Broadcast
messages
will play a larger role than the use of unicast messages in a Vehicular
Ad Hoc
Network (VANET). A large portion of the messages sent in a vehicular
network
will be broadcast messages. Some of the uses for broadcast messages
are:
sending emergency warning messages, periodically broadcasting vehicle
state
information, etc. The lower layer
technology used in VANETs will be a variant of IEEE 802.11a
technology.
However, the 802.11 technology is known for
not being able to manage the medium resources very efficiently,
especially in
case of broadcast messages. Providing
reliable delivery of broadcast messages in a VANET introduces several
key
technical challenges:
- No
retransmission is possible for failed broadcast transmissions since
they
can not be detected. A failed unicast
transmission is usually detected through the acknowledgement (ACK) from
the
receiver. However, it is
not practical to receive an ACK from each node for a broadcast
message.
If ACKs were used, a broadcast “storm” problem
would occur.
- The
contention window size, CW, cannot change because there is no
MAC-level recovery on broadcast frames. In order to control
congestion, the contention window size (CW) is exponentially increased
each
time a failed transmission is detected. Since there is no detection of
failed
broadcast transmissions, the size of the CW fails to change for
broadcast
traffic as it does for unicast traffic. This may result in
excessive collisions, if a
large number of nodes are contending for access.
- The hidden
terminal problem exists because the RTS/CTS exchange cannot
be used. The hidden terminal problem [1] is the main cause
of collisions in a wireless network. The IEEE 802.11 protocols use an
optional
RTS/CTS handshake followed by an acknowledgment to guarantee the
delivery of a
unicast packet. Broadcast messages, on the other hand, cannot use the
RTS/CTS
exchange because it would flood the network with traffic.
- The vehicular
network should support the ability to prioritize messages.
When emergency warning messages are broadcast, they
should be given a higher access priority than common data messages.
Objectives
The goal
of
this project is to develop broadcast protocols that improve the
reliability of delivering
broadcast messages in VANETs. We rely on the observation that a
node in a
VANET is able to detect collisions and congestion by simply analyzing
the
packets it has recently received. In a
VANET, each node will broadcast its status
to its neighbors at least 10 times every second. While a node
does not
know if the packets it sent are correctly delivered or not, it knows
the exact percentage
of packets sent to
him from neighboring nodes are successfully received. Based on
the percentage of packets that are
successfully received in the last few seconds, a node is able to
determine the current
local conditions of the network and roughly estimate the number of
neighbors in
its communication range. Therefore, a
node is able to dynamically adjust the parameters it uses, such as
contention
window size, transmission rate, and transmission power, to improve the
delivery
rate of broadcast messages.
The novelty of
this approach is that no communication control overhead
is involved. In addition, the proposed
technique doe not require changing the existing 802.11 standard instead
it
focuses on optimizing the parameters used by 802.11. For that
reason, we believe that this approach
will have very good chances to be commercially deployed. In
particular, this project will concentrate
on the development of:
(1)
prioritized access protocols,
(2)
adaptive adjustment of contention window
protocols,
(3)
adaptive transmission range control protocols,
(3)
dynamic transmission power control protocols
