop is added to the existing path, the cost of the path
should never decrease
The path metric should account for the reduction in
throughput due to interference among links that
operate on the same channel
Computing path metric
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The protocol assigns a weight to each link that is equal to the
expected amount of time it would take to successfully transmit a
packet of some fixed size S
This time depends on the link bandwidth and loss rate
Now, the ETT of a link i between x and y nodes is denoted by
ETTi
Using the above notation, the WCETT can be derived as:
n
WCETT =
Σ ETT
i=1
i
Computing path metric II
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It is desirable for the WCETT to consider the impact
of channel diversity
In a two-hop path, if the hops are interfering, then
the effective bandwidth of the channel is reduced to
half due to the fact that only one hop can operate at
a time
The assumption that the hops that are nearby and in
the same channel always interfere holds almost true
for short paths, but it might be somewhat pessimistic
for longer paths
Computing path metric III
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Assuming a n hop path and that the system
has a total of k channels, we define Xj as:
Xj =

Σ
Hop i is on channel j
ETTi
1≤j ≤k
WCETT is taken as max(Xj)
Computing path metric IV
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The metric, WCETT = max(Xj) favors paths along diverse
channels
This metric achieves the third design goal, but not the second
design goal
To achieve both the design goals, we can combine the two
equations as follows:
n
WCETT = (1 – β) *
Σ ETT + β * max X
i=1
i
1≤j ≤k
j
Interpreting the expression
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Two possible ways:
1.
2.
The first term reflects the sum of the
transmission times along all hops in the network.
The second term reflects the set of all hops that
will have the most impact on the throughput of
this path.
We can view the equation as a tradeoff between
throughput and delay.
Measuring ETT
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ETT is defined as bandwidth-adjusted ETX
Hence, ETT is given by
–
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ETT = ETX * (S / B)
To accurately calculate the ETT, we need to
know the forward and reverse loss rates (pf
and pr) and the bandwidth of each link
This can be achieved by using broadcast
packet technique described by De Couto et
al [2]
Measuring ETT - Determining bandwidth
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Determining bandwidth is complex
One possibility is to set the bandwidth of
each 802.11 radio to a fixed value
Another possibility is to allow 802.11 radios
to select the bandwidth automatically by
enabling them to operate at autorate mode
Measuring ETT - Determining bandwidth II
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The technique of packet pairs is used in this case to determine
the bandwidth
Each node sends a back-to-back probe packet of sizes 137
bytes and 1137 bytes to each of its neighbor every minute
The neighbor measures the time difference between the receipt
of the first and the second packet and communicates it back to
the sender
The sender takes the minimum 10 consecutive samples and
estimates the bandwidth by dividing the size of the second
probe packet by the minimum sample
N1
P1
P2
P1
P2
N3
Sender
N2
N4
P1
P2
P1
P2
Implementation of MR-LQSR
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Implemented in an ad-hoc routing framework called the Mesh
Connectivity Layer (MCL)
MCL is a loadable windows driver and implements a virtual network
adapter within
To the rest of the system, the ad-hoc network appears as an additional
network link
It internally routes the packets using the LQSR protocol
IPv4
IPv6
IPX
…
MCL (with LQSR and WCETT)
Ethernet 802.11
Note: The above diagram has been borrowed from [1]
802.16
…
Implementation - Advantages
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Higher layer software runs unmodified over
the ad-hoc network. Hence, no modification
to the network stack is required
The virtual MCL network adapter can
multiplex several physical network adapters.
Hence, the ad-hoc routing runs over
heterogeneous link layers.
Testing
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The implementation has been tested on a testbed
consisting of 23 wireless nodes
The testbed is located in an office floor and the
nodes are placed in cubicles, conference rooms and
labs
All nodes are HP machines with latest configuration
and with Microsoft Windows XP as their operating
system
Each node has two 802.11 radios connected to the
PC via PCD-TP-202CS PCI-to-Cardbus adapter
cards and each node has a NetGear WAG 511 or
NetGear WAB 501 card
Testbed…
Note: The above diagram has been borrowed from [1]
Results
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The results have been classified as
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Accuracy of bandwidth estimation
Baseline scenario – Single radio
Two radios
The impact of β
Two simultaneous connections
Results - Accuracy of bandwidth
estimation
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Two of the testbed nodes
were used
The time between
successive pair of packets
was 2 seconds
Each bandwidth estimate
was obtained by taking the
minimum of 50 such pairs
The estimation is not
accurate for higher rates.
Note: The above diagram has been borrowed from [1]
Results - Baseline scenario - Single
radio
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Out of 506 sender-receiver
pairs, 100 pairs were picked
at random
A 2-minute TCP transfer was
carried out between the
selected pair of nodes
The experiment was carried
out for WCETT, ETX and for
basic shortest-path routing
Since each node had a
single radio, the throughput
difference between the three
protocols were not that
significant
Note: The above diagram has been borrowed from [1]
Results – Two radio
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One 802.11a radio and one
802.11g radio per node was
used
The same TCP transfer was
used with the parameter β set
to 0.5 for WCETT
As shown in the figure, WCETT
outperformed the other
protocols by a huge margin
This is due to the fact that
WCETT takes into
consideration the channel
diversity of the link too in
addition to bandwidth of the link
Note: The above diagram has been borrowed from [1]
Results – One and two radios
Note: The above diagram has been borrowed from [1]
Results - The impact of β
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β plays an important role in
the WCETT calculation
When β is set to 0, WCETT
selects the link based only
on the ETT or the latency,
without regard to the
channel diversity
Setting the value of β to 1
makes little sense
The metric selects the paths
with less channel diversity
when β is low
Note: The above diagram has been borrowed from [1]
Results - Two simultaneous
connections
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For WCETT metric, the experiment
was repeated four times with β = 0,
0.1, 0.5 and 0.9
The measured median throughput
was multiplied by 2 since there
were two connections. The product
was called the Multiplied Median
Throughput (MMT)
It must be noted that WCETT
performs better than ETX for all
values of β
The conclusion is that at higher
loads, the throughput is maximized
by having lower values of β
Note: The above diagram has been borrowed from [1]
Related work
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One way to improve the capacity of wireless
networks is by using improved MAC
–
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To exploit multiple non-interfering frequency channels
An alternative way to improve the capacity is to
stripe traffic over multiple network interfaces
Another approach is to use directional antennas
The capacity of wireless network can also be
improved by taking advantage of full spectrum by
using rapid channel switching
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This can be quiet slow with the existing hardware
Can be implemented if hardware support is achieved
Conclusion
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It is shown that when nodes are equipped with multiple
heterogeneous radios, it is important to select channel diverse
paths in addition to taking care of latency and bandwidth for
links
The results show that WCETT outperforms the existing
protocols in this particular scenario where channel diversity is
involved
WCETT is flexible in the sense that it allows us to tradeoff the
channel diversity by setting the value for β
The implementation calls for no change in hardware or the
networking software. This allows the user to seamlessly use
this protocol with the existing system setup
References
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[1] Richard Draves, Jitendra Padhye and
Brian Zill “Routing in Multi-Radio, Multi-Hop
Wireless Mesh Networks”
[2] D. De Couto, D. Aguayo, J. Bicket, and R.
Morris: "High-throughput path metric for
multi-hop wireless routing", In MOBICOM,
2003.
Questions, corrections and
suggestions?
Thank you