Military Communications and Information Technology: A Trusted ...

Chapter 1: Concepts and Solutions for Communications and Information Systems
99
In the case of CANaerospace protocol, when all 8 bytes of data are transmitted,
mean length of the data frame 123 bits is taken in a count. The data frame length
without bit stuffing mechanism application is 111 bits and in case of maximum
bit stuffing is 135 bits.
The duration of one bit transmission τ is defined by reciprocal value of bus
bit rate according to the equation:
1
(4)
bitrate
The transmission message period is very important parameter for the following
calculations. In this case it is transmission message period at hundred percent
of bus utilization, thus when the messages are transmitted in close behind. For
the transmission message period p U100% is defined equation:
p
U100%
n (5)
Hundred percent of bus utilization is supposed during the bus capacity calculation
C CANAS , when the transmission message period is given by n D τ. The bus
capacity determines the number of messages that is possible to send over the bus
per second. Then the bus capacity is defined by:
1
CCANAS
(6)
p
D
U100%
Maximum bus capacity reaches at the minimum data frame length and
the maximum bit rate. The value of maximum bus capacity is the 8771 messages/s.
The value of maximum bus capacity depends on the number of inserted complementary
bits in the process of bit stuffing. The occurrence of these bits is very stochastic.
Bus utilization by synchronous messages is defined by:
U
S
M
1
nS 100
(7)
p
i1
where U S is bus utilization by synchronous messages, n S is frame length of synchronously
transmitted message, p S is transmission period of synchronous message
frames.
The maximum bus utilization by synchronous messages is reached at the state
of maximum bit rate 1 Mbit/s and at small transmission message period.
Bus utilization by asynchronous messages is defined by:
M
S
UAS nAS xAS
100
(8)
i
where U AS is bus utilization by asynchronous messages, n AS is frame length of asynchronously
transmitted message, x AS is a number of asynchronous message transmitted
frames.