Protocol Design and Implementation of Vehicular Ad-hoc Networks

module, a 2 byte value is also received though it
may not be used in some cases.
When the system is first powered up, a
function is called which initializes the RMF12
module by giving it a number of commands to
bring it to the desired settings and enabling all
required features. To transmit a byte of data, the
FM module is given the command “B8XXH”
where XX is the byte to be transmitted. Before
transmitting our data payload, we must first
transmit the preamble “AAAAAAH” followed by
the synchronization bit sequence “2DD4H” [8].
When the module receives a byte of data, it stores
it in its receiver FIFO register. To read this
register, the module is given the command
B000H. The data will be in the lower byte of the
two byte data received. The received data must be
read immediately after receiving; otherwise the
register will be overwritten if more data is
received.
B. Protocol Details
The RFM12 module is only capable of
performing one function at a time; that is, it is
unable to transmit and receive at the same time.
Thus, it is only put into transmitting mode when
some data is to be sent, and then shifted back to
receiving or listening mode. Another point of
importance is the switching of the module
between channels. With channel 1 at 433MHz and
channel 2 at 439.75MHz, The FM transceiver is
tuned to listen at the channel where the next frame
is expected to arrive.
The Broadcasting Unit of the Roadside
Module is programmed to transmit a DR frame
followed by 49 SB frames with delays of 100ms
between them and the cycle is repeated
continuously. This results in a DR frame being
transmitted once every 5 seconds, and at this
moment, the BU triggers an interrupt on the
Responding Unit. The RU then opens up the
UART connection between it and the
Broadcasting Unit and the BU obtains the vehicle
count in the region from the RU over the
connection. The BU then uses this updated value
in the DR and SB frames it is broadcasting. The
BU does not expect to receive anything so it
remains in the transmitting mode. Also, all the
frames it transmits are in ch1.
It is the job of the Responding Unit of the
Roadside Module to wait for vehicles to contact it,
and when they do, to acknowledge their
transmission. The RU keeps a one byte global
722
variable rx_frame which is initially initialized to
zero. When the FM transceiver receives data, the
nIRQ pin of the module goes low and this triggers
an interrupt on the microcontroller. The Interrupt
service routine calls a function which stores the
received frame in the rx_frame variable. The RU
only expects to receive frames in channel2.
Therefore, it initially tunes to ch2 in the receiving
mode and waits, continuously polling the valid bit
of rx_frame. The valid bit going high indicates a
frame being received, and the RU saves the
required data from the frame before clearing it to
make it ready for the next received frame. If the
received frame is a VR frame, the RU will enter
the transmission mode briefly to transmit the ACK
frame (in ch2) and enter the vID of the vehicle in
the array v_list. This array contains entries of
vehicles registered to the station. The RU will also
trigger an interrupt on the BU signalling it to
listen to the UART connection opened by the RU,
over which the RU will send the updated vehicle
count to the BU. It may be possible that the
vehicle may not receive an ACK for some reason,
in which case it will transmit the VR frame. The
RU must detect and ignore duplicate VR frames.
The RU treats E frames in a similar manner. It
maintains an emergency list where it keeps
records of vehicles facing an emergency. Each
entry is valid only for 30s after which it is
removed.
The Vehicular Module is undoubtedly the
most complex of the three. It must listen at both
the channels for specific frames as well as
properly handle and forward Emergency frames.
The VM initially listens at ch1 for the SB, DR or
E frame and remains in this state for the majority
of the time. To transmit the VR frame, the VM
switches to ch2. It then starts a timer and waits for
the ACK frame. If it is received before the timer
overflows, the VM saves the sID of the station. As
soon as an ACK is received, or the timer
overflows, the VM will switch back to ch1 and
continue listening. If the vehicle receives an E
frame, and the frame is found not to be a
duplicate, the VM will enter the vID of the
received frame in its emergency list. It will then
check the TTL value of the frame, and if it is zero,
it will take no further action. If the TTL value is
non-zero, the VM will decrement it and transmit
the modified E frame in ch1 where it will be
received by other nearby vehicles and
retransmitted. If a vehicle has an accident it will