Understanding the Performance of Short-lived Control Broadcast Packets in 802.11p/WAVE Vehicular Networks Claudia Campolo * , Antonella Molinaro * , and Alexey Vinel † * Department DIMET, Universit` a Mediterranea di Reggio Calabria, Italy Email: claudia.campolo@unirc.it, antonella.molinaro@unirc.it † Department of Communication Engineering, Tampere University of Technology, Finland Email: alexey.vinel@tut.fi Abstract—The 802.11p standard has been recently standard- ized to provide Wireless Access in Vehicular Environments (WAVE). A multi-channel architecture is envisioned to concur- rently support both time-sensitive safety-related applications and value-added informative and entertainment services. According to the WAVE specifications, an alternating channel access scheme allows single-radio devices to tune into a common frequency during the control channel (CCH) interval, to exchange safety and control packets, and to subsequently switch to one of the available service channels (SCHs) for non-safety related data exchange. Broadcasting of short-lived packets is largely expected to be used on the CCH to deliver periodic status updates from vehicles and network initialization information. This paper inves- tigates the joint impact of different packet generation patterns and contention window sizes on the delivery of broadcast packets on the CCH by accounting for WAVE channel switching under different traffic load conditions, data rate and packet size values. Index Terms—802.11p, VANET, WAVE, broadcast, contention window, beacon, WSA I. I NTRODUCTION Vehicular Ad-hoc NETworks (VANETs) are expected to be a key enabling technology for the delivery of a wide range of services, spanning from safety alerting to traffic efficiency and entertainment. The IEEE 802.11p standard [1] has been recently ratified as an amendment of 802.11 to operate with the WAVE (Wireless Access in Vehicular Environments) sys- tem and better cope with the high dynamicity in vehicular environments. Seven 10 MHz-wide channels are available in the frequency band of 5.85-5.925 GHz for the Dedicated Short Range Com- munications (DSRC) in US, with one channel designated as control channel (CCH) and the remaining 6 channels defined as service channels (SCHs). Similarly, in Europe a 50 MHz- wide spectrum has been allocated by the ETSI (European Telecommunications Standards Institute) in a slightly different frequency band. According to the IEEE 1609.4 [2] multi-channel architec- ture, safety and control messages must be delivered on the CCH during a common time interval. The rest of the time, vehicles switch over one of the available SCHs for exchanging non-safety related data. Safety messages and control packets are to be exchanged on the CCH. In particular, vehicles constantly broadcast one- hop short heartbeat messages, the beacons, to carry position, speed, direction, and basic sensor information in order to make the drivers aware of surrounding road situation and to act accordingly. Due to the high mobility of vehicles, beacons are typically transmitted with frequency of 5-10 Hz [3] to ensure up-to-date information at the receiver side. Therefore, they can heavily load the CCH as the vehicle density increases. The management and design of beaconing applications has fostered a great interest both in the ETSI and DSRC community, where they are currently respectively specified as Cooperative Awareness Messages (CAM) [4] and Basic Safety Messages (BSM) [5]. Emergency Vehicle Warning, Intersection Collision Warn- ing, and Emergency Electronic Brake light Indication are just a few identified use cases for which the timely and successful exchange of beacons is expected to be crucial. Additionally, beacons also support topology construction and maintenance in multi-hop routing [6] and may help safety messages dissemination [7]. WAVE service advertisement (WSA) messages are also expected to be periodically broadcasted on the CCH by nodes, both road-side units and vehicles, wishing to offer non- safety data services (e.g., e-maps download, web browsing, multimedia streaming) during the SCH interval. WSAs are transmitted in order to initialize a Basic Service Set (BSS) and to advertise its main operational parameters. Broadcast packets are never acknowledged, so failed trans- missions of beacons and WSAs, either due to collisions with other packets or channel impairments, cannot be detected. As a result, their contention window does not change to cope with high traffic load, this leading to poor performances. While a WSA message failure hinders the successful BSS initialization and the consequent exchange of non-safety data during the SCH interval, an unsuccessful beacon delivery could undermine some crucial VANET applications, like co- operative driving and cruise control. The WAVE channel switching is expected to further penalize broadcast transmissions, by reducing transmission opportuni-