CRaSCH: A Cooperative Scheme for Service Channel Reservation in 802.11p/WAVE Vehicular Ad Hoc Networks Claudia Campolo, Alessandro Cortese, Antonella Molinaro ARTS Laboratory – Department DIMET University Mediterranea of Reggio Calabria Loc. Feo di Vito, 89060 Reggio Calabria – ITALY Email:
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Abstract—IEEE 802.11p is an emerging standard intended to support wireless access in the vehicular environment and to deliver both safety and non-safety applications to vehicles on the roads. Despite the massive research effort related to the design of reliable and timely schemes for dissemination of safety messages, only a few works have investigated on-the-road delivery of non-safety data traffic, such as comfort and entertainment applications (e.g., multimedia, web browsing, e-mails, e-maps), by considering the standard features and capabilities. The IEEE802.11p/WAVE (Wireless Access for Vehicular Environment) standard foresees that safety and control messages are carried over a dedicated control channel, while non safety messages can be delivered over one of a set of available service channels. In this paper, we propose a cooperative reservation scheme for service channels to be carried out by wireless nodes acting as WAVE providers. The proposal is fully compliant with the IEEE802.11p/WAVE standard and supports both V2I (Vehicle-to-Infrastructure) and V2V (Vehicle-to-Vehicle) communications. We expect that the cooperative scheme can outperform the legacy IEEE 802.11p/WAVE protocol, by reducing the cases where two or more providers choose the same service channel for non-safety traffic delivery. This would result in improved system performance without adversely affecting the delivery of safety applications on the control channel. Index Terms—vehicular network, WAVE, CCH, SCH
I. I NTRODUCTION Vehicular Ad-Hoc Networks (VANETs) are a special case of mobile ad-hoc networks, where wireless-capable vehicles spontaneously form a network while traveling on the roadway. Vehicular networks can support both Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure (V2I) communications, and are targeted at the delivery of both road safety applications and comfort applications, which aim to provide information and entertainment to people on traveling [1]. Vehicular communication poses new challenges to the Medium Access Control (MAC) design, due to the quickly changing and unstable nature of wireless links among vehicles and between vehicles and the roadside infrastructure. The high mobility of nodes, the short connection lifetime, and the multihop nature of communications make the current IEEE 802.11 standard a non viable solution for a vehicular environment. To better cope with the dynamic nature of V2V
and V2I links, IEEE 802.11p [2] has been proposed as a draft amendment to the IEEE 802.11 standard. Its core mechanism is based on the prioritized distributed channel access scheme of 802.11e [3] and on the WAVE (Wireless Access for Vehicular Environment) system [4], which provides for multichannel operation to deliver various safety and commercial applications to vehicles on the roadway. A single shared channel (CCH) is used for safety messages and control frames delivery, while multiple service channels (SCHs) are for non-safety applications. Since the standard provides for multi-channel operations, a straightforward approach in designing WAVE-compliant MAC protocols has to exploit as much as possible this capability. In fact, it is wellknown that by exploiting multiple channels, higher network throughput can be achieved, because parallel transmissions by neighboring nodes can take place without reciprocal interference [5]. The WAVE standard suggests each provider to choose the least interfered SCH for service delivery in its WBSS (WAVEmode Basic Service Set), but lets unspecified how performing this choice. The few proposed MAC protocols, which take into account the IEEE 802.11p/WAVE specifications and address the support of non-safety applications [6]–[8], have not explicitly investigated how a provider chooses the SCH for its WBSS and how nearby providers interact in order to set up channel-disjoint WBSSs. Given such shortcomings, we propose CRaSCH (Cooperative Reservation of SCH), a channel reservation scheme that is specifically tailored for the multi-channel architecture of the WAVE system. It aims at enhancing the SCH selection procedure in order to improve the delivery of non-safety applications without affecting safety messages distribution on the CCH. CRaSCH enables WAVE providers to self-coordinate in order to minimize contention; cooperation relies solely on exchange of information about every providers immediate neighborhood. The remainder of the paper is organized as follows. Section II provides a basic description of the WAVE architecture. An overview of the proposed CRaSCH scheme is given in
9781-4244-3941-6/09/$25.00 2009 IEEE
Fig. 2. Fig. 1.
