Military Communications and Information Technology: A Trusted ...

98 Military Communications and Information Technology...
This time interval is set into the comparative timer register of the microcontroller.
The message is sent only once in the time-triggered mode, no further
arbitration is performed.
Each row of the matrix is begun by synchronization message [6, 7], followed by
the CAN ID that characterize the various flight parameters. Free frames reserved for
asynchronous messages of the CANaerospace protocol can be found here. The time
required for the matrix reconfiguration depends on its dimension, e.g. reconfiguration
of the matrix with size of 10 x 6, takes 77.76 ms. This value is not critical.
However, complete on-board aircraft electronic system provides far greater number
of messages. For imagination, the time required for the Cycle matrix reconfiguration
with maximum possible number of elements, which enables CANaerospace
protocol specification [5], it is the matrix of size 256 x 256, is 1 min 25 s. This time,
when the system does not provide any data, is quite critical and can have fatal consequences
for flight safety. Maximal system bit rate reduces the value to 10.62 s. This
time is also still quite critical, and therefore it is necessary to select a compromise
among the transmission period of synchronous messages, the number of necessary
broadcast messages and the number of elements in the Cycle matrix. In the case,
that is impossible to avoid the number of elements of the Cycle matrix approaching
to the possible maximum according to the CANaerospace specification, it is necessary
to use some other reconfiguration algorithm of the Cycle matrix [6, 7 and 9].
IV. Mathematical analysis
Certain parameters, which quantitatively express the level of communication
over the bus is necessary to establish to evaluate the communication behavior on
the bus. The most important parameters for the mathematical analysis of the bus
behavior are bus capacity C CANAS and bus utilization U. To obtain these parameters
is important to come out of the following values.
The data frame length n D is considered for 11-bit identifier. It is possible to
work with up to 2 11 messages, which is 2048 with 11-bit identifier. Such number
of messages is excessively adequate for designed on-board aircraft electronic system.
Data frame length n D in this case is given by:
348N
D
nDround
478N
D
5
(3)
where N D is number of data byte.
Round function signifies cutting off the decimal part. Division by five in the argument
of the function is due to the application of stuff bits insertion mechanism.
Number of stuff bits is not possible or hardly ever to analytically determine. Inserting
or not inserting of stuff bits depends on a combination of bits in the CAN
identifier, data length control field, data field and CRC sequence [4, 6], when more
than five bits of the same level must not follow, so here is the stuff bit inserted.