distance adaptive (dad) broadcasting for ad hoc networks

Distance ADaptive (DAD) Broadcasting for Ad Hoc Networks

Copyright: S. Krishnamurthy, UCR

The Paper X. Chen, M. Faloutsos & S.V.

Krishnamurthy, “Distance Adaptive Broadcasting for Ad Hoc Networks”, in IEEE MILCOM 2002.


Objective To find a good way to perform

effective broadcasting in an ad hoc network such that: Fewer number of rebroadcasts are

needed. Achieve a higher coverage Achieve power efficiency Result in a fewer number of collisions


Our Approach Only outmost nodes rebroadcast

Outmost nodes are more likely to reach new nodes

We achieve a reduction in contention Each node identifies the outmost nodes

within its range based on some neighborhood information that is exchanged.


Roadmap Description of problem Metrics of Interest Previous work Our approach Results and Discussion Future work


Broadcasting in Ad hoc network

Definition A session in which information is to reach all

nodes. Multiple rebroadcasts (local) would be needed

Objective Perform broadcasting in an efficient way so as

to use fewer rebroadcasts while maintaining the requisite coverage


Metrics and parameters Metrics

Coverage – fraction of nodes reached Broadcast Efficiency Broadcast Latency or Duration

Parameters Mobility -- speed Node density


Previous work Flooding Expanding Ring Search –

application specific. S.Y.Ni, Broadcasting storm problem

Probabilistic Scheme wherein a node re-broadcasts a received packet with a certain probability.


General probabilistic broadcast (GEN)

Parameter k as the target rebroadcast size

When node receives a packet, it randomly generates a number n that is between 0 and its neighborhood size

If n < k, it will rebroadcast, otherwise it discards the packet.

The protocol attempts to have ‘k’ new rebroadcasts for every broadcast.


Broadcasting and outmost nodes

Not every node is needed to rebroadcast

It’s more efficient to let the outmost nodes rebroadcast

Outmost nodes span the desired area more quickly

E.g. outmost nodes 4,5,6,7,8 rebroadcast, it is not necessary for nodes 1,2,3 to rebroadcast.


Our approach Using power level to decide outmost

nodes Distance ADaptive: based on local

information, a node decides certain number of outmost nodes that are to rebroadcast.

Two variants DAD-NUM, DAD-PER


DAD-NUM• Fixed number of outmost nodes performing

rebroadcast• Node keeps a neighbor table to records the received

power level from each neighbor.• This table is sorted to decide the threshold power

level that identifies the outmost nodes• Include this threshold in the broadcast packet • When the packet is received, the receiver compares

the threshold in the packet and the received power strength to decide whether it should rebroadcast


DAD-NUM State diagram

Init_state

Pkt_recv.Set timer Pkt_Gen

Finished

Receiveda packet

Time out

If receiving power is less than threshold power in packet, ignore packet.

Time out

Find threshold power and putit in packet, broadcast packet.


DAD-PER Only difference from DAD-NUM:

• A fixed percentage of total neighboring nodes performing rebroadcast

Not good for topologies wherein node density is small or variant.


Simulation results System Setup

• Glomosim 2.0• 802.11 MAC CSMA/CA• Hello Message: every 5 seconds• Network topology in 3000m x 3000m• Transmission radius 223m• Result is average on 200 random topologies


Broadcast efficiency DAD-NUM has the

highest efficiency DAD-PER is better

than GEN when the rebroadcast size is small.


Coverage Bars represent the

improvement in coverage of DAD-NUM over GEN.

DAD-NUM can achieve up to a 20% increase in coverage.


Efficiency v.s. Coverage DAD-NUM can

achieve a better Coverage than GEN while attaining a higher broadcast efficiency.


Latency DAD-NUM takes a

short time to complete the broadcast session than GEN

Improvement can be up to 21%


Conclusion and future work

Conclusion DAD is better than GEN with regards to:

• Coverage• Efficiency• Latency

Future work Apply DAD to in power-heterogeneous

ad hoc networks.


Distributed Power Control in Ad Hoc Networks


The Paper S.Agarwal, S.V.Krishnamurthy,

R.H.Katz and S.Dao, “Distributed Power Control in Ad-hoc Wireless Networks”, IEEE PIMRC 2001.


The IEEE 802.11 MAC

A B

CRTS

CTS

D

•RTS – CTS – DATA – ACK

•Solves the hidden and exposed terminal problem in most cases.

E


Why is Power Control Hard? No centralized controller as in

cellular networks. Distributed decisions on what power

to use.


Benefits Energy Conservation Frequency Re-use – more number of

simultaneous transmissions possible – translates into an increase in the network capacity.


Transmission Range Models Typically models assume a circular

range – 250 meters is the transmission range – within this range, data can be decoded.

Interference range – larger than the transmission range – data cannot be decoded – only the interference can be sensed.


Clustering Elect clusterheads for a group of

nodes. The clusterhead is responsible for

the transmit power for each node within its cluster.

Imposing a cellular infrastructure onto an ad hoc framework.

Refer to paper for reference.


Power Control Extensions to the IEEE 802.11 MAC Ten Quantized Power Levels The levels vary linearly the

difference between levels is about a tenth of the maximum power level.

Implement a power control loop between a communicating pair.


Modifications to control messages RTS/CTS messages modified to include a

new field. When a node receives the RTS message it

measures received signal strength (There is usually what is called a Received Signal Strength Indicator or RSSI in hardware).

The receiver indicates the ratio of the received strength to the minimum acceptable strength in the CTS header.


The Power Loop Closed The transmitter (the originator of data)

then does a similar computation with the received CTS message.

It then includes a similar ratio in the header of the DATA message.

So both the transmitter and the receiver are now aware of the power situation on the link – how well are we doing!


Basic Idea Increase power if the power requirements

are not satisfied – packet loss. Decrease power if power requirements

are satisfied Maintain table for each neighbor – to

know the power level to be used in order to communicate with that neighbor.


Nuances A single power measurement will not

suffice. One would need to dampen fluctuations. Once a power level is chosen, ten

transmissions at that level (a heuristic parameter).

The power control loop is only used for unicast transmissions – routing updates etc. that are broadcast do not use this.

For further details – read paper.


Sample Simulation Results Simulations were done in ns 2.0 Various mobility models were

considered. TCP Throughput (actually goodput –

does not take into account duplicates) is the metric of interest.


Parameters


Sample Simulation Results• Througput improvement is due to an increase in capacity – higher frequency re-use.


Sample Simulation Results (Cont).

• Decrease in overall energy consumption (on average).


Why can performance be worse ?

•Node 3 is receiving Data from Node 4

Node 2 does not know about the data transferThe high power CTS collides with the Data at Node 3

•Node 1 establishes a high power link

1 2

RTS

CTS

3

Data 4


Power Control leads to Asymmetry There is an inherent asymmetry

resulting from power control. Simply changing power levels can

lead to unfairness – and collisions and can in some scenarios degrade performance.


Problems at the routing layer Traditional routing protocols may no

longer be used. Uni-directional links are formed. How

can they be used ? Neighbor discovery a challenge.


Interesting topics for projects Few papers try to do power control Paper by Monks in INFOCOM 2001 Paper by Jung and Vaidya – Mobicom 2002. However, no capacity increase, use highest

power for transmission of control signals. We will see these in next class. Open area – tough problems – but

opportunities.

distance adaptive (dad) broadcasting for ad hoc networks

Documents

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