The WAVE channel switching
The IEEE 802.11p/WAVE protocol stack
Section III, while details are provided in Section IV and V. We evaluate CRaSCH in Section VI, by comparing its simulated performances against the legacy IEEE 802.11p/WAVE solution. Finally, in Section VII we give conclusive remarks and hints for the future research. II. T HE WAVE S YSTEM The IEEE 802.11p task group [2] is currently working on a set of specifications to permit communications in the rapidly changing vehicular environment, which operates in the DSRC (Dedicated Short Range Communication) frequency band of 5.85-5.925 GHz. 802.11p cooperates with the IEEE 1609 standard family [4] to define a new operational mode for wireless access from vehicles (referred to as the WAVE mode), which allows for multi-channel operation. As shown in Figure 1, 802.11p supports both the TCP/UDP/IP protocol stack and a new lightweight WAVEmode short message protocol (WSMP) for the exchange of small packets carrying safety or road messages. This kind of traffic can be exchanged directly among WAVE devices without the IP overhead. IEEE 802.11p/WAVE foresees the presence of one control channel (CCH), which is reserved for system control and safety messages, and up to six service channels (SCHs) used to exchange non-safety data packets (e.g., IP traffic) and WAVEmode short messages. Coordination between channels exploits a global time reference, such as the Coordinated Universal Time (UTC), which can be provided by a global navigation satellite system. As shown in Figure 2, the channel time is divided into synchronization intervals with a fixed length of 100 ms, consisting of a CCH interval and a SCH interval, each of 50 ms. According to the multi-channel operation, all vehicular devices have to monitor the CCH during common time intervals (the CCH intervals), and to (optionally) switch to one SCH during the SCH intervals. The described operation allows the safety warning messages to be transmitted on CCH, while non-safety data applications may run over SCHs. The IEEE 802.11p physical layer is an amended version of the 802.11a specifications, based on OFDM (Orthogonal Frequency-Division Multiplexing), but with 10MHz channels and a maximum data rate of 27Mbps. The IEEE 802.11p MAC layer has the same core mechanism of the Enhanced Distributed Channel Access (EDCA) specified in 802.11e [3],
which is based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme. In the WAVE mode, data packets transmission is allowed within a WBSS, which is an independent basic service set (IBSS) established ad-hoc among vehicles, but without any active scanning, association or authentication procedure. A node that initiates a WBSS is called provider, while a node that joins a WBSS is called user. To establish a WBSS, the provider has to periodically broadcast on the CCH a WBSS announcement message that includes the WAVE Service Advertisement (WSA). WSAs contain all the information identifying the WAVE application and the network parameters necessary to join a WBSS (e.g., the ID of the WBSS, the SCH this WBSS will use, timing information for synchronization purposes). WSAs are broadcasted by providers without any feedback on their successful reception; thus, the standard suggests that each provider sends more WSAs in the CCH interval for reliability purposes. However, the standard specifies neither the number of WSAs to be sent nor how scheduling their transmissions. A node should monitor all WSAs on the CCH to learn about the existence and the operational parameters of available WBSSs. After that, the node may join the WBSS by simply switching to the SCH used by this WBSS, on the subsequent SCH interval. III. CR A SCH: AN OVERVIEW The WAVE standard suggests that the WAVE management entity (WME) in each wireless station keeps track of SCHs that are in use by nearby WAVE devices. The objective is that, when called upon to do so, the wireless device can choose the least congested SCH for its WBSS set up. Nevertheless, the standard does not specify how selecting the SCH; it only suggests to measure the congestion level of SCHs by monitoring WSAs received on the CCH from providers in visibility. By doing so, a wireless node can potentially know about the status of the SCHs reserved by providers at one hop distance. However, due to either potential collisions between WSA frames sent by nearby providers or to collisions with safety messages delivered on the CCH, a provider could not be able to know about the status of the SCHs reserved by all 1-hop neighbor providers. Additionally, a provider can be unaware of potentially interfering 2-hop neighbor providers due to the lack of visibility.
