Link Quality Source Routing (LQSR)

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27 Οκτ 2013 (πριν από 3 χρόνια και 10 μήνες)

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Link Quality Source Routing
(LQSR)

Girish Nandagudi

Reference


This presentation is based on the paper
“Routing in Multi
-
Radio, Multi
-
Hop Wireless
Mesh Networks”

by Richard Draves, Jitendra
Padhye and Brian Zill.

Goal


The aim is to improve the network capacity or the performance
of individual transfers
(by means of an efficient routing
algorithm)


Challenge


Reduction in total capacity of the network due to interference
between multiple simultaneous transmissions


Possible solutions


Provide two radios per node, enabling the node to transmit and
receive simultaneously


Having two (or more) radios can improve robustness, connectivity
and performance


Nodes can utilize more of the radio spectrum.

Other alternative solutions


Using directional antennas


Improved MACs


Channel switching

Diagnosing the multiple radio scenario


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?


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


ETX uses the underlying packet loss probability, both forward
and reverse, denoted by p
f

and p
r

respectively to measure the
expected number of transmissions including re
-
transmissions.


ETX is denoted by:


ETX =
Σ

k * s(k) =


1

1
-

p

K = 1





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


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.


LQSR protocol


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.

LQSR protocol (2)


Source
-
routed, link
-
state protocol


Derived from DSR


Each node measures quality of its link to its
neighbor.


The info regarding link quality propagates through
the mesh (updates in link
-
state routing).


Source selects route with the best cumulative metric.


Packets are source
-
routed using this route.


LQSR: Assumptions


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.


LQSR: Design Goals


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.



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
ETT
i


Using the above notation, the WCETT can be derived as:


Computing path metric

WCETT =
Σ

ETT
i

i = 1


n

Computing path metric II


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


Assuming a
n

hop path and that the system
has a total of
k

channels, we define X
j
as:

X
j


=
Σ

ETT
i
1≤
j

k

Hop i is on channel j


WCETT is taken as max(X
j
).

Computing path metric IV


The metric, WCETT = max(X
j
) 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:

WCETT = (1


β
) *

Σ

ETT
i
+

β

* max X
j

i = 1


n

1≤
j

k

Interpreting the expression


Two possible ways:

1.
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.

2.
We can view the equation as a tradeoff between
throughput and delay.


Measuring ETT


ETT is defined as
bandwidth
-
adjusted ETX


Hence, ETT is given by


ETT = ETX * (S / B)


To accurately calculate the ETT, we need to know
the forward and reverse loss rates (p
f

and p
r
) 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


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


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

Sender

N2

N3

N4

P1

P2

P1

P2

P1

P2

P1

P2

Implementation of MR
-
LQSR


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

802.16



Note: The above diagram has been borrowed from [1]

Implementation
-

Advantages


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


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


The results have been classified as


Accuracy of bandwidth estimation


Baseline scenario


Single radio


Two radios


The impact of
β


Two simultaneous connections

Results
-

Accuracy of bandwidth
estimation


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


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 radios


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


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


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


One way to improve the capacity of wireless
networks is by using improved MAC.


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.


This can be quiet slow with the existing hardware.


Can be implemented if hardware support is achieved.


Conclusion


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

[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.