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 08 Sep 24 th 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 Fading vehicle channel Shadowing due to static and mobile obstructions 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 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?
Compare common MAC ARQ schemes with spatial diversity. 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 are OR-ed Both FW1 and FW2 can forward. The forwarder with the good channel forwards. Channel = (FW1->CAR1) OR (FW2->CAR1)
Use Freeway measurements for trace based emulation Trace used for simulation.
Spatial Diversity Gains over STOP and WAIT 1 Empirical CDF CDF 0.8 0.6 0.4 Up to 10 tries required Spatial Diversity FixFWReTX STOP and WAIT (FW2) Spatial Div: Max 6 tries required 40% improvements in max delay 0.2 Spatial Div: 80% get delivered in one try! 20% gains over STOP and WAIT 0 0 2 4 6 8 10 12 No. of Transmissions before successful reception, unless dropped Total Number of Transmissions
Real Implementations have 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. 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 (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) 802.11 MAC (broadcast & monitor) 802.11 PHY
GeoMAC Timing
Backoff calculation TX FW1 FW2 FW3 FW4 RX d 4 d 1 d 2 d 3 Backoff at FW n = (d n /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 WAIT. Neighbor lists are updated via beacons (1 per sec) We compare throughput-delay characteristics
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 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 (msec) 150 100 AODV Non Rt AODV Rt GPSR Non Rt GPSR Rt GeoMAC Mean Delay 50 0 2 4 6 8 Maximum no. of transmissions allowed per packet
Elimination of routing overhead minimizes Delay Mean Delay (msec) 150 100 50 AODV Non Rt AODV Rt GPSR Non Rt GPSR Rt GeoMAC Minimum average delay achieved by AODV is for 18 max. no. of TXs. 0 14 16 18 20 Maximum no. of transmissions allowed per packet
1 AODV vs GeoMAC Delay Distribution Comparison CD DF 0.8 0.6 0.4 GeoMAC achieves low and bounded delays. AODV has a large delay spread even for small mean delays. 0.2 AODV Max. No. Of Allowed Transmissions = 18 GeoMAC Max. No. Of Allowed Transmissions = 8 0 50 100 150 Delay (msec)
Spatial diversity leads to high packet 100 delivery rates too! Percentage Packe ets Rcvd 80 60 40 20 0 AODV GPSR GeoMAC 2 4 6 8 Max. No. Of Transmissions allowed per packet
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 Multihop Wireless 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 STOP and WAIT ARQ schemes.
Co-operative ARQ can exploit spatial diversity. 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. 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 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 Jitter (msec) Log Scale 1000 500 100 20 10 5 2 1 AODV GPSR GeoMAC Ideal 1 2 4 6 8 10 12 14 16 18 20 No. Of Transmissions (Maximum Allowed)