It could happen that WBSSs, which have been initialized for running on the same SCH by providers not in reciprocal visibility, overlap due to the mobility of the involved nodes with consequent increase in the interference level and collision events. Our convincement is that, by letting providers cooperate in order to set up frequency-disjoint WBSSs, the performance of traffic delivery over the SCHs can be significantly improved. Coordination among providers has the additional benefit to enable a kind of load balancing of provider-user pairs on the available SCHs, so that collisions due to hidden terminals can be significantly reduced and throughput can be improved. This can be particularly effective for time-bounded applications delivered over SCHs, for which retransmissions of lost frames due to collisions could be useless. Moreover, by forcing a cooperative SCH selection instead of a blind one, bandwidth can be more effectively utilized, because cooperation reduces the risks of having two hidden providers establishing their WBSS on the same SCH, although other service channels are unused. The main idea of our solution is to exploit, as much as possible, the WSA frames sent on the CCH interval to spread out information about the SCH occupancy among 1-hop and 2-hop neighboring providers, in order to ensure set up of channel-disjoint WBSSs. Thus, we propose CRaSCH (Cooperative Reservation of SCH), a gossip-based reservation mechanism that relies on cooperation among nearby providers. WAVE providers, through properly modified WSA frames, exchange information about the perceived SCH reservation status, instead of only advertising their own SCH as suggested in the standard. Specifically, we propose two incremental approaches: • Proactive Gossiping: every provider advertises in enhanced WSA frames the information about its own SCH and the SCHs reserved by nearby providers whose WSAs have been heard. • Reactive Gossiping: every provider, besides spreading out the perceived SCH status information, explicitly reacts to a detected SCH-overlapping event (when hearing two or more providers reserving the same service channel) by sending a collision warning frame, which triggers the SCH change by one (or more) providers. The details of the proposed schemes are described in the following sections. IV. P ROACTIVE G OSSIPING A provider, which aims to advertise its presence and the services it offers, seizes the channel during the CCH interval according to the EDCA rules and sends a WSA frame. According to the standard, by simply broadcasting the WSAs with its choice of SCH, a provider initiates its own managed WBSS. Nearby nodes, which are interested in profiting from the capabilities of the provider, can join the WBSS during the subsequent SCH period. In the CRaSCH scheme, each provider sends an enhanced WSA, which includes all fields of legacy WSAs plus some
additional information. Legacy fields specify the provider’s capabilities, stored in the Provider Service Table field, and its position in terms of GPS (Global Positioning System) coordinates, stored in the Latitude and Longitude subfields. The additional field added by the CRaSCH scheme to manage coordination among providers is a ChannelGossip field, which contains GossipType and SCHBitmap subfields, as shown in Figure 3(a). The ChannelGossip field only accounts for 1-byte overhead, i.e., 2 bits for the GossipType subfield and 6 bits for the SCHBitmap. The first subfield refers to the WSA frame type (WSA or CCW, as clarified in the following); the second subfield indicates the current occupancy status of every SCH, as perceived by the provider sending the WSA. Specifically, a bit 1 in position i, i = 1, . . . , 6, of the bitmap signals a busy status for the i-th service channel, while a bit zero means that the channel is not in use by any provider. A provider fills in the SCHBitmap subfield of its WSA with information about the occupancy of the SCHs reserved by providers at one hop and of its own SCH. Figure 4 shows an example of the way providers update the SCHBitmap and choose the SCH for their own WBSS. Provider A advertises in the WSA both its own SCH, e.g., SCH 3, in the ChannelNumber subfield already foreseen in the legacy WSA frame, and the perceived SCH occupancy status in the SCHBitmap. In the example, SCH 5 and SCH 6 are perceived as busy, having been advertised in the ChannelNumber subfield of the WSAs sent by neighboring providers. This information is exploited by Provider B to select a free channel, e.g., SCH 1, among the available ones advertised in the SCHBitmap by Provider A, namely SCH 1, 2, and 4. Then, Provider B can advertise