Reducing Broadcast Latency in Wireless Mesh Networks (WMNs)

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Reducing Broadcast Latency in Wireless Mesh
Networks (WMNs)

• Cyrus Minwalla
• Maan Musleh
• COSC 6590

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Presentation Layout

• Overview
• Broadcasting in wireless mesh networks (WMNs)
• Broadcast configurations in WMNs
• Fully multi-rate multicast (FMM)
• Single best-rate multicast (SBM)
• Performance Evaluation
• Conclusion

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Brief overview of Wireless Mesh Networks (WMNs)

• Network Topology
• Properties of WMNs

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Network Topology in WMNs
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Properties of Wireless Mesh Networks

• Nodes
• Wireless but static
• Connected in an ad-hoc manner
• Energy a non-issue (nodes generally plugged in,
  or easily rechargeable)
• Network
• Topology is cluster-based
• Static routers connect subsets of the network.
• Routers can serve as source nodes for sub-trees
  (useful for topology construction, scheduling,
  etc.)

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Why Broadcasting in WMNs

• Motivation
• Carried over from wired networks
• Useful for many applications
• OS updates
• Video conferencing/streaming
• Multiplayer gaming
• Have fewer packet transmissions due to wireless
  broadcast advantage (WBA)

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What is Wireless Broadcast Advantage (WBA)?

• Refers to a unique quality belonging to wireless
  networks
• Wired networks perform broadcast by separate
  unicasts across the network (separate to each
  root node in a tree)
• In wireless networks
• Direct neighbours of the source node require only
  one tx
• Multiple unicast tx in wired 1 broadcast tx in
  wireless
• Potential Energy and bandwidth savings!

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Exploiting WBA for Broadcast

• Achievement of WBA in broadcast transmissions ?
  configuration changes at link level
• Link level changes involve
• Number of radios/channels
• Rates
• Radio power (for channel reuse)
• Antenna gain (direction)

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Node Configuration

• Various node configurations in literature
• Authors discuss the following two configurations
• Single-radio single-channel multi-rate
• Multi-radio multi-channel multi-rate

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What is Minimum Latency Broadcasting (MLB)?

• Definition
• To provide the best QoS by minimizing latency at
  the slowest node
• Goal
• All destination nodes must receive packet within
  same time frame
• Maximize the transmission rate of the slowest
  node
• Metric
• RAP (Rate-Area Product)

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Why do we care about MLB?

• Motivation
• Want to guarantee quality of service (QoS) to all
  users in the multicast session
• Want to decrease the latency encountered by the
  slowest link.

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Overview of Techniques

• Both techniques involve the idea of using
  multicasts across partitioned nodes to achieve
  broadcast
• Single-channel multi-rate
• Also known as fully multi-rate multicast (FFM)
• Multi-channel multi-rate
• Referred to as the single best-rate multicast
  (SBM)

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Multi-rate vs. Multi-radio

• FMM
• Uses an optimum rate per link to maximize
  throughput and minimize latency
• Attempts to minimize the number of transmissions
• Needs scheduling per transmission to avoid
  interference
• SBM
• Determines a single best-rate metric for the
  entire network
• Simplifies the construction algorithm by using
  one rate
• Uses multiple channels, thus simplifying the
  scheduling algorithm

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What about Energy Efficiency?

• Both techniques transmit a packet multiple times
  from the same node
• Multi-rate uses multiple rates for various
  neighbours (based on RAP)
• Multi-channel uses multiple channels (channel
  diversity ? non-interference)
• The goal To minimize broadcast latency, not
  energy efficiency

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• Fully Multi-rate Multicasting
• (FMM)

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Fully Multi-rate Multicasting (FMM)

• Topic Layout
• What is fully multi-rate multicast?
• Why we want to use it
• How it works
• Topology Construction Algorithm
• Multicast Grouping Algorithm
• (Simplified) Transmission Scheduling
• Maximum end-to-end throughput
• Pros and Cons
• Recap

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What is Fully Multi-Rate Multicast ?

