GeoMAC: Geo-backoff based Co-operative MAC for

V2V networks.

Sanjit Kaul and Marco Gruteser

WINLAB, Rutgers University.

Ryokichi Onishi and Rama Vuyyuru

Toyota InfoTechnology Center.

ICVES’08Sep 24th 2008

Target Applications

• Safety Applications in Vehicular

networks

• Emergency Vehicle approaching warning – “Seconds

Can Save a Life!”

• Collision Avoidance/Warning

Distress Warning Message

Dissemination

Playground: The vehicle to

vehicle channel• Fading

• Shadowing due to static and mobile obstructionsobstructions

• Channel coherence time

(at times tens of msecs)– Long for MAC retries to be effective

– Too short for Routing protocol adaptation

Goals

• Maximizing Reliability

• Minimizing Delay

Talk Objectives

• Show overheads in dynamic routing lead

to slow adaptation to the channel.

– High reliability incurs large delays and – High reliability incurs large delays and

jitter.

• Show exploiting spatial diversity leads to

high reliability and low delays.

Spatial Diversity – What?

•All other cars that receive a message can help deliver it•Diversity as the different channels are uncorrelated

Potentially how large are

spatial diversity gains?spatial diversity gains?

• Compare common MAC ARQ schemes

with spatial diversity.

• Ignore protocol implementation overheads.• Ignore protocol implementation overheads.

– Packet transmission times are considered

– Packet transmissions start at fixed intervals

– After a total of 16 transmissions (15 retries) a packet is

dropped.

Fixed Forwarder STOP and WAIT

Fix a forwarder (say FW1) using a routing protocol like AODV

Use STOP and WAIT ARQ between FW1 and CAR1

Spatial Diversity

Channels

Both FW1 and FW2 can forward.

The forwarder with the good channel forwards.•Channel = (FW1->CAR1) OR (FW2->CAR1)

Channels are OR-ed

Use Freeway measurements for trace

based emulation

Trace used for simulation.Trace used for simulation.

Spatial Diversity Gains over STOP and WAIT

0.6

0.8

1Empirical CDF

Spatial Diversity

FixFWReTX (FW2)

Up to 10 tries required

Spatial DiversitySTOP and WAIT

0 2 4 6 8 10 120

0.2

0.4

0.6

No. of Transmissions before successful reception, unless dropped

CD

F

FixFWReTX (FW2)

Spatial Div: 80% get delivered in one try!20% gains over STOP and WAIT

Spatial Div: Max 6 tries required 40% improvements in max delay

Total Number of Transmissions

STOP and WAIT

Real Implementations have

overheads…overheads…

GeoMAC

How it enables spatial diversity…

How do we Exploit Spatial Diversity?

• Co-operative ARQ

– Cars that receive a message take turns forwarding the

message until it is delivered successfully.message until it is delivered successfully.

• GeoMAC enables taking of turns in a distributed

manner using Geo-backoff

Geo-Backoff

• Use Euclidean distance to destination as a

heuristic.

– We assume that nodes have location information – We assume that nodes have location information

(for example GPS)

• Each node sets a back-off timer which is an

increasing function of its distance from

destination.

GeoMAC illustration

GeoMAC illustration

GeoMAC illustration

GeoMAC illustration

GeoMAC illustration

Rewind a bit…

GeoMAC illustration

GeoMAC illustration

GeoMAC illustration

Stack Architecture

(Layer3 and above)

Location MAC(click router based implementation)(click router based implementation)

802.11 MAC(broadcast & monitor)

802.11 PHY

GeoMAC Timing

Backoff calculation

• Backoff at FW = (d /δ) * (slot time)

TX RXFW4FW3FW2FW1d4

d3d2

d1

• Backoff at FWn = (dn/δ) * (slot time)– δ is the expected minimum spatial separation

between any two forwarders.

• In vehicular networks δ ~ 5m on packed freeways.

GeoMAC Evaluation

• AODV, GPSR vs. GeoMAC.

• AODV uses Dynamic Routing + STOP and WAIT

• GPSR uses Neighbor lists + STOP and • GPSR uses Neighbor lists + STOP and WAIT.