its WSA by setting the SCHBitmap with a bit ’1’ in the positions of SCH 1 (its own channel) and SCH 3 (the channel reserved by its neighbor Provider A). Provider C selects SCH 6 as its own service channel and propagates information about the occupancy of the channel reserved by Provider B only. This way, our CRaSCH mechanism allows propagation of the SCH status information only within two hops. In addition to the SCHBitmap, CRaSCH foresees that the WME of every provider maintains an SCH status vector, where it stores information about the service channel status occupancy. Again, a bit 1 in position i, i = 1, . . . , 6, of the vector signals that the i-th SCH is busy, while a zero in position i means that the i-th SCH is not in use. This vector is updated by any node at the reception of a WSA frame, as follows. The WME performs an OR operation between the values contained in the SCHBitmap of every received WSA frame and the bits stored in its SCH status vector. The vector is re-initialized to zero at the beginning of every CCH interval. Every provider is hence enabled to build its personal view of the SCHs reserved both by 1-hop providers, by keeping track of the advertised SCHs in the received WSAs, and by 2-hop providers, by reading the SCHBitmap subfield there included. The WME of provider nodes, by reading the SCH status vector, chooses among the SCHs that are perceived as free
(i.e., they have a bit zero in the SCH status vector) and sends WSA frames by reserving the chosen channel. It is worth noticing that a provider uses the information contained in its own SCH status vector only to decide on which service channel it initializes the WBSS. It does not store such information in the SCHBitmap subfield of the transmitted WSA frame, otherwise information about channel status occupancy would spread out more than two hops away, this causing bandwidth waste and inefficient channel reuse. Furthermore, both users and providers, when receiving WSA frames, store in a Provider table the following information: (i) the MAC address of the provider sending the WSA, (ii) the service channel chosen by the provider to initialize its WBSS, (iii) the services it offers. Such information is used by the WME of a user to join the target WBSS or to avoid joining a WBSS which transmits on a potentially conflicting channel, and by the WME of a provider to detect conflicting choices as explained in the next section. In summary, the Proactive Gossiping mechanism helps providers in cooperatively selecting their SCHs in such a way to avoid channel-overlapped WBSSs. Nevertheless, in some situations Proactive Gossiping cannot avoid overlapping WBSSs. In the example illustrated in Figure 5, if two hidden providers, labeled as PrA and PrB, choose the same channel, they do not have any chance to know of the potential frequency overlapping. This can be a problem for users in the overlapping area, labeled as U 1 and U 2, which are interested in the services advertised by either one or both the providers. It can also be a problem if the WBSSs of the two providers overlap due to the providers’ mobility. This shortcoming could be solved if a third provider aware of the conflict, labeled as PrC, informs the two colliding providers of the conflicting choice. The proposed solution is explained in the following section.
TABLE I F RAMES DETAILS Frame Type
Priority
Safety Alert CCW WSA
3 2 1
CCW is a new type of WSA frame identified by GossipType ’01’. The CCW frame format is shown in Figure 3(b). In particular, we add to the ChannelGossip field already foreseen in the extended WSA described in the previous section, a 3bit long UnsafeSCHid and a 6-byte long OwnerId subfields. The first one contains the identifier of the SCH that has been selected by two or more providers, the second subfield refers to the MAC address of the provider that has been considered by the gossip provider as the owner of the advertised SCH1 . The owner is the provider which has been heard by the gossip provider as the first sender of the WSA frame reserving the channel. In order to ensure the CCW frame to seize the channel before any other transmission, CRaSCH assigns it a higher priority than the WSA frame, but lower than the safety messages carried over the same CCH. Details about the frames transmitted on the CCH are reported in Table I.
(a) New WSA format WAVE VERSION
PROVIDER SERVICE TABLE
CHANNEL GOSSIP
WRA LENGTH
WAVE ROUTING ADVERTISEMENT
optional GOSSIP TYPE
SCH BITMAP
Fig. 4.
Proactive Gossiping
(b) CCW format WAVE VERSION
PROVIDER SERVICE TABLE
CHANNEL GOSSIP
WRA LENGTH
WAVE ROUTING ADVERTISEMENT
optional GOSSIP TYPE
SCH BITMAP
Fig. 3.
