Thursday, September 28, 2017
Summary of Properties of the MAC Layer in Safety Vehicular Ad Hoc Networks
Summary of Properties of the MAC Layer in Safety Vehicular Ad Hoc Networks
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:
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.
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:
- IPv6 introduces high overhead for safety message can not be tolerated by the applications.
- 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.
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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