Università degli Studi di Catania Dipartimento di Ingegneria Informatica e delle Telecomunicazioni VANETs

Transcription

1 VANETs Ing. Alessandro Leonardi

2 VANETs Vehicular Ad-hoc NETworks (VANETs) can be considered as a subset of Mobile Ad hoc NETworks (MANETs) with unique characteristics. A typical VANET consists of vehicles and access points along the road. Vehicles move on the roads sharing information between themselves and with the Internet through the access points.

3 Vehicles communication Vehicle-to-vehicle communication Multi-hop communication Vehicle-to-infrastructure communication

4 VANETs vs. MANETs Vehicles mobility Vehicles often move at high speed but their mobility is rather regular and predictable Network topology High speed movement creates scenarios characterized by a very dynamic network topology No significant power constraint Vehicles can always rely on recharging batteries Localization An accurate estimate of vehicles position can be made available through GPS systems or on-board sensors

5 PHY/MAC

6 DSRC/802.11p (1/2) Dedicated Short Range Communication (DSRC) was released in 2002 by the American Society for Testing and Materials (ASTM) In 2003 the standardization moved to IEEE Forum and changed the name from DSRC to WAVE (Wireless Ability in Vehicular Environments) also known as p

7 DSRC/802.11p (2/2) The standard is based on IEEE a PHY layer and IEEE MAC layer Seven 10 MHz channels at 5.9GHz one control channel and six service channels

8 DSRC/802.11p vs a (1/3) a is designed for high data rate multimedia communications in indoor environment with low user mobility DSRC PHY uses a variation of OFDM modulation scheme to multiplex data high spectral efficiency, simple transceiver design and avoids multipath fading

9 DSRC/802.11p vs a (2/3) DSRC/802.11p reduces the signal bandwidth from 20MHz to 10MHz all parameter values are doubled in time domain in order to increase the robustness to ISI caused by the multi-path delay spread and Doppler spread effect Data rates from 6 up to 27 Mbps Transmit power level are changed to fit requirements of outdoor vehicular communications communication ranges up to 1000 meters

10 DSRC/802.11p vs a (3/3)

11 DSRC/802.11p MAC MAC layer of DSRC is very similar to the IEEE MAC based on CSMA/CA with some minor modifications DSRC involves vehicle-to-vehicle and vehicle-toinfrastructure communications

12 DSRC/802.11p scenarios relative speed between vehicles is low despite high absolute speed the effect of mobility can be ignored and considered only multi-hop aspects the key attribute necessary for communications in this kind of scenario is the need for very high download rates over a short duration

13 MAC problems Scenario a) Starvation due to hidden terminal exposed terminal Solutions: Scenario b) limiting data transmission duration by using small maximum TCP window size MAC layer approaches (MACAW) Low efficiency and instability due to shortened connection time Solutions: low overhead protocol (no RTS/CTS handshake) no variable rates because this leads to overestimation or underestimation of the NAV used in future

14 MAC issues Priority, response time and reliability should be assured because are basic requirements of safety applications Approaches: Different CWs and backoff increase parameters for different priority classes in data traffic (e.g., DFS) Assigning shorter CWs to low-delay real-time service Static slot assignment for high-priority traffic and dynamic assignment for the low-priority traffic

15 Routing

16 Routing Protocols for VANETs (1/3) Proactive algorithms (table-driven) better performance in terms of delay but they need a considerable amount of control traffic (e.g., OLSR) Reactive algorithms (on-demand) minimize the number of broadcast packets by creating routes only on demand (e.g., AODV) Several studies demonstrate that neither of the two protocol classes outperform the other in every vehicular scenarios.