UNSAFE SCH_ID
OWNER_ID
WSA and CCW frame format
V. R EACTIVE G OSSIPING To solve potential conflicting choices of hidden providers, CRaSCH includes the Reactive Gossiping mechanism. As soon as a provider hears two (or more) WSA frames from different providers which reserved the same SCH, it explicitly reacts to this event by broadcasting a warning message. We named this frame Channel Collision Warning (CCW).
Fig. 5.
1 The
Proactive Gossiping failing
MAC address is retrieved in the Provider table.
Fig. 6.
Reactive Gossiping
CRaSCH rebroadcasts in the CCW frame all data included in WSAs in order to ensure information spreading on a wider extent. In fact, since there is no assurance on the reception of broadcasted WSAs, then, by piggybacking WSA information in the CCW frame, the nodes that missed previous WSAs can be aware of the presence of providers offering connectivity services. Every provider hearing a CCW frame acts as follows: • if its MAC address equals the value carried in the OwnerId subfield, then it realizes to be considered by the gossip provider as the owner of the UnsafeSCHid channel, thus it ignores the warning message; • if its MAC address is different from the one carried in the OwnerId subfield and it has chosen the UnsafeSCHid channel as its own SCH, then it realizes that its channel selection can lead to frequency-overlapped WBSS. Thus, it selects a new SCH for its WBSS and sends a WSA advertising the new channel selection, granting that the WSA can be scheduled before the end of the current CCH interval. Otherwise, the provider can either refrain from transmitting in the subsequent SCH interval or transmitting but risking collisions with nearby providers. Such a choice depends on the type of service the provider is going to deliver, delay-tolerant or throughput-sensitive traffic, respectively. To clarify, the behaviour of Reactive Gossiping is shown in Figure 6. Thereby, the Reactive Gossiping mechanism allows to reduce the number of WBSSs initialized to work on the same SCH. The main shortcoming of Reactive Gossiping is that it requires the presence of a provider which acts as a gossip provider; however, in dense network scenarios, such a provider exists with high probability. Another weakness is that two or more providers could simultaneously detect the same SCHoverlapping event and decide to act as gossip providers. In
this case, the providers contend for seizing the medium and could simultaneously send their CCW frame, this resulting in collision at the intended receivers2 . In order to solve this competition problem among gossip providers, CRaSCH provides an additional position-based waiting time before the CCW transmission. Before sending its own CCW, a gossip provider waits for a time interval computed according to its own estimated distance from the provider whose WSA triggered the CCW generation (i.e., the provider which sent a WSA reserving an SCH already reserved by another provider), in such a way that the closer is the provider located the shorter it waits. During this time interval, a gossip provider listens to CCWs for the same event (i.e., the same values in the UnsafeSCHid and OwnerId subfields) transmitted by other gossip providers. If while waiting, the gossip provider detects any other CCW intended to solve the same SCH-overlapping event, then it drops its own CCW, otherwise it transmits the scheduled CCW. As an additional countermeasure to the set up of potentially interfering WBSSs, CRaSCH foresees the active involvement of those WAVE users that detected the service channel overlapping. If a user hears WSAs from two (or more) providers reserving the same SCH or it hears a CCW frame not followed by a new WSA advertising the change of SCH, it realizes that joining one of the conflicting WBSSs may be at risk of collisions. Thus, the user can either refrain from joining the WBSS or join it and use virtual carrier sensing (RTS/CTS) for protecting its transmissions from hidden terminals. VI. P ERFORMANCE E VALUATION A. Simulation Settings In order to deploy a realistic simulation scenario, the VanetMobiSim [9] vehicular traffic simulator has been used to give mobility traces as inputs to the ns-2 [10] network simulator. Specifically, we used the modified PHY and MAC support proposed in [11], which provides for cumulative SINR (Signalto-Noise Interference Ratio) computation, header and frame body capture, structured and modular MAC procedures. We modified this extension in order to deploy the EDCA-based MAC envisioned by the IEEE 802.11p, the multi-channel operational mode of WAVE, and the proposed CRaSCH scheme. We consider two kinds of scenarios: highway and urban. In the highway scenario, four lanes, two for each direction, are deployed. The length of the highway is 2 km. Vehicles moving with speed ranging from 80 km/h to 130 km/h are simulated. A network infrastructure is supplied in the middle of the road with static nodes (RSUs, Roadside Units) acting as WAVE providers. Traveling vehicles can connect to RSUs, as depicted in Figure 7. Distance between nearby RSUs is fixed to 200 m. The urban scenario, shown in Figure 8, is a Manhattan grid of 400m x 400m, where vehicles move according to an Intelligent Driver Model with Lane Changing [12] at an 2 The message is broadcasted, however by intended receivers we mean the providers involved in the SCH selection at risk of overlapping.
