Information Technology Reference
In-Depth Information
for the vehicles. We discuss the details this dynamic channel selection algorithm
in Section 4.2.
4.2 Dynamic Channel Assignment Algorithm
The analysis in Section 4.1 is based on the transmission probability. In reality,
the transmission probability can be estimated by the channel usage time. By
recording the active transmission time
ΔT
on channel
c
in every
T
period by
the node itself, it can estimate its current channel usage level. We still denote it
using
P
(
n,c
)as
P
t
(
n,c
)=
ΔT
T
(2)
for node
n
on channel
c
in time period
t
. In algorithm, the Exponentially
Weighted Moving Average (EWMA) of
P
t
is used to filter out the random
fluctuations:
P
t
+1
(
n,c
)=
αP
t
+1
(
n,c
)+(1
−
α
)
P
t
(
n,c
)
.
(3)
α
is the EWMA parameter. The initial value of
P
is set to
P
0
(
n,c
)=0.This
channel usage level in Equation (2) and (3) is used to predict the transmission
probability in a short period of time. They are exchanged by hello/beacon mes-
sages between nodes and substituted into Equation (1) to calculate the conflict
probability.
With the estimated conflict probability on every channels, the node can select
the channels with lower conflict probability to use in the next time period. Con-
sider that network interfaces have channel switch delays, too frequent switches
waste the channel time. Moreover, too frequent switches will cause the thrashing
problem. Also, if all the channel usages are low, channel switching is unneces-
sary. In order to minimize the channel switching cost and make full use of the
channels, interface will switch their channel only when the conflict probability
of the current channel is larger than a threshold, denoted as
P
threshold
.Weset
P
threshold
=0
.
2275. It can be seen in Table 1 that 0.2275 is the value of maxi-
mized
P
conflict
at
q
=0
.
9and
m
=+
∞
. According to the monotonic property
of the conflict probability we analyzed in Section 4.1, the threshold can keep the
conflict within tolerance and the summed channel usage
q
below 1.0.
The algorithm is fully distributed. The communication cost is the channel
usage information and the active channel announcements of the nodes exchanged
by the hello/beacon messages. 2 bytes are enough to indicate the channels in use
(1 bit for each channel). One byte (256 levels) is enough to indicate the usage
level of each channel. Since the number of channels is quite small (for example,
12 for 802.11a), the total message overheads in one hello message are within
twenty bytes.
The neighbor table is required to store the extra 1-byte channel usage levels
and a 4-byte updating timestamp of every channel for every neighbor. For a
network with
C
channels and a node with
M
neighbors, the total storage cost
is
M
5
Bytes
. For a dense VANET with 256 neighbors and the 802.11a
network with 12 channels, the extra storage cost on each node is 15KB, which
×
C
×
