localized operations for distributed minimum energy multicast algorithm in mobile ad hoc networks
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Paper by Song Guo and Oliver Yang; supporting images and definitions from Wikipedia
Presentation prepared by Al Funk, VT CS 6204, 10/30/07
Table of Contents
Background and related work Models: system, network, mobility DMEM algorithm Operations Performance Conclusions
Background and related work Multicast: communication technique which
enables a source to send a single packet to reach multiple receivers.
Objective: Create a distributed algorithm to solve the Minimum Energy Multicast (MEM) problem Definition of MEM: Find a route for multicast
transmission with the minimum total energy consumption for a given communication session.
Challenges: MANET changing network topology, lack of central authority; problem is NP-hard
Background and related work Prior research focused on:
Creating centralized, not distributed, algorithms
Efficient heuristic algorithm design Weaknesses of prior research
Examination of static, not dynamic, network topologies
Little examination of performance impact of node mobility
Models: System Model
Discrete Power Level Management Model Transmission range based on
power level, but power level increases at an exponential rate as distance increases
Identify discrete power levels appropriate to reach nodes at various distances from the transmitter Vary transmitter power in
granular increments to balance power use with the bandwidth usage necessary to constantly adjust transmission strength
Models: System Model
Pvu = Power level required to transmit from node v to node u
lvu = Layer (concentric ring from prior slide) of u relative to v
K = Number of discrete power levels of the transmitter (and therefore number of layers)
rK = Distance of ring K α = Parameter (2 to 4) representing rate
of signal attenuation
Models: Network Model
Represent network as a directed graph, G(N,A,p) N = set of nodes, A = set of arcs, p =
function representing power required for each arc
Rooted tree: directed acyclic graph with a source node that has no incoming arcs and where other nodes have a single incoming arc
Leaf vs. internal/relay nodes
Models: Network Model
For any node v in the rooted tree,there exists a single acyclic source route πv
Our goal is to set lv, the transmission layer of node v, to the minimum necessary for v to reach all of its child nodes
Once this is known, we can calculate pv, the necessary power level for the node
Models: Mobility
Mobility is a differentiator for the contribution, as alternative models require the significant overhead associated with central coordination.
Authors use “Random Waypoint Model” Calculate random speeds bounded by Vmin
and Vmax; assume random start and end points; introduce pause between journeys.
Objective: calculate the steady-state average speed:
Algorithm: Data Structure We need to store the forwarding
state at each tree node v. Membership status – sender, receiver,
forwarder (can be receiver and forwarder)
Source route π – directed path from the source to node v (used to avoid loops)
Tree neighborhood table TNv – stores neighbors, along with whether is a father, child or other, along with layer lvu
Algorithm: Tree Construction Minimum Spanning Tree: Given a
connected, undirected graph with weighted edges, an MST is a subgraph which connects all vertices together resulting in the minimum total weight.
Algorithm: Tree Construction MULTICAST-JOIN-REQUEST (MJREQ):
Broadcast message initiated by the source used when no route information is known
MULTICAST-JOIN-REPLY (MJREP):Response message sent to previous hop node
MJREQ: Transmitted at maximum transmission power
MJREP: Returned at necessary power Necessary power determined by strength of
the original MJREQ message
Algorithm: Tree Flood
MULTICAST-ALIVE (MA): Message sent periodically during session to refresh the tree (otherwise tree routes are cleared) Message sent at maximum power Used to adjust power dynamically Only sent if received from father (but then
always sent) Supports tree repair and energy saving
operations Nodes update neighborhood information to
identify nearby nodes
Localized Operations
Normal Energy Saving (NES): Upon receipt of MA from children, node adjusts its transmission power to the minimum necessary. Reactive approach which could lower
total power utilization Keeps the tree connected but not with
maximum efficiency
Localized Operations: SHO Soft Hand-Off (SHO): Initiated by a
node that detects it is leaving its father’s transmission range (K). Goal is to identify a new father s.t.
and power utilization is minimized Node severs link with previous father
(via MULTICAST-LEAVE (ML) message), selects the new father
Tree is maintained.
