the broadcasting problem in wireless ad hoc network
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National Taiwan University Department of Computer Science and Information Engineering
The Broadcast Function in Wireless Ad-Hoc Network
2002.9.2Speaker:peter
National Taiwan University
Department of Computer Science and Information Engineering
Outline
An overview of the broadcast function
Difference between wired and wireless networks
The essence of broadcast problem
Categorization of present protocols
Ultra WideBand technology
Broadcast protocol for Ultra WideBand ?
National Taiwan University
Department of Computer Science and Information Engineering
Broadcast
Function: paging a particular host sending an alarm signal finding a route to a particular host
Two type: Be notified -> topology change Be shortest -> finding route
Objective: Reliability (all nodes have received the broadcast packet) Optimization
National Taiwan University
Department of Computer Science and Information Engineering
The difference of two type
source
Be notified Be shortest
5 forwarding nodes4 hop time
source
6 forwarding nodes3 hop time
National Taiwan University
Department of Computer Science and Information Engineering
Difference between wired and wireless networks
Broadcast property Reliability:
CSMA/CD vs. CSMA/CA RTS/CTS/data/ACK procedure is too cumbersome to
implement for broadcast Simultaneous transmission and hidden node problem
Expensive bandwidth
Therefore the flooding which is the simple broadcast mechanism in wired networks is not suitable in wireless networks (broadcast storm problem)
National Taiwan University
Department of Computer Science and Information Engineering
Broadcast Storm
Many retransmissions are redundant Because radio propagation is omnidirectional and a physical loca
tion may be covered by the transmission ranges of several nodes Heavy contention could exist
Because retransmitting nodes are probably close to each other Collisions are more likely to occur
Because the RTS/CTS dialogue is inapplicable and the timing of retransmissions is highly correlated
National Taiwan University
Department of Computer Science and Information Engineering
The essence of broadcast problem
Reliability Reliable MAC negotiation Collision avoidance
Reduce redundant rebroadcasts Avoid Simultaneous transmission
Optimization Minimize the forwarding nodes Minimize the power consumption
National Taiwan University
Department of Computer Science and Information Engineering
Reliable MAC negotiation
Extend RTS/CTS for broadcast Waiting for all neighbors to be ready
RTS collision ? Forwarding ?
Sending broadcast packets anyway Affect other transmission
Repeatedly broadcast until all neighbor received Acknowledgement
AThe neighborhood state of mobilenodes is under control by RTS/CTS
National Taiwan University
Department of Computer Science and Information Engineering
Collision avoidance
Reduce redundant rebroadcasts (minimize forwarding nodes)
Be shortest minimum forwarding set Be notified minimum broadcasting set
Avoid Simultaneous transmission Different timing of rebroadcasts
National Taiwan University
Department of Computer Science and Information Engineering
Minimum Forwarding Set Problem
Define: Given a source A let D and P be the sets of k and k+1 hop neighbors of A Find a minimum-size subset F of D such that every node in P i
s within the coverage area of at least one node from F
In general graph: NP-complete: reduce “Set Cover” to it Approximation ratio: logn
In unit disk graph: Unknown Approximation ratio: constant (by reference [1])
National Taiwan University
Department of Computer Science and Information Engineering
Minimum Broadcasting Set Problem
Define: Given a source A Find a spanning tree T such that the number of internal
nodes is minimum
In general graph: NP-hard: hard to “Minimum Connected Dominating Set” Approximation ratio: log ( is the maximum node degree)
In unit disk graph: NP-hard Approximation ratio: constant (by reference [2])
National Taiwan University
Department of Computer Science and Information Engineering
Optimization
Minimize the power consumption (Minimum-Energy Broadcast Tree Problem)
Define: Given a wireless ad hoc network M = (N,L) A source node s to broadcast a message from s to all
the other nodes such that the sum of transmission powers at all nodes is minimized
Same as the Steiner Tree problem in directed graph: NP-complete: reduce “3-CNF SAT” to it Approximation ratio: n
National Taiwan University
Department of Computer Science and Information Engineering
Categorization of present protocols
Simple flooding Area based method (by reference [3])
Counter based scheme Distance based scheme Location based scheme
Neighbor knowledge method Neighborhood base Set cover base MCDS base
National Taiwan University
Department of Computer Science and Information Engineering
Flooding
Each node forwarding the broadcasting packets exactly one time Using Process ID
National Taiwan University
Department of Computer Science and Information Engineering
Area Based Method1
National Taiwan University
Department of Computer Science and Information Engineering
Area Based Method2
Maximum additional coverage of previous transmission:
Average additional coverage:
≈ 0.41r2
Average additional coverage after having received a broadcast
message twice: ≈ 0.19r2
222 61.0)2
3
3()( rrrINTCr
National Taiwan University
Department of Computer Science and Information Engineering
Area Based Method3
The expected additional coverage after hearing the message k times, is expected to decrease quickly as k increases.
