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VANET comprise two kinds of communication, one between two vehicles (V2V) and another between a vehicle and a road side unit (RSU) (V2I). The 5.9 GHZ band is already reserved for VANET communications in both United States and Europe. One of the challenging issue on designing actual VANET is providing adequate MAC protocol. Popular wireless local area network medium access control protocol, IEEE 802.11, is selected because of availability, maturity, and cost consideration as an standard for MAC layer in VANETs.

The spectrum in VANETs divided into multiple 10MHz wide channel so that one channel, Control CHannel (CCH), is reserved for sending safety messages and it can only be used by road safety applications. The rest of the channels that is called Service CHannels (SCH) can be used by different applications such as safety and non-safety applications (focused for example on traffic efficiency or passenger entertainment). This differentiation in channel usage is another issue that has not addressed by IEEE 802.11 because it assumes to have just one type of application.

IEEE 802.11p, an amendment to the IEEE802.11, can only work on a single channel like its original version. The Wireless Access in Vehicular Environment (WAVE) architecture contains another standard, IEEE 1609.4, to handle some issues like channel switching and per-packet channel routing. MAC sub-layer is divided into two parts, the upper part is defined by IEEE 1609.4 and the lower part is defined by IEEE 802.11p.

Most of the points in this paper are covered by "A Tutorial Survey on Vehicular Ad Hoc Networks" that are related to addressing the problems of beaconing (it is proved that the communication in control channel in  VANETs is more than simple broadcasting) in CCH and its consecutive effects on network throughput.

Non-safety application that are mostly IP-based, still using TCP/IP protocol stack but safety applications can not use TCP/IP architecture because of some reasons are summarized as follow:

1. IPv6 introduces high overhead for safety message can not be tolerated by the applications.
2. Different safety applications need the same information. Therefore, it is required to send a large number of messages with the same content can lead to a congested channel.

As a result, ETSI ITS architecture introduces a new layer, facilities layer, between the application and transport layer to addressed the problem of overhead in safety applications with gathering all information in the facilities layer and alleviate the necessity of application-to-application communication.

In VANET, sending a message in control channel can be causes by two reasons, to  announce cars status and unexpected hazard. This information are likely interesting for all the surrounding neighbours. this issues appears the necessity of broadcast communication in control channel. In order to announce these events, VANET generates two types of message, CAM and DEN. DEN is just transmitted by happening an unexpected hazard and CAM is periodically transmitted to improve road safety. The DEN messages is just transmitted if there is a long time until the next CAM message, otherwise it can be merged with the CAM message.

Despite broadcast nature of control channel as already mention, previous designed MAC protocols for different networks such as MANETs or sensor networks that are designed with the idea of unicast communication in mind are analyzed and is showed that cannot be used directly.

Although IEEE 802.11DCF can manage both unicast and broadcast communication, it cannot be able to use regular schemes in unicast communication like ACK packets and RTS/CTS in the broadcast mode. This inability causes that this popular standard cannot be able to detect the collision and alleviate the hidden nodes problem, respectively. It is obvious that it is not sufficient enough for communication in control channel and needs to more revision based on the characteristics of VANETs. It needs to be mentioned that while the hidden terminals problem is problematic, exposed terminal does not appear because in the transmission zone all the nodes, including those trying to access the channel are interested by the safety messages.

As a result of lack of collision detection ability in control channel, the IEEE 802.11 DCF mechanism for adapting the back-off time with traffic density cannot work properly and the contention window remains minimum. This problem causes more collisions and reduces the beacon reception probability. Therefore, finding an optimal minimum contention window is a challenging issue that has a large impact on the reception ratio.

One important point that makes different beaconing in VANETs from regular broadcasting is the limited lifetime of safety messages. It means that if each safety message can not be transmitted in predefined time period, it must be dropped and the updated one will be send indeed. It also shows that the delay for safety messages is bounded by the beacons lifetime, therefore measuring average delay becomes less significant.

Moreover, beacons in VANETs has an important role while in MANETs they are assumed as an overhead and is tried to be minimised in order to improve the network efficiency. Although, beacons are still assumed as control messages, they provide valuable information and decreasing the amount of them leads to a reduction in the efficiency of safety-applications.

Data rate adaptation has been proposed as a solution for congestion control in VANET. Using higher data rate decreases channel occupancy that allowing more transmissions to take place. But, with increase in the data rate, higher SIR is needed at the receiver side that shows limitations in data rate adaptation. (?)

Distinguishing between collision and lost packets in consequence of noisy channel. (?)

Information about DEN messages are mostly neglected.

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IEEE 802.11 is an standard for wireless LANs that specifies the characteristics of physical and MAC layer. Availability and capability of IEEE 802.11 makes it the first choice for car-to-car communication in vehicular ad hoc networks (VANETs). Also, the result of different studies shows that contention-based solutions like IEEE 802.11 DCF outperforms contention-free solutions [no reference yet].