– Neighbor lists are updated via beacons (1 per sec)

• We compare throughput-delaycharacteristics

Distress Warning Message

Dissemination

GeoMAC Evaluation

• Channel assumptions

– Channels between the SRC and the forwarders are assumed perfect.

– The channel between FW1 and DST is emulated by the trace Car1by the trace Car1

– The channel between FW2 and DST is emulated by the trace Car2

• 5000 packets, 10 pkts/sec CBR, 512 byte each

GeoMAC achieves low Packet Transmission Delay

100

150

Me

an

De

lay

(m

se

c)

AODV Non−Rt

AODV Rt

GPSR Non−Rt

GPSR Rt

GeoMAC

2 4 6 80

50

Maximum no. of transmissions allowed per packet

Me

an

De

lay

(m

se

c)

Elimination of routing overhead minimizes Delay

100

150

Me

an

De

lay

(m

se

c)

AODV Non−Rt

AODV Rt

GPSR Non−Rt

GPSR Rt

GeoMAC

14 16 18 200

50

Maximum no. of transmissions allowed per packet

Me

an

De

lay

(m

se

c)

Minimum average delay achieved by AODV is for 18 max. no. of TXs.

0.6

0.8

1

CD

F

AODV vs GeoMAC −− Delay Distribution Comparison

GeoMAC achieves low and bounded delays.

AODV has a large delay spread even

0 50 100 1500

0.2

0.4

Delay (msec)

CD

F

AODV Max. No. Of Allowed Transmissions = 18

GeoMAC Max. No. Of Allowed Transmissions = 8

AODV has a large delay spread even for small mean delays.

Spatial diversity leads to high packet

delivery rates too!

80

100

Perc

en

tag

e P

ackets

Rcvd

2 4 6 80

20

40

60

Max. No. Of Transmissions allowed per packet

Perc

en

tag

e P

ackets

Rcvd

AODV

GPSR

GeoMAC

Related Work� C. E. Perkins and E. M. Royer, “Ad-hoc on-demand

distance vector routing,” in Proceedings of the 2nd IEEE Workshop on Mobile Computing Systems and Applications, $ew Orleans, LA., February 1999

� D. Johnson, D. Maltz, and J. Broch, “DSR: The Dynamic Source Routing Protocol for MultihopDynamic Source Routing Protocol for MultihopWireless Ad Hoc $etworks”. Addison-Wesley, 2001.

� B. Karp and H. T. Kung, “Gpsr: greedy perimeter stateless routing for wireless networks,” in MobiCom’00.

� S. Biswas and R. Morris, “Opportunistic routing in multi-hop wireless networks,” SIGCOMM’04

In Summary…

• Spatial diversity can achieve low

and bounded delays and high

delivery rates in comparison to typical delivery rates in comparison to typical

STOP and WAIT ARQ schemes.

• Co-operative ARQ can exploit

spatial diversity.

• Proposed GeoMAC that • Proposed GeoMAC that

implements Co-operative ARQ

using Geo Backoff

• GeoMAC incurs low mean delays

of 12.4–16.5msec, mean AODV

delays range from 24.7–300msec.delays range from 24.7–300msec.

• GeoMAC achieves packet delivery

gains of up to 50% over GPSR

and up to 25% over AODV.

Thanks…

Questions?

The schemes…

• Fixed Forwarder Retransmit

– Pre-select a forwarder and use STOP and WAIT ARQ to the destination.

• Blind Forwarder Selection Retransmit• Blind Forwarder Selection Retransmit

– Retry via the other forwarder on channel error

• Spatial Diversity

– Don’t preselect forwarders.

– The forwarder with good channel to destination transmits.

Distress Warning Message

Dissemination

Packet Transmission Jitter

100

500

1000

Jit

ter

(msec)

−−

Lo

g S

cale

AODV

GPSR

GeoMAC

Ideal

1 2 4 6 8 10 12 14 16 18 201

2

5

10

20

No. Of Transmissions

Jit

ter

(msec)

−−

Lo

g S

cale

(Maximum Allowed)