TABLE II S IMULATION S ETTINGS Category PHY
Fig. 7.
Highway scenario
MAC
Fig. 8.
Urban scenario
average speed of 50 Km/h. Some of the vehicles (OBUs, Onboard Units) are randomly chosen to act as WAVE providers. In both scenarios, the number of vehicles acting as users is set to 60, while the number of WAVE providers is a varying simulation parameter. In order to initialize their WBSS, during the CCH interval the providers broadcast WSA frames that advertise both the offered services and the chosen service channel. For the purpose of assessing the effects of CRaSCH on the distribution of safety messages over the CCH, simulations have been carried out both in ideal conditions, i.e., without safety traffic on the CCH, and under high safety traffic load conditions. The traffic pattern used for broadcasting of safety messages is the following: at every sync interval, a 100 byteslong3 safety message is generated by a randomly chosen vehicle and is relayed to nearby nodes according to a naive flooding approach. The PHY and MAC layer parameters are carefully configured according to IEEE 802.11p protocol and are summarized in Table II. Propagation is modeled according to the well-known Tworay ground model, all nodes have a common transmission range of 250 m. Each simulation lasts 200 seconds; the results are averaged over 15 independent runs. B. Simulation Results In order to assess the performance of the proposed cooperative service channel reservation scheme, we run our 3 It is the approximate middle value of reasonable safety messages sizes for VANETs [13].
Parameter
Value
Frequency Channel bandwidth Basic transmission rate Transmission power Power monitor threshold Noise floor Carrier sense threshold SINR preamble capture SINR data capture Slot time SIFS time Header length
5.9 GHz 10 MHz 3Mbps 1 mW -102 dBm -99 dBm -85 dBm 4 dB 10 dB 13 µs 32 µs 40 µs
CRaSCH proactive and reactive gossiping schemes against the 802.11p/WAVE legacy scheme under the described topologies. The metrics used to evaluate the algorithms are the following: • the SCH at Risk, computed as the percentage of SCH intervals in which two (or more) providers with overlapping radio coverage initialize their WBSS on the same service channel. It assesses the capability of the compared schemes to cope with frequency overlapping of nearby WBSSs. • the CCW effectiveness, computed as the ratio between the number of successful CCW frames and the total number of transmitted CCW frames. Successful CCW frames are those which are able to solve the conflicts among providers advertising the same SCH and to enable the set-up of frequency-disjoint WBSSs. Preliminarily to performance comparisons, we separately studied how the number of WSA frames sent during the CCH interval could impact on the performances of the proposed CRaSCH solution. It is of clear understanding that the higher the number of WSAs sent per CCH interval the higher the probability that users are aware of nearby providers offering connectivity services. Nevertheless, a higher number of WSAs sent on the CCH interval increases the traffic load and, consequently, the collision probability. For the tuning of the number of WSAs for CCH interval, we used the Reactive gossiping scheme, since it foresees higher load on the CCH interval compared to the Proactive gossiping scheme. In fact, besides the enhanced WSAs transmitted by the proactive scheme, in the reactive scheme the transmission of CCW frames can trigger the subsequent transmission of new WSAs advertising the reservation of different service channels. The tuning has been decided by analyzing the SCH at Risk metric with safety traffic on the CCH. The best performance is achieved by the reactive gossiping when two WSA frames are sent for CCH interval in the highway scenario or when three WSA frames are sent for CCH interval in the urban scenario (as shown in Figure 9). Henceforth we conduct our study by considering these optimal number of WSA frames in
REACTIVE GOSSIPING - WITH SAFETY MESSAGES 9 1 WSA/CCH Int 2 WSA/CCH Int 3 WSA/CCH Int
8.5
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30 28 SCH At Risk - Total (%)
each simulated scenario. The different behavior in the two scenarios is due to the fact that in the urban environment, vehicles moving with lower speeds and stopping at the intersections, can pile up. Thus, WSA frames may experience collisions in areas dense of providers which might select SCHs already in use by nearby WBSSs. Figures 10 and 11 show the comparison of CRaSCH schemes and the legacy protocol in terms of SCH at Risk in the highway and urban scenario, respectively. We observe that the SCH at Risk metric increases as the number of providers increases. This is expected because when increasing the number of providers the number of WSAs sent during the CCH interval increases and may cause collisions. When safety messages contend for the access to CCH the performances of all schemes certainly worsen compared to the case of absence of safety traffic. In fact, safety messages are sent with higher priority than both WSA