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Routing in MultiRadio, MultiHop Wireless Mesh Networks

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Title: Routing in MultiRadio, MultiHop Wireless Mesh Networks


1
Routing in Multi-Radio, Multi-Hop Wireless Mesh
Networks
  • Girish Nandagudi

2
Acknowledgements
  • This presentation is based on the paper Routing
    in Multi-Radio, Multi-Hop Wireless Mesh Networks
    by Richard Draves, Jitendra Padhye and Brian Zill

3
Introduction
  • 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
4
Point of interest
  • The aim is to improve the network capacity or the
    performance of individual transfers by means of
    an efficient routing algorithm
  • Challenge
  • To cope up with the problem of reduction in total
    capacity of the network due to interference
    between multiple simultaneous transmissions
  • Possible Solution
  • 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

5
Other alternative solutions
  • Using directional antennas
  • Improved MACs
  • Channel switching

6
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

7
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

8
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

9
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

10
The MR-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

11
MR-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

12
MR-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

13
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
14
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

15
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)

16
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
17
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.

18
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

19
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

20
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
21
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
22
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.

23
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

24
Testbed
Note The above diagram has been borrowed from 1
25
Results
  • The results have been classified as
  • Accuracy of bandwidth estimation
  • Baseline scenario Single radio
  • Two radios
  • The impact of ß
  • Two simultaneous connections

26
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
27
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
28
Results Two radio
  • 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
29
Results One and two radios
Note The above diagram has been borrowed from 1
30
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
31
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
32
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

33
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

34
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.

35
Questions, corrections and suggestions?
36
Thank you
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