17 Routing Protocols for VANETs (2/3) Traditional MANETs routing protocols require an explicit route establishment phase before the data transmission begins They are not adequate to low delivery-latency requirement for safety applications Routing protocols for VANETs should rely on packet forwarding based on geographic location of sender and receiver should be broadcast oriented

18 Routing Protocols for VANETs (3/3) Position-based unicast routing (geographical forwarding) these routing protocols exploit the availability of accurate location information more suited to dense networks and to frequent network disconnections (e.g., GPSR) Geocast routing a kind of multicast routing where the destination nodes are characterized by their geographical coordinates in VANETs nodes interested in notifications of traffic congestions or warnings are located in the same place

19 Broadcast (1/2) Majority of applications for VANETs rely on broadcast dissemination of information in the applications area Blind flooding is the first approach to achieve broadcasting since it does not require local or global topology information Broadcasting has a strong influence on network performance Serious redundancy, contention and collision problems can occur as a result of flooding An efficient broadcast protocol should minimize the total number of packet retransmissions, while at the same time preserving network connectivity

20 Broadcast (2/2) A trade-off between robustness and redundancy should be found Different approaches: probability-based a node forwards the packet with a probability p which depends on network scenario location-based the idea is to select as relays those nodes that permit to cover the widest additional area neighbor-based the nodes decide whether broadcasting the message or not on the status of their neighbors cluster-based nodes are grouped into small clusters each one managed by a particualr node called cluster-head which has the task of retransmitting broadcast messages

21 Classes of Applications Assistance for Safe Navigation Internet Connectivity

22 Assistance for Safe Navigation (1/4) Traffic safety It is possible to detect dangerous situations through on- board sensors Warning messages propagate to other vehicles through ad- hoc networking Traffic management services Access points along the road distribute information about traffic congestion, weather forecast,, road works,, etc.

23 Assistance for Safe Navigation (2/4) Smart vehicle s onboard instrumentation: Newer cars include features as recording,, processing, positioning and location capabilities. Communication facility supports wireless data exchange with other vehicles or fixed stations. Forward radar Event data recorder (EDR) Positioning system Communication facility Display Computing platform Rear radar

24 Assistance for Safe Navigation (3/4) A potential danger scenario : The system automatically triggers the propagation of warning messages by exploiting multi-hop ad-hoc communications Vehicle-to-roadside communication Inter-vehicle communication Roadside base station Emergency event

25 Assistance for Safe Navigation (4/4) Overview of some vehicular safety and communication projects: European Community has recently funded a forum to support the development of a network system offering assistance to the drivers. Many interesting features are presented in the following projects: CarTalk 2000 FleetNet

26 CarTalk 2000 (1/5) Objectives: - Development of co-operative driver assistance systems based upon inter-vehicle communication - Development of a mobile ad-hoc radio network as a communication platform with the aim of preparing a future standard PARTNERS: Centro Ricerche Fiat, DaimlerChrysler, Siemens Mobile, Bosch

27 Applications: CarTalk 2000 (2/5) - Information and Warning Functions: transmission of warning messages during breakdown,, high traffic density or congestion,, or dangerous road surface conditions. This allows an early warning of the driver of following vehicles on the same road.

28 CarTalk 2000 (3/5) Applications: - Communication-based Longitudinal Control System: Existing Adaptive Cruise Control systems only react on the vehicle directly in front. By integrating communication, these systems may adapt longitudinal control to the traffic in front and can allow anticipating to an early braking maneuver when an invisible vehicle beyond the direct predecessor in front is braking. This leads to a more natural following behavior.

29 CarTalk 2000 (4/5) Applications: - Co-operative Assistance Systems: A typical scenario for co-operation is the highway entry and merging scenario. Today, misunderstandings between drivers on the highway and on the entry-lane cause critical situations. By exchanging information up to simple trajectory plans, critical situations can be foreseen and solved by the vehicles themselves.

30 CarTalk 2000 (5/5) Technologies: - Communication System: A key technology for CarTALK 2000 is the development of a mobile multi-hop ad-hoc radio communication system that meets the requirements set by the applications. It will be based on a UMTS radio access technology, using a spatial-aware position-based multi-hop routing protocol.

31 FleetNet (1/6) Objectives: - Development of a communication platform for inter-vehicle communications - Implementation of demonstrator applications - Development and standardization of promising strategies and solutions in order to improve drivers and passengers safety and comfort PARTNERS: DaimlerChrysler AG, Fraunhofer FOKUS, NEC Europe Ltd, Robert Bosch GmbH, Siemens AG, TEMIC Telefunken, microelectronic GmbH, Universities of Mannheim, Hamburg-Harburg, Karlsruhe,, Hannover.