and CCW frames, thus they may seize the channel by hindering the transmission of other frames. Additionally, a higher load on the CCH can cause collisions of WSA and CCW frames that cannot reach nearby providers, which consequently fail in updating their 2-hop neighbors view of the network. However, both in highway and urban scenarios, the two proposed CRaSCH schemes significantly outperform the legacy solution also under high safety traffic load. Furthermore, the reactive gossiping technique fares better than the proactive gossiping solution thanks to the active involvement of third providers aiming at reducing the number of conflicting nearby WBSSs. In the urban scenario with mobile providers performances of all algorithms are worse than in the highway scenario with roadside providers. Again, this is due to the urban mobility pattern according to which vehicles and, consequently, providers can pile up and transmit WSAs which may collide. Moreover, differently from the highway scenario, where the deployment of a chain of providers guarantees the presence of a provider acting as gossiper, in the urban scenario with moving providers such a condition is not so likely to be met. For this reason, in the urban scenario, the reactive gossiping has more chances to fail, thus hindering the providers to know which SCHs have already been chosen for the setup of nearby WBSSs. As regards the CCW effectiveness parameter, by looking at Table III, we can notice that it decreases as the number of providers increases. In fact, a higher number of providers directly translates into a higher load in terms of WSAs transmitted on the CCH. Thus, the CCW frames can fail in resolving the SCH overlapping because of collisions with WSA frames (even if CCW frames are transmitted with a higher priority than the one assigned to WSA frames). Furthermore, due to the short duration of the CCH interval, providers receiving the CCW frame could not be able to broadcast a WSA frame advertising the change of the SCH. Additionally, vehicles may move out of each other’s coverage, this hindering the reception of the CCW frames by the interested providers.
26 24 22 20 18 16 14 3
4
5
6
Number of Providers
Fig. 9. SCH intervals at risk with safety load in the highway (top) and urban (bottom) scenario by varying the number of WSAs for CCH interval
Slightly lower values of CCW effectiveness are noticed in the presence of safety traffic. In this case, CCW frames may collide with safety messages or loose the contention for seizing the channel due to their lower priority compared to safety messages. We also notice that the CCW effectiveness in the highway scenario is higher than in the urban grid topology for the same reasons explained above. Such a behaviour witnesses that when a gossip provider capable of triggering the service channel re-selection exists, conflicting providers are made capable of setting up channel-disjoint WBSSs. Finally, in order to evaluate the impact of the proposed CRaSCH solutions on safety messages, we compute the average delivery delay for this type of traffic. Interestingly, achieved results, not included due to length constraints, show that the CRaSCH schemes do not affect the delivery of safety messages over the CCH, which are always delivered within their expected deadline, i.e., 100 msec [13]. Moreover, no additional delays are perceived with respect to the legacy scenario. The same trends are observed both in the highway and the urban scenarios, with slightly higher delay values in the urban scenario for the reasons explained above. VII. C ONCLUSIONS AND F UTURE W ORK In this paper, we introduced CRaSCH, a cooperative scheme for the reservation of service channels in 802.11p/WAVEbased vehicular networks. The proposed scheme targets vehicle-to-infrastructure and vehicle-to-vehicle communications, where nodes, either roadside or on-board units, acting as
2 WSA / CCH INTERVAL - WITHOUT SAFETY MESSAGES
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Fig. 10. SCH intervals at risk without safety (top) and with safety load (bottom) in the highway scenario: CRaSCH and legacy comparison
Fig. 11. SCH intervals at risk without safety (top) and with safety load (bottom) in the urban scenario: CRaSCH and legacy comparison
WAVE providers choose a service channel for their initialized WBSS. Achieved results show that CRaSCH outperforms the legacy solution in all the considered scenarios. Moreover, the additional information carried out in the WSA frames in order to enforce the gossip-based reservation, adds negligible overhead (i.e., few bytes) on the CCH interval. This guarantees that the delivery of safety messages does not experience degradations or additional delays. Work is underway to jointly test the performances of the proposed mechanism with some techniques of smart broadcasting for safety messages dissemination. Furthermore, research efforts are required to investigate additional countermeasures taken by providers or users to protect data transmissions in the SCH interval against collisions due to the hidden nodes.
