Link Quality Source Routing (LQSR)

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Link Quality Source Routing (LQSR)

• Girish Nandagudi

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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.

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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.

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Other alternative solutions

• Using directional antennas
• Improved MACs
• Channel switching

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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.

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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.

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ETX

• 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 S k s(k)
1
8
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.

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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.

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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.
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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.

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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.

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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.

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Computing path metric

• 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 S ETTi
i 1
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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.

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Computing path metric III

• Assuming a n hop path and that the system has a
  total of k channels, we define Xj as

Xj S ETTi 1j k
Hop i is on channel j

• WCETT is taken as max(Xj).

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Computing path metric IV

• 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 ß) S ETTi ß max Xj
i 1
1j k
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Interpreting the expression

• Two possible ways
• 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.

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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 (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.

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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.

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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
N3
P1
P2
P1
P2
Sender
N2
N4
P1
P2
P1
P2
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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
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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.

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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.

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Testbed
Note The above diagram has been borrowed from 1
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Results

• The results have been classified as
• Accuracy of bandwidth estimation
• Baseline scenario Single radio
• Two radios
• The impact of ß
• Two simultaneous connections

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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
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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
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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
  ß 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
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Results One and two radios
Note The above diagram has been borrowed from 1
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Results - The impact of ß

• ß plays an important role in the WCETT
  calculation.
• When ß 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 ß to 1 makes little sense.
• The metric selects the paths with less channel
  diversity when ß is low.

Note The above diagram has been borrowed from 1
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Results - Two simultaneous connections

• For WCETT metric, the experiment was repeated
  four times with ß 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 ß.
• The conclusion is that at higher loads, the
  throughput is maximized by having lower values of
  ß.

Note The above diagram has been borrowed from 1
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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.

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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 ß.
• 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.

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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.