Ali Balador

Platooning is both a challenging and rewarding application. Challenging since strict timing and reliability requirements are imposed by the distributed control system. Rewarding since considerable fuel reductions are possible. As platooning takes place in a vehicular ad hoc network, the use of IEEE 802.11p is close to mandatory. IEEE 802.11p has problems with its medium access method causing packet collisions and random delays. Therefore, we suggest a token-passing medium access method, where the next token holder is selected based on beacon data age. This has the advantage of allowing beacons to be re-broadcasted in case of extra time in each beacon interval. We show that our token-based method is able to reduce the data age and increase reliability considerably compared to the standard.

Lately, all the top truck manufacturers
are investing considerable resources in the research
and development of platooning systems which would
allow vehicles to save fuel and improve safety by
travelling in a close-following manner. The platoon-
ing system requires frequent and reliable vehicle-to-
vehicle communications. As platooning takes place in
a vehicular ad hoc network, the use of IEEE 802.11p
is close to mandatory. However, the 802.11p medium
access method suers from packet collisions and ran-
dom delays. Most ongoing research suggests using
TDMA on top of 802.11p as a potential remedy. How-
ever, TDMA requires synchronization and is not very

exible if the beacon frequency needs to be updated,
the number of platoon members changes, or if re-
transmissions for increased reliability are required.
We therefore suggest a token-passing medium access
method where the next token holder is selected based
on beacon data age. This has the advantage of al-
lowing beacons to be re-broadcasted in each beacon
interval whenever time and bandwidth are available.
We show that our token-based method is able to re-
duce the data age and considerably increase reliability
considerably compared to pure 802.11p.

Intelligent Transportation Systems (ITS)
have attractive potential in order to decrease the ordi-
nary trac jams and avoid transportation disasters.
Also, they are able to provide various infotainment
services like browsing, reading e-mail or using social
networks that makes a trip more interesting. In or-
der to make it more ecient in real vehicular envi-
ronments, achieving a well-designed Medium Access
Control (MAC) protocol is a challenging issue due
to the dynamic nature of VANETs, scalability issues,
and the variety of application requirements. Dier-
ent standardization organizations have selected IEEE
802.11 as the rst choice for VANET environments
considering its availability, maturity, and cost. The
research results for IEEE 802.11 MAC protocol show
the importance of contention window adjustment on
the communications performance. The impact of ad-
justing the contention window has been studied in
MANETs, but the vehicular communication commu-
nity has not yet addressed this issue thoroughly.
This paper proposes e-HBCWC, a new contention
window control scheme for VANET environments
based on estimating the network condition. Analy-
sis and simulation results using OMNeT++ in urban
scenarios show that e-HBCWC clearly outperforms
802.11 DCF, even in very high network density, by
increasing the packet delivery rate while decreasing
the number of collisions and the end-to-end delay for
unicast applications.

In mobile ad hoc networks (MANETs), the IEEE 802.11 CSMA/CA is deployed as the primary medium ­access control (MAC) layer protocol to schedule the access to the wireless medium. The IEEE 802.11 standard was designed with the assumption that nodes would never deviate from the protocol. However, MANET nodes may purposefully show misbehavior at the MAC layer to obtain more bandwidth or degrade the network performance and disrupt the network services. This paper reviews and classifies the most important strategies generating MAC layer misbehavior based on their objectives and operating principles. Then, it examines some of the recent proposed solutions and mechanisms for detecting and preventing MAC layer misbehavior. A comparison of the studied solutions is carried out using a set of critical evaluation metrics. Finally, the paper concludes with a brief summary of key ideas and a general direction that can provide a basis for future work.

In the IEEE 802.11 standard, network nodes experiencing collisions on the shared medium need a mechanism
that can prevent collisions and improve the throughput. Furthermore, a backoff mechanism is used that
uniformly selects a random period of time from the contention window (cw) that is dynamically controlled
by the Binary Exponential Backoff (BEB) algorithm. Prior research has proved that the BEB scheme suffers
from a fairness problem and low throughput, especially under high traffic load. In this paper, we present a
new backoff control mechanism that is used with the IEEE 802.11 distributed coordination function (DCF).
In particular, we propose a dynamic, deterministic contention window control (DDCWC) scheme, in which
the backoff range is divided into several small backoff sub-ranges. In the proposed scheme, several network
levels are introduced, based on an introduced channel state vector that keeps network history. After
successful transmissions and collisions, network nodes change their cw based on their network levels. Our
extensive simulation studies show that the DDCWC scheme outperforms four other well-known schemes:
Multiplicative Increase and Linear Decrease, Double Increment Double Decrement, Exponential Increase
Exponential Decrease, and Linear/Multiplicative Increase and Linear Decrease. Moreover, the proposed
scheme, compared with the IEEE 802.11 DCF, gives 30.77% improvement in packet delivery ratio, 31.76%
in delay, and 30.81% in throughput.