National Taiwan University
Department of Computer Science and Information Engineering
Area Based Method4
Counter based scheme: Using “Random Assessment Delay” (RAD) The counter is incremented by one for each redundant
packet received If the counter is less than a threshold value when the
RAD expires, the packet is rebroadcast. Otherwise, it is simply dropped
National Taiwan University
Department of Computer Science and Information Engineering
Area Based Method5
Distance based scheme: Using “Random Assessment Delay” (RAD) Estimating the distance d between sender and receiver by s
ignal strength Calculate the additional coverage by d (additional coverage = ) If the additional coverage which is calculated by the minim
um distance is more than a threshold value when the RAD expires, the packet is rebroadcast. Otherwise, it is simply dropped
)(2 dINTCr
National Taiwan University
Department of Computer Science and Information Engineering
Area Based Method6
Location based scheme: Using “Random Assessment Delay” (RAD) Adding location information to the header of the broadcast
packets Calculate the additional coverage by k location informatio
n which are received during RAD Difficult to calculate exactly Using grid-filling approximation
If the additional coverage is more than a threshold value, the packet is rebroadcast. Otherwise, it is simply dropped
National Taiwan University
Department of Computer Science and Information Engineering
Neighbor Knowledge Method
Neighborhood information How to decision forwarding nodes
Neighborhood base SBA, Self pruning
Set cover base Multipoint relaying, Dominant pruning, AHBP
MCDS base
National Taiwan University
Department of Computer Science and Information Engineering
Scalable Broadcast Algorithm (SBA)
Information: Hello message (2-hop)
Forwarding node decision: Node vj who receives the packet from vi checks whether the set N
(vj)-N(vi)-{vi} is empty Node vj schedules the packet for delivery with a RAD (Random Assessment Delay) Dynamically adjust the RAD to
(nodes with the most neighbors usually broadcast before the others)
me
Nd
d max
National Taiwan University
Department of Computer Science and Information Engineering
Self pruning
Information: Hello message (1-hop) Piggyback adjacent node list in broadcast packets (2-hop) Store adjacent node list in cache
Forwarding node decision: Node vj who receives the packet from vi checks whether th
e set N(vj)-N(vi)-{vi} is empty
vi vj
National Taiwan University
Department of Computer Science and Information Engineering
Multipoint relaying
Information: Hello message (2-hop)
Forwarding node decision: The sending node A selects forwarding nodes from it’s
adjacent nodes A select a minimum node set F N(A) such that:
A node set U = N(N(A)) – N(A) Piggyback forward list in “Hello” packets
UUfNFf
i
i
))((
National Taiwan University
Department of Computer Science and Information Engineering
Dominant pruning
Information: Hello message (2-hop)
Forwarding node decision: The sending node selects forwarding nodes from it’s adjacent n
odes Node vj who receives the packet from vi , vj select a minimum nod
e set F N(vj) - N(vi) such that:
A node set U = N(N(vj)) – N(vi) – N(vj) Piggyback forward list in broadcast packets
UUfNFf
i
i
))((
National Taiwan University
Department of Computer Science and Information Engineering
Dominant pruning
vi vj
N(N(vj))
B(vi,vj)
U
N(vi) N(vj)
National Taiwan University
Department of Computer Science and Information Engineering
The drawback of present set cover based protocols1
…
i-1
i
i+1
i+2
vi-1
vi
s
When a node vi receivedthe broadcast packet fromnode vi-1, it will select some forwarding nodes from N(vi)-N(vi-1) to cover all nodes in U.
However, some nodes in U are not i+2 level nodes, and some nodes in N(vi)-N(vj) are not i+1level nodes.
National Taiwan University
Department of Computer Science and Information Engineering
The drawback of present set cover based protocols2
…
i-1
i
i+1
i+2
vi-1
vi1s
When we will select somelevel i+1 nodes to cover alllevel i+2 nodes, the number of forwarding nodes selected by distributed algorithmcan not be bounded tosome ratio of the optimal solution ?
vi2
National Taiwan University
Department of Computer Science and Information Engineering
Forwarding Set Problem in unit disk graph
Q1Q2
Q3 Q4
OPTOPTOPTOPTOPTFFFF
OPTOPTOPTF
OPTOPTOPTF
OPTOPTOPTF
OPTOPTOPTF
3)(3
)(
)(
)(
)(
43214321
1434
4323
3212
2141
Fi is the output of the -approximation algorithm which select some nodes in blue area to cover all nodes in Qi
OPTi is the optimal solution of ForwardingSet problem and lie on Ai
A1A2
A3 A4
National Taiwan University
Department of Computer Science and Information Engineering
MCDS based algorithm
Approximation Algorithm: Definition: A piece is defined as a white node or a
black connected component Initialize: all nodes are white Procedure:
At each step we pick a node u that gives the maximum (non-zero) reduction in the number of pieces.
coloring u black and coloring all adjacent white nodes gray.