• Broadcast achieved via sequential multicasts
• Multicast to separate subsets in network
• Algorithm in four steps
• Construct a tree to span the entire network
• Calculate the optimum rate at every link
• Provide scheduling for all transmissions
• Recalculate maximum end-to-end throughput
• Caveat Most of the solutions are NP-hard

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Why choose FMM

• Motivation
• Multi-rate allows us to minimize the MLB
• Current radios work with setup
• RAP metric is easy to calculate

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Current 802.11 metrics

• Transmit rates and ranges for 802.11b
• Obtained via Qualnet simulation
• Consider network topology in next slide

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A Motivational Example

• Node 1 wants to broadcast to 2, 3, 4 and 5.
• Node 2 _at_ 11 Mbps, node 5 _at_ 1 Mbps
• One single transmission at lowest rate or two
  transmissions (one at either rate)

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Motivational Example The Single Transmission
Case

• Node 1 broadcasts to nodes 2 and 5
• Transmission rate slowest link i.e. 1Mbps
• Transmission to node 2 _at_ 1Mbps ? 4 is starved
  until 33 u.t.

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Motivational Example The Multiple Transmission
Case

• Node 1 makes two transmissions
• Transmission 1 to node 2 _at_ 11 Mbps
• Transmission sequence 2 ? 3 ? 4
• Node 1 ? 5 only occurs when 2 ? 3 is complete
• Node 4 receives packet at 23 u.t.

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Topology Construction in FMM

• We want to reach all nodes within the network
• Build a connected dominating set (CDS)
• Defn of CDS
• In a graph G(v,e), the connected-dominating set
  is a set of edges Se all non-leaf nodes v are
  connected. All other (leaf) nodes are one hop
  away from at least one node in CDS

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Connected-Dominating Set (CDS)

• What this means
• In a CDS, the source has a path to all relaying
  nodes in the network
• Calculate all possible CDSs in the network
• Obtain the CDS with the minimum cost
• Steps
• Calculate the set of possible CDSs
• Attach a cost metric per CDS
• Pick one that minimizes that cost (use Djikstra)

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Problems with CDS

• Problem 1
• For k nodes, 2k possible sets to consider
• Solution
• Use Djikstra with an approximation criteria
• Problem becomes polynomial
• Problem 2
• Minimum connected set will assume slowest rate to
  maximize downlink neighbours per node
• Same as using slowest rate for all nodes
• Solution
• Account for the rate metric max (no. of nodes x
  transmission rate)
• This is defined by the RAP

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Topology Construction in FMM

• Algorithm steps
• Keep a set C of all covered nodes.
• C starts with just source node s
• Pick optimal product of rate x no. of nodes
  covered
• Add covered nodes within optimal area to C
• Continue until C satisfies CDS quality for G
• This process ensures a minimum-cost,
  minimum-spanning tree

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Sample Network Topology
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Example Minimum WCDS Tree
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Example Minimum WCDS Tree with rates
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Multicast Grouping in FMM

• Once the broadcast tree is constructed, need to
  determine two things for each node
• No. of times to multicast
• No. of nodes covered by multi-cast
• Need to find transmission delay to reach all
  downstream nodes with minimum latency
• Every nodes latency depends on what happens
  downstream ? follow bottom-up topology

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Bottom-up Topology

• Algorithm Steps
• Start with leaf nodes
• Calculate the minimum latency to the relay (based
  on optimal rate in previous step)
• Latency maximum at relay node is stored in
  Cardinality Value (CV)
• CV helps determine the transmission delay at
  relay node R

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Example Multicast Grouping
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Example Multicast Grouping
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Example Multicast Grouping
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Example Multicast Grouping
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Example Multicast Grouping
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Bottom-up Topology (2)

• CV values along nodes build up a transmission
  sequence
• For k rates, there are 2k-1 possible valid
  transmission sequences (VTS)
• Pick the VTS with the shortest possible
  transmission delay
• Assumption
• Grouping does not deal with nodal interference

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(Simplified) Transmission Scheduling

• Transmission sequence determined by CV
• Higher CV higher latency ? more critical
  transmission
• All nodes assigned a start-time and a stop time
• Nodes must have packet before start time
• The goal is to avoid nodal interference
• In our example, time is measured in packet time
• Packet tx _at_ 11 Mbps 1 u.t.

