Proceedings Template - WORD - Twente Student Conference on IT

RSU so they have a higher priority. Also the RSSI (Received
signal Strength Indication) and transmission power can be used
to measure the distance between the RSU and the vehicles.
Based on this distance a priority class from EDCA will be
chosen. Vehicles that are closer to the RSU, and within the near
area, will use a higher EDCA priority class, than vehicles in the
far area. Lower priority traffic has to perform carrier sensing for
a longer time, so it can happen that higher priority traffic has
already claimed the channel, see Section 2.1
packets at the start of the SCH, because the nodes switch
independently of each other.
Figure 15. Broadcast coverage of the nodes in the near part,
copied from [9]
This scheme is not focussing on minimizing the impact of
channel switching on the IEEE 802.11p beaconing (i.e., CCH)
performance. Furthermore, it is not standardized and its
complexity is high. The Exploitable node-assisted WBSS
Broadcasting mechanism makes it possible for newly coming
vehicles to connect faster to a RSU. The RSU have to send less
‘administrative packets’ and can send more useful packets to
enhance to cooperative awareness. However it comes with the
cost that nodes need to calculate their relative location to the
RSU. Furthermore, in order to make full use of the Exploitable
Node-Assisted WBSS Broadcast Mechanism all vehicles should
support this mechanism.
4.5 Adaptive Independent Channel
Switching Mechanism
While the number of nodes increases, more packets are
generated and the channel could become congested. This means
that the queuing and contention delay of packets increases, as
the channel gets more congested. Simulations experiments in
[18] indicated that for a congestion window greater than 64 and
a number of nodes greater then 20, the transmission times for
the nodes to complete their transmissions becomes higher than
50 ms. This means that the CCH interval defined in IEEE
1609.4 is too short. In [18] a different channel switching
mechanism is presented, namely the Adaptive Independent
Channel Switching Mechanism (AICSM). In the AICSM all
nodes can independently of each other change their average
switching time, based on the vehicle density. As soon as a node
finishes its CCH transmissions and its average switching time
has elapsed it changes to the SCH. The AICSM is illustrated in
Figure 16.
In this switching schema the SCH performance is much better,
at the cost of CCH performance, for several reasons. The SCH
interval will often be longer than the 50 ms. specified in IEEE
1609.4. This means that more packets can be sent during the
SCH transmission time, because nodes will switch faster to the
SCH than others. Also, this results in fewer collisions between
Figure 16. Adaptive Independent Channel Switching
Mechanism, copied from [18]
This scheme is not focussing on minimizing the impact of
channel switching on the IEEE 802.11p beaconing (i.e., CCH)
performance. Moreover, this solution is not standardized.
Because of the fact that nodes can independently of each other
implement an adaptive independent channel switching
mechanism, it is very scalable. Therefore its complexity is low,
because a node only has to find out how many nodes are using
the channel and change its average switching time based on its
findings.
5. CONCLUSIONS AND FUTURE WORK
This paper provided an analysis of the beaconing performance
of IEEE 802.11p using the channel switching procedures of
IEEE 1609.4. Both the continuous scenario and alternating
scenario are evaluated in OMNeT++, using a varied number of
nodes, queue lengths and buffering mechanisms. Also an
overview is given of solutions that improve the performance of
IEEE 802.11p using the IEEE 1609.4 channel switching
procedures.
In Section 2 an answer is given to research questions (1) and (2)
by providing an overview of the specifications of IEEE 802.11p
and IEEE 1609.4. Section 3 answers the research questions (3)
and (4) by providing an overview of the simulation experiments
in which the impact of the channel switching procedures,
defined in IEEE 1609.4 are evaluated. Section 4 answers
research question (5) by providing an overview of the solutions
that are designed to minimize the impact of the IEEE 1609.4
channel switching procedures on the IEEE 802.11p (mainly on
SCH) performance.
As seen in section 3, using the alternating scenario results in a
lower reception probability and a higher end-to-end delay of
beacons already for a small number of nodes. The reception
probability decreases and the end-to-end delay increases as the
number of nodes increases. This is because the channel
becomes more congested as more nodes are transmitting
beacons. If the number of nodes exceeds 120 nodes the average
end-to-end delay is smaller for the alternating scenario than the
continuous scenario, because the channel has saturated faster.
As the number of nodes increases, the end to end delay for the
continuous scenario also increases. However as the number of
nodes increases, for both continuous and alternating scenario
fewer beacons are received successfully.
In the simulations, two buffering mechanisms are compared,
namely OPD and NPD. It can be concluded that both buffering
mechanisms perform equally well under the test circumstances.
In the simulations different queue lengths are used, namely a
queue length of 1, 2 and 5. From the simulations it can be
concluded that a different queue length has no influence on the
reception probability and the end-to-end delay of the beacons,