Recursively connect pairs of black components by choosing a chain of two vertices.
Approximation ratio: 3+log (reference [4])
National Taiwan University
Department of Computer Science and Information Engineering
MCDS based algorithm in unit disk graph1
Idea: Any MIS (maximal independent set) is also a DS, and convers
ely, any independent DS must be an MIS The size of any MIS in a unit disk graph is at most four times
of the size of the MCDS The shortest distance between a node in MIS and it’s neare
st node in MIS is at most three Algorithm:
Find any MIS Spanning all nodes in MIS
Approximation ratio: 12 (reference [2])
National Taiwan University
Department of Computer Science and Information Engineering
MCDS based algorithm in unit disk graph2
Lemma: The size of any MIS in a unit disk graph is at most four times of the size of the MCDS
proof: U is any MIS, T is a spanning tree of MCDS v1,v2,…,v|T| be an arbitrary preorder traversal of T Ui is the set of nodes in U that are adjacent to vi but none of v1, v2,
…,vi-1
Then U1,U2,…,U|T| form a partition of U |U1|5, |Ui|4, 2i|T|
14)1(45||
1
TTUUT
ii
National Taiwan University
Department of Computer Science and Information Engineering
MCDS based algorithm in unit disk graph3
A node is adjacent to at most five independent nodes in unit disk graph
at most 240
vivj
j=1~i-1
Ui lie in a sector of at most 240 degree within the coverage rangeof node vi, this implies that |Ui|4
National Taiwan University
Department of Computer Science and Information Engineering
Comparison 350x350 r:100
Ultra WideBand Technology(UWB)
By Chiang Jui-Hao
What is Ultra Wideband?
Originally referred to “baseband”, “carrier-free”, or impulse
Any wireless transmission scheme occupies a bandwidth of more than 25% of a center fre
quency, or more than 1.5GHz
Compare with narrowband and wideband
UWB systems have two characteristicsBandwidth is much greater,
Defined by the Federal Communications Commission (FCC), is more than 25% of a center frequency or more than 1.5GHz
Carrierless fashion “narrowband” and “wideband” use RFUWB directly modulate an "impulse" that
has a very sharp rise and fall time
UWB in Short Range Wireless
cyclemeter
bpsM
50
12*54
cyclemeter
bpsM
10
6*50
Spatial capacity : (bps/m2)higher bit rates concentrated in smaller areas
For users gather in crowded spaces, the most critical parameter of a wireless system will be its spatial capacity
[1]
Compare with IEEE 802.11 and Bluetooth (cont.)
UWB have greater spatial capacity From the Hartley-
Shannon law Potential
for support of future high-capacity wireless systems
Notice of Proposed Rule Making
In May of 2000, the FCC issued a Notice of Proposed Rule Making (NPRM)
limit UWB transmitted power spectral density for
frequencies greater than 2GHz.
Ultra-Wideband transceiver
Advantages:UWB is a “carrierless” system,thus we can remo
ve traditional blocks such as carrier recovery loop,mixer…etc.
High data rate and number of users.
Robustness to multi-path fading.
[3]
UWB Advantages
Extremely difficult to interceptShort pulse excitation generates wideband s
pectra – low energy densitiesLow energy density also minimizes interfere
nce to other servicesMultipath immunity
Time-gated detector can excise delayed returns - time separation
UWB Advantages (cont.) Commonality of signal generation and processing
architectures Communications
LPI/D, High Data Rates, Multipath Tolerance
Radar Inherent high precision – sub-centimeter ranging Wideband excitation for detection of complex, low RCS targets
Low cost Nearly “all-digital” architecture
Ideal for microminiaturization into a chipset
Frequency diversity with minimal hardware modifications
UWB Signal in multi-path fading channel
Multi-path fading results from the destructive interference caused by the sum of several received paths that may be out of phase with each other.The very narrow pulses of UWB waveforms result in the multiple reflections being resolved independently rather than combining destructively.
[4]
UWB Applications
UWB operation and technology
Imaging SystemsGround Penetrating Radar Systems Wall Imaging SystemsThrough-wall Imaging SystemsMedical SystemsSurveillance Systems
Vehicular Radar SystemsCommunications and Measurement
Systems
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