A dominating set (DS) is a subset of all the nodes such that each node is either in the DS or adjacent to some node in the DS.

A connected dominating set (CDS) is a subset of the nodes such that it forms a DS and all the nodes in the DS are connected.

Applications of CDS: Virtual backbone
Virtual Backbone Flooding
Reduction of communication overhead
Reliability
Redundancy
Contention
Collision
Unreliability
CDS is used as a virtual backbone in wireless networks.

Applications of CDS: Broadcast
 Only nodes in CDS relay messages
 Reduce communication cost
 Reduce redundant traffic

 Only nodes in CDS maintain routing tables
 Routing information localized
 Save storage space
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Area Coverage Problem
CDS provides connectivity

Target Coverage Problem
CDS provides connectivity

CDS plays an important role in wireless networks.
Challenges
How to construct a CDS?
How to make the size of a CDS small?

Definition & Preliminaries
Minimum connected dominating set
 Given: a graph G=(V,E).
Goal: find the smallest CDS.
 NP-hard
Approximation algorithms
 Performance ratio (PR) = |C|/|C*|
 Smaller PR, better algorithm.

Definition and Preliminaries (Cont.)
Notations
Given a graph G and a DS C, all nodes in G can be divided into three classes.
Black nodes : Nodes belong to C.
Grey nodes : Nodes are not in C but adjacent to C.
White nodes : Nodes are neither in C nor adjacent to C.
C

Greedy Algorithm in General Graph
Guha’s algorithm 1
Select the node with the max number of neighbors as a dominating node.
Iteratively scans the grey nodes and their white neighbors. Select the grey node or the pair of nodes with the max number of white neighbors.
PR = 2(1 + H( Δ ))

Greedy Algorithm in General Graph
Guha’s algorithm 2
Iteratively select the node with the max number of white neighbors as a dominating node.
The first phase terminates when there are no white nodes.
Color some grey nodes black to connect all the black nodes.
PR = 3 + ln( Δ )

Maximal Independent Set (MIS) is a maximal set of pair-wise nonadjacent nodes.
MIS DS

 MIS DS
 Idea: connect MIS CDS

Centralized Algorithm
Alzoubi’s Algorithm
Construct a rooted spanning tree from the original network topology

Centralized Algorithm
Alzoubi’s Algorithm
Color each node to be black or grey based on its rank
(level. ID). The node with the lowest rank marks itself black. All the black nodes form an Maximal
Independent Set (MIS).

 Each node exchanges its neighborhood information with all of its one-hop neighbors.
 Any node with two unconnected neighbors becomes black.
 The set of all the black nodes form a CDS.

For each node u
r(u) = the number of 2-hop-away neighbors – d(u) where d(u) is the degree of node u
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Node u with the smallest <r, deg, id> within its neighborhood becomes black and broadcast a BLACK message where deg is the effective degree .
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If v receives a BLACK message from u, v becomes grey and broadcasts a GREY message containing (v, u).
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 black node w receives a GREY message (v, u)
 w not connected to u
Color v blue
(5, 0)
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 v has received a GREY message (x, y)
 v receives a BLACK message from u
 y & u not connected
C olor v and x blue
BLACK
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