the broadcasting problem in wireless ad hoc network

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 ?



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



The difference of two type

source

Be notified Be shortest

5 forwarding nodes4 hop time

source

6 forwarding nodes3 hop time



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)



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



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



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



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



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])



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])



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



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



Flooding

Each node forwarding the broadcasting packets exactly one time Using Process ID



Area Based Method1



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



Area Based Method3

The expected additional coverage after hearing the message k times, is expected to decrease quickly as k increases.



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



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



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



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



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



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



Multipoint relaying


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

))((



Dominant pruning


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

))((



Dominant pruning

vi vj

N(N(vj))

B(vi,vj)

U

N(vi) N(vj)



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.



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



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



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])



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])




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




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



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

the broadcasting problem in wireless ad hoc network

Technology

broadcast packets

broadcast message

forwarding nodes

coverage area

broadcast packet optimization

broadcast function difference

simple broadcast mechanism

minimum forwarding set