In IEEE 802.11 standard, RTS/CTS handshaking eliminates the collision but this mechanism can not be used in broadcast communication because in this case there are multiple nodes would need to answer to the same RTS message that results in collision. Moreover, the same problem appears in the case of ACK message. Consequently, not only it can not eliminate the hidden nodes, but collision detection is also impossible. Therefore, it results in low reliability in broadcast communication comparison to unicast communication.

In mobile ad hoc networks (MANETs), research community mainly focused on multi-hop communication and not one-hop broadcast messages. But oliveira et al. [1] Shows that DCF performances significantly drops when broadcast traffic occupies more than 50% of the total traffic. This situation can not be found in MANETs but safety applications in VANETs have the same characteristic and also they demand high level of reliability.

In the case that 802.11 does not have capability of collision detection, contention window (CW) will never be increased for broadcast messages and always uses the minimum CW (CWmin). This increases the importance of having a dynamic CWmin based on the network density (number of contending neighbors).

There are lots of research in ad hoc networks that they tried to estimate the number of contending nodes but it is very difficult to obtain a good estimation. Also, there are exist another approach that is trying to calculate CW based on the number of idle and busy (due to collision) slots. However, all of these studies assume that IEEE 802.11 can detect collision with ACK messages that is not valid in broadcast communication.

If CWmin (or CW because CWmax does not used in this case) will be selected very small it causes high collision probability because it increases probability of that multiple nodes select the same time slot to transmit. Also, if it will be selected very large, it causes high beacon drop probability because each beacon has limited time to be received in destination and selecting a large CW increases the total delivery delay (it must be received before arriving next beacon). In consequence, CW must be calculated precisely based on the number of nodes in neighborhood.

References:

The concept of using wireless communication to provide safety and comfort in vehicles has appeared since 1980s. In the last few years, the research in this field shows a large increase because of these reasons:

The main goals of VANET summarized as follow:

One-hop broadcast communication plays an important role to provide information for safety-related applications. For this reason, two different messages are used as follow:

are sent when an hazardous event happens.

are sent periodically to inform the surrounding cars about the cars knowledge such as: geographical position, speed, acceleration and so on. It is suggested that these messages send 10 times per second with maximum latency of 100ms and minimum range of 150 m.

Unfortunately, using broadcast communication causes that we can not use the predefined schemes in IEEE 802.11 to alleviate the collision among the contending nodes. In this case, schemes like RTS/CTS and ACK packets cause more collisions. As a result, IEEE 802.11 converts to a simple CSMA scheme for broadcast communication. Therefore, well-known problem of hidden and exposed terminals is problematic and must be revised. Because the periodic messages overloaded the control channel, adaptive transmit power and rate control are required.

Advances in wireless communication technologies and increasing demand for wide variety of applications lead to the fact that Vehicular Ad Hoc Networks (VANETs) become an attractive research area in academic and industry. In order to reach an actual Intelligent Transportation System (ITS), a well-designed Medium Access Control (MAC) layer protocol is a challenging issue. Unfortunately, the statistical review has been done by this paper on the data that is extracted from the IEEE Xplore database proves that the area of MAC support for VANETs has received less attention than other areas. Also, most of these relatively few research have focused on V2I communications. As a result, many significant research challenges still remain unexplored in MAC support for V2V communications in VANETs.

The most important challenges of a MAC layer for VANET can be summarized as follows [1]: the dynamic nature of VANETs including the changing causes by vehicles or in channel characteristics, scalability in various traffic densities, and supporting different applications with different requirements.

MAC protocols based on how the nodes access to the channel are controlled, can be categorized into contention-free and contention-based approaches. Contention-based approaches assume no or little coordination so that this assumption makes them more suitable for VANETs. However, unbounded delay and their low performance in high dense networks is problematic for safety messages as well as some infotainment applications with minimum requirements. Contention-free approaches can satisfy better (guarantee) Qos requirements for timely-critical applications but they suffer from high coordination especially in high variable conditions such as VANETs.

1. Kenney, J.: �Standards and regulations�, in Hartenstein, H., Laberteaux, K.P. (Eds.): �VANET: vehicular applications and inter-networking technologies� (Wiley, 2010), Ch. 10, pp. 365� 428

This paper evaluates the performance of IEEE 802.11p* in comparison to 802.11 with only considering non-safety applications. The parameters that is used in this simulation summarized as follow:

Protocol: DSDV

Transmission rate: 9.6 Kbps
Data Payload: 512 bytes
Traffic Load Packet Sent in Every: 1 ms

PHY layer Parameters:

Phy/WirelessPhyExt: Pt_ 5.0e-2
Phy/WirelessPhyExt: freq_ 5.9e+9

MAC layer Parameters:

Mac/802_11Ext: CWMin_ 15
Mac/802_11Ext: CWMax_ 1023

Mac/802_11Ext: RTSThreshold_ 2346

* The simulation is conducted in NS-2. Also, the 802.11 and 802.11Ext is used for 802.11 and 802.11p MAC layer, respectively.