reducing broadcast latency in wireless mesh networks (wmns)
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Reducing Broadcast Latency in Wireless Mesh Networks (WMNs)
Cyrus MinwallaMaan MuslehCOSC 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 @ 11 Mbps, node 5 @ 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 @ 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 @ 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): Def’n of CDS:
In a graph G(v,e), the connected-dominating set is a set of edges S{e} | 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 node’s 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 @ 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 => set of transmission rates, which if used returns a connected network.1.Use the highest link-layer multicast
rate in R. “Quickest rate”.2.Use the transmission rate with the
highest RAP value of all Rates in R.
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Topology Construction
Two Heuristics Proposed1.Connected Dominating Set (CDS):
Simplified Minimum Connecting Dominating Set Problem.
2.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
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