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Example Transmission Scheduling
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Example Transmission Scheduling
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Example Transmission Scheduling
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Example Transmission Scheduling
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Example Transmission Scheduling
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Problems with Transmission Scheduling

• Problem 1
• Absolute times require centralized clock
• Solution
• Algorithm assumes a centralized clock within
  source node
• Problem 2
• Node schedules are broadcast throughout the
  network.. to set up broadcasting
• Solution 2
• ...........

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Pros and Cons

• Advantages
• Obtains lower latency compared to standard
  techniques
• Works with current hardware
• Disadvantages
• Algorithms are NP-hard
• Scheduling problem has no apparent solution

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Recap

• The technique FMM
• uses selected multicasts to achieve broadcast
  over network
• Minimizes latency in the network
• Algorithms required to achieve optimal solution
  NP-hard
• Need a centralized station for clock
  synchronization scheduling
• The next technique resolves some of these issues

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• Single Best-Rate Multicasting
• (SBM)

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Single Best-rate Multicast (SBM)

• Decides a single transmission rate for all link
  layer data multicast.
• Depends on the network's topological properties.
• Simplifies broadcasting algorithms.

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Decisions To Be Made

• Selecting 'best' transmission rate to use for all
  link layer broadcasts.
• Deciding whether a certain node should transmit.
• Deciding 'Interface Grouping'.
• Scheduling each node's transmissions.

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'Best' link-layer multicast rate selection

• Can be predicted reasonably by the product of the
  transmission rate and transmission coverage area
  (rate-area product or RAP).
• Higher RAP means more broadcast-efficient for
  SR-SC MR WMNs.

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Methods of Selection

• R gt set of transmission rates, which if used
  returns a connected network.
• Use the highest link-layer multicast rate in R.
  Quickest rate.
• Use the transmission rate with the highest RAP
  value of all Rates in R.

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Topology Construction

• Two Heuristics Proposed
• Connected Dominating Set (CDS)
• Simplified Minimum Connecting Dominating Set
  Problem.
• Parallelized Connected Dominating Set (PCDS)
• Adaptive to the radio resources available
  (interfaces and channels).
• Uses two more parameters ( priority and label).

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Example CDS Construction
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Interface Grouping and Transmission Scheduling

• Broadcast performance can be improved by delaying
  the choice of interface to use till the
  scheduling stage
• WMN can then maximally exploit the channel
  diversity in the system.

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Interface Grouping and Transmission Scheduling

• During scheduling, an appropriate choice of the
  interface to use is made
• Depending on other transmissions at that time
• The algorithm aims to find a start time and end
  time of each transmission node
• For this algorithm, nodes are sorted in
  descending order according to height of node.
• Height is distance from the node to its furthest
  leaf.

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Interface Grouping and Transmission Scheduling

• The choice of channel to be used for a particular
  transmission is motivated by the desire to
  include as many parallel transmissions as
  possible.
• The algorithm completes execution when all
  transmissions are scheduled.

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Normalized Broadcast Latency
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Review of Presentation

• Topics Covered
• Broadcasting in WMNs
• What is WBA?
• What is MLB?
• Techniques with examples
• Fully multi-rate multicast (FMM)
• Single best-rate multicast (SBM)
• Performance Comparison

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Future Work

• Sleep
• Actually, to find a feasible solution for the
  scheduling algorithm

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Bibliography

• 1 C.T.Chou, A. Misra and J. Qadir. Low latency
  broadcast in multi-rate wireless mesh networks.
  IEEE JSAC special issue on wireless mesh
  networks, 2006.
• 2 J. Qadir, C.T.Chou and A. Misra. Exploiting
  rate diversity for multicasting in multi-radio
  wireless mesh networks. IEEE, 2006.
• 3 R. Draves, J. Padhye, and B. Zill. Routing in
  multi-radio, multi-hop wireless mesh networks. In
  Mobicom, pages 114-118, 2004
• 4 H. Lim and C. Kim. Flooding in wireless ad
  hoc networks. Computer Communications, 24(3-4)
  353, 2001

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• Thank you for your time and patience
• Questions/Comments?