Localized Operations: MTR Multicast Tree Repair (MTR): In the
case where loss of a node results in a tree partition, we need a way to repair the multicast tree. Occurs when a forwarder or receiver fails
to receive successive MAs from its father Nodes furthest from the source attempt
to reconnect first MULTICAST-JOIN-SOLICITATION (MJS):
Hop-limited message
Localized Operations: MTR Disconnected node closest to source
notifies the subtree that it is initiating repair procedures using an MA message
The closest node to the source initiates an MJREP message and attempts to reconnect the subtree back to the multicast tree
If an appropriate node responds, the tree is reconnected; if not, other nodes in the subtree attempt to reconnect, and the node(s) that failed must rejoin through a network flood.
Localized Operations: AES Advanced Energy Savings (AES): A proactive
method of reallocating child nodes s.t. overall power utilization of the system is reduced. The major contribution of the paper We must be able to retain the MST structure for
multicast Operation performed as part of MA Approach: Each child node attempts to
extend its transmission range to become the parent of a current child of its father – but only if such a change reduces the total power utilization of the system
More sophisticated than NES They are not mutually exclusive
Localized Operations: AES Using the MA message header means that no
separate message is necessary for the operation
Use of MA messages fits the algorithm -- father to child propagation enables communication of power levels and supports child decision-making. At each transmission from its father, a node
modifies header with its own information and propagates to its neighbors
Because MA messages are at full power, neighbors of multicast tree nodes will receive. As a result, non-multicast tree nodes can join, but
must consider potential added cost of the link from a father node
Localized Operations: AES AES-REQUEST: When a node
identifies a power savings, it sends an AES-REQUEST to the source
Source reviews AES-REQUEST messages and sends AES-REPLY to the node with the greatest power savings
Localized Operations: AES Finalizing the update
Selected node sends local broadcast TREE-UPDATE and assigns itself as father to the node to move
Moving node leaves father, sending MULTICAST-LEAVE.
If selected node is a non-tree node, it must find a father It will be a forwarding node only, otherwise it
would have been part of the original tree Multiple nodes may become children of the
selected node if power savings justify
Localized Operations: AES Examples of AES tree revision
Performance Evaluation
Simulations Ad hoc network with size 1,000 meters
sq. Each node can transmit 250 meters K=10 α = 2 Modeled max node movement speeds of:
1, 5, 10, 15, 20 and 25 m/s Multicast groups 5, 25, 50, 75, 100 Static networks considered 50 scenarios for each multicast group
Performance Evaluation
Measures Relative tree power: Ratio of actual total
tree power for heuristic algorithm vs. ideal of MST algorithm
Average tree power: Power used over time for the tree
Communication overheads: Overhead for AES, SHO and MTR as a total number of these operations over each simulation
Performance Evaluation
Static network evaluation Compared DMEM against prior work Not key to the paper, but demonstrates
that DMEM is a useful heuristic compared with prior research
Performance Evaluation
Mobile network evaluation: Consider with and without optional protocol components
Performance Evaluation
Examine AES performance considering node speed and multicast group size.
Performance Evaluation
Examine SHO operations given node speed and multicast group size.
Performance Evaluation MTR operations considering node
speed and multicast group size.
Conclusions
In a static network, DMEM is superior to alternative algorithms for medium and large multicast groups. Measures heuristics, but major contribution is on
dynamic network DMEM is efficient in reducing energy
utilization AES provides significant value relative to base
case SHO is mostly redundant when using AES
DMEM proven correct for maintaining tree structure using localized operations
Critique
Graphs are not presented in such a way to visually support the analysis e.g., authors require visual comparison of separate
charts to compare AES and SHO, rather than presenting a single chart
Is it scalable? Authors indicate that AES becomes saturated; this seems to occur rapidly in “large” networks even at slow speed. Authors indicate that it is scalable with regard to
mobility – but AES saturation seems to put this in question, as do some of their comments right before the conclusion
If scalability is an issue, possible approaches to address it would have been welcome
Do the arbitration performed by the source node along with the broadcast approach amount to centralization that reduces scalability and creates a bottleneck?
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