2, pp. 88-105, 2008. [2] “IEEE 802.11p/D3.0,” in IEEE Standard Activities Department, July 2007. [3] Standards Committee, “Wireless LAN Medium Access Control (MAC) and Physical Layer (Phy) Specifications: Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements,” in IEEE Computer Society, 2005. [4] “IEEE 1609.4. Trial-Use Standard for Wireless Accesses in Vehicular Environments (WAVE)- Multi-channel Operation,” in IEEE Vehicular Technology Society, October 2006. [5] J. So, N. Vaidya, “Multi-channel MAC for ad hoc networks: handling multi-channel hidden terminals using a single transceiver,” in ACM International Symposium on Mobile Ad Hoc Networking and Computing (MOBIHOC), May 2004, pp. 222-233. [6] Y. Zang, L. Stibor, B. Walke, H. Reumerman, A. Barroso, “Towards Broadband Vehicular Ad-Hoc Networks The Vehicular Mesh Network (VMESH) MAC Protocol,” in Proc. of IEEE Wireless Communications and Networking Conference, pp. 417-422, Hong Kong, 2007. [7] N. Choi, S. Choi, Y. Seok, T. Kwon, and Y. Choi, “A Solicitation-based IEEE 802.11p MAC Protocol for Roadside to Vehicular Networks,” in Proc. of IEEE Mobile Networking for Vehicular Environments, pp. 9196, Anchorage, Alaska, USA, 2007. [8] M. Amadeo, C. Campolo, A. Molinaro, and G. Ruggeri, “A WAVEcompliant MAC protocol for Vehicle-to-Infrastructure Non-Safety Applications,” in IEEE Vehi-Mobi 2009, co-located with IEEE ICC 2009, June 14-18, Dresden, Germany. [9] VanetMobiSim Project. http://vanet.eurecom.fr. [10] The Network Simulator ns2. http://www.isi.edu/nsnam/ns/. [11] Q. Chen, F. Schmidt-Eisenlohr, D. Jiang, M. Torrent-Moreno, L. Delgrossi, and H. Hartenstein, “Overhaul of IEEE 802.11 modeling and simulation in ns-2,” in International Workshop on Modeling Analysis and Simulation of Wireless and Mobile Systems, pp. 159168, October 2007. [12] M. Fiore, J. Harri, F. Filali, and C. Bonnet, “Vehicular mobility simulation for VANETs,” in Proc. of 40th IEEE/SCS Annual Simulation Symposium, 2007. [13] Q. Xu, T. Mak, J. Ko, and R. Sengupta, “Vehicle-to-vehicle safety messaging in DSRC,” in Proc. of 1st ACM International Workshop on Vehicular Ad Hoc Networks (VANET 2004), 2004.
R EFERENCES [1] M.L. Sichitiu, and M. Kihl, “Inter-Vehicle Communication Systems: A Survey,” in IEEE Communications Surveys and Tutorials, vol. 10, no.
TABLE III CCW E FFECTIVENESS HIGHWAY (2WSA/CCH int) 3 4 5 6
Pr Pr Pr Pr
URBAN (3WSA/CCH int)
with safety
without safety
with safety
without safety
99.55 98.59 94.69 92.20
100 99.58 95.32 93.69
99.69 96.60 91.71 88.85
100 99.23 98.05 96.27
