Mac Support for Broadcast-based Ad hoc Forwarding

Scheme

Ashikur Rahman and Pawel Gburzynski

Department of Computing ScienceUniversity of Alberta

Email: ashikur@cs.ualberta.ca

Agenda

• Protocol overview• Initial experimental result• Problem of IEEE 802.11-based Mac Layer• Fuzzy Acknowledgement• Performance Improvement• Future direction

Features & Assumptions

• Controlled Flooding as a routing scheme

• Simple• Flexible• Low cost• No control packets• Some tunable parameters• Fixed communication range (symmetric)• Bi-directional flows

Protocol in Brief

• Reactive Broadcast-based • Converge to a narrow strip of nodes along the

shortest path

S D

Two controlling rules

• Duplicate Discard (DD) rule

Retransmission Count

Source ID

Sequence noSession ID

Destination ID

knsDS ,,,,• Packet signature

• Packet Header,

Packet signature + r + h + some more fields

• Packet signatures are stored in DD Cache.

First Rule (Cont.)

hrtT avgr

• Expiry time of an entry, Tr

hrtFT avgcr • Average transmission time, tavg

taavgaavg tCtCt )1(

Initially

C

Lctavg

Second Rule• Sub Optimal Path Discard (SPD) rule

• Target Avoid forwarding via sub optimal path

• Intermediate node K will keep track of, dssddksk CChhDS ,,,,,

hSKhDK

hbS

K

D

bDKSKds hhhC • Number of Packets to be discarded by K,

bDKSKbds hhhmC

Second Rule (Cont.)

• Lower value of mb will cause more flooding.

• Higher value of mb will narrow down the width of

the forwarding node strip.

bDKSKtbds hhhtdmC )(

Dynamic quantity

•The constant need not to be static

• mb is called mobility factor

• Value of mb indicates aggressiveness

Initial Experimental Result

Problem in MAC Layer

• Hidden Node Problem

A B C

• Four way handshake RTS/CTS/DATA/ACK ineffective.

• Does not even use two-way handshake DATA/ACK.

• Without feedback retransmission is impossible.

Fuzzy ACK

Fuzzy Ack (Cont.)• Before broadcasting

– Sender will set a timer with expiry time: Data tx time + SIFS + ACK tx time +2×MAX_PROP_DELAY

• Sender switches to monitor the channel immediately• Exactly 3 things can happen

– No activity – A valid ACK– A short period of bust activities followed by a silence.

• Duration of silence period SIFS duration.

• Duration of the activity tx time of ACK packet.

• Fuzzy ACK not vary reliable, merely gives a hint.

Sample Scenario

Cross Layer Interaction

• For Recipient MAC Should I send ACK?• For Sender MAC Should I retransmit?• Retransmission decision is done statistically:

• RF related to probability of lying on the optimal path

• If RF > RFthreshold retransmit.

)(

)()(

fwdSKD

ackSKD

SKD n

nRF

Effect of RFthreshold

Performance Improvement

Future Direction

• Solution to Hidden node problem yet to be

uncovered.• Incorporation of power consumption into heuristics

facilitating path identification.

Question/Comment