32 FleetNet (2/6) Applications: - Cooperative driver assistance : - Emergency notification - Overtaking assistance - Obstacle warning

33 FleetNet (3/6) Applications: - Decentralized floating car data : - Traffic jam monitor - Dynamic navigation - Route weather forecast

34 FleetNet (4/6) Applications: - User communications and information services : - Hot-spot Internet access - Inter-vehicle chat - Distributed games

35 FleetNet (5/6) Key Features: - Wireless multi-hop ad hoc networking - Unlicensed radio frequency bands - Allows for low cost data transmission - Position-based routing and location-based services - Internet access and integration - Open solution

36 FleetNet (6/6) Technologies: - FleetNet plans to use a UTRA TDD radio hardware: UMTS Terrestrial Radio Access system will be enhanced or replaced by FleetNet protocols enabling operation ad hoc mode. - Radar-based communication currently under feasibility study will focus on safety-related applications. - During the FleetNet development process IEEE wireless LAN components will be used intermediately for testing and verification purposes.

37 Internet Connectivity (1/2) Internet-based services Access to ,, web browsing,, etc. Video and audio applications P2P Chatting, multiplayer games On board file sharing

38 Internet Connectivity (2/2) Some proposed systems : Drive-thru Internet CarTorrent

39 Proposed Architecture : Drive-thru (1/5) One or more locally interconnected access points form a so- called connectivity-island, several of them along a road or in the same area may be interconnected and cooperate to provide network access with intermittent connectivity to drivers in a larger area.

40 Drive-thru (2/5) Features: - Investigation about the feasibility of providing network connectivity and, ultimately,, Internet access to mobile users in vehicles by testing UDP and TCP performance in a real vehicular scenario using IEEE b and g hardware - Hot spots along the road will provide WLAN access for some (relatively( short) period of time to vehicles driving by. - Conventional client-server approach

41 Drive-thru (3/5) Measurement scenario and findings:

42 Drive-thru (4/5) Findings from measurements: - Both UDP and TCP tests performed over IEEE b and g hardware suggest to subdivide a connectivity session into three distinct phases: Entry phase Production phase Exit phase Only in the production phase a stable connectivity jointly with a maximum throughput value is possible, while in the entry and exit phases performances decrease due to a higher number of lost packets and link-layer retransmissions.

43 Drive-thru (5/5) Results and impact on Internet applications: - Usability of exsisting IEEE radio hardware to transmit a significant amount of data during a connectivity session - Limited connectivity period and variable transmission characteristics limit the usage of the system for continous communications such as IP telefony while transaction based application like sending an could be performed exploiting more than one connectivity window

44 CarTorrent (1/11) Key Features: - Cooperative P2P vehicular strategy proposed for content delivery of large multimedia files - Communication efficient swarming protocol which uses a gossip mechanism to propagate content avalaibility informations that leverages the broadcast nature of wireless medium - Piece-selection strategy which takes proximity into account for the exchange of information pieces

45 CarTorrent (2/11) Key factors for using opportuninistic ad hoc networking models: - Problems: Stopping at gas station for full download is a nuisance Downloading from GPRS/3G too slow and quite expensive - Observation: Many other drivers are interested in download sharing (like in the Internet) - Solution: Co-operative P2P Downloading via Car-Torrent

46 CarTorrent (3/11) Infostation model : downloads only at infostation Download pieces Roadside base station Vehicle-to-roadside communication Internet

47 CarTorrent (4/11) Basic idea: Co-operative download phase 1 Download pieces Roadside base station Vehicle-to-roadside communication Internet

48 CarTorrent (5/11) Basic idea: Co-operative download phase 2 Inter-vehicle communication Roadside base station Internet

49 CarTorrent (6/11) A Bit Torrent review: - Swarming: Parallel downloads among a mesh of cooperating peers - Scalable: System capacity increases with increase in number of peers - Tracker: Handles peer discovery Centralized Tracker: single point of failure Observation: Might not work for Wireless scenarios, because of intermittent connectivity Issue: Mobility increases churn of nodes participating in a download

50 CarTorrent (7/11) A picture of Bit Torrent system: Uploader/downloader Uploader/downloader Uploader/downloader Tracker Uploader/downloader Uploader/downloader

51 CarTorrent (8/11) Gossip protocol : FIles are divided in chunks. A Gossip message containing Torrent ID, Chunk list and Timestamp is propagated by each peer.

