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

Questions/Comments?