52 CarTorrent (9/11) Peer Selection Strategies: Problem: how to select the peer for downloading? Possible selections schemes: 1) Rarest First: BitTorrent-like policy of searching for the rarest bitfield in your peerlist and downloading it 2) Closest Rarest: download closest missing piece (break ties on rarity) 3) Rarer vs Closer: weights the rare pieces based on the distance to the closest peer who has that piece.

53 CarTorrent (10/11) Impact of Selection Strategy :

54 CarTorrent (11/11) Attractive aspects of CarTorrent solutions: - Bandwidth at the infostation is limited and not convenient - It can become congested if all vehicles stop - It is a nuisance as I must stop and waste time - GPRS and 3G bandwidth is also limited and expensive - The car to car bandwidth on the freeway is huge and practically unlimited! - Car to car radios already paid for by safe navigation requirement - CarTorrent transmissions are reliable - they involve only few hops (proximity routing)

55 Vehicular mobility models and simulators Modeling vehicular traffic Vehicular simulators

56 Issues: Modeling vehicular traffic (1/5) One key problem in the study of VANETs is the need to characterize vehicular nodes mobility and traffic patterns in order to produce mobility models that reflect as close as possible the real behaviour of mobile systems Mobility model affects the simulation results

57 Approaches: Modeling vehicular traffic (2/5) Based on a level-of- detail classification, traditionally, three types of approaches have been used: Microscopic models Kinetic models Macroscopic models

58 Modeling vehicular traffic (3/5) Microscopic models: The most basic and detailed approaches are microscopic or car following models, modeling the actual response of individual vehicles to their predecessor Each vehicle is described on the microscopic scale by its space x(t) and speed v(t) coordinates at time t

59 Kinetic models: Modeling vehicular traffic (4/5) Present an intermediate step between the other two models Partially derived from microscopic models and used to obtain fluid dynamic equations,, so leading to macroscopic models Effective approach for the derivation of valid vehicular traffic models which takes also into account for the psycho- physiological nature of driver interaction (microscopic( level) A distribution function f(x,v,t) describes the number of vehicles with a certain location and speed at time t

60 Modeling vehicular traffic (5/5) Macroscopic models: Coarsest level of description In most applications one is neither interested in the exact evolution of the singles vehicle nor in the distribution function f Use fluid dynamic equations Main quantities are the density, the mean speed variance of vehicles and speed

61 Issues: Vehicular simulators (1/3) While it is crucial to test and evaluate protocol implementations in a real testbed environment, simulations are still commonly used as a first step in any protocol development for VANETs research VANETs imply specific motion patterns that make the common random mobility models used in unrealistic MANETs

62 A quick overview : Vehicular simulators (2/3) Recently new open-source tools are available for generation of vehicular mobility patterns. Most of them are capable of producing traces for network simulators such as ns-2 or OpNet STRAW: The Street Random Waypoint tool provides road topology extraction from the maps of the TIGER database as well as micromobility support.. The main drawback is that it produces traces not usable by different well-known network simulators but only by the SWANS platform on which it is based

63 A quick overview : Vehicular simulators (3/3) CanuMobiSim: is a tool for the generation of movement traces in a variety of conditions. Extrapolation of real topologies from detailed Geographical Data Files (GDF) are possible, many different mobility models are implemented,, a GUI is present,, and the tool can generate mobility traces for ns-2 and other simulators. Micromobility is considerated, implementing several car-to- car models.

64 Future research and challenges Solutions proposed for VANETs are inspired to well-known paradigms defined for ad hoc networks and they do not fully exploit the peculiar features characterizing VANETs Most current research on multi-hop networks assumes fixed or very slow varying network topology the effect of high mobility in ad hoc networks should be fully investigated new mobility models should be developed New algorithms should be designed taking into consideration severe delayconstraints of safety applications and real-time applications new MAC schemes routing schemes should provide a good tradeoff between reliability and overhead Location privacy schemes should be designed to prevent or mitigate tracking of vehicles during their communication with other vehicles or the road-side infrastructure