Image Processing Reference
In-Depth Information
by one FlexRay CC for transmission of a frame. his ownership only relates to one channel. On other
channels, in the same slot either the same or another controller can transmit a frame. The identi-
fication of the transmitting controllers in one slot is also determined by configuration parameters
of the FlexRay controllers. This piece of information is local to the sending controller. The receiv-
ing controllers do not possess any knowledge on the transmitter of a frame nor on its content; they
are configured solely to receive in a specific slot. Hence the content of a frame is determined by its
positions in the communication cycle.
Alternatively, a message ID in the payload section of the frame can be used to identify a message.
he message ID uniquely tells the receivers which information is contained in a frame. By providing
a filter mechanism for message IDs, a controller can pick specific data.
The static segment provides deterministic communication timing, since it is exactly known when
a frame is transmitted on the channel, giving a strong guarantee for the communication latency. his
strong guarantee in the static segment comes for the trade-off of fixed bandwidth reservation.
The dynamic segment has fixed duration, which is subdivided into so-called minislots. A minislot
has a fixed length that is substantially shorter than that of a static slot. he length of a minislot is not
sufficient to accommodate a frame; a minislot only defines a potential start time of a transmission
in the dynamic segment. Similar to static slots, each minislot is exclusively owned by one FlexRay
controller for the transmission of a frame. During the dynamic segment, all controllers in the net-
work maintain a consistent view about the current minislot. If a controller wants to transmit in a
minislot, the controller accesses the medium and starts transmitting the frame. This is detected by
all other controllers, which interrupt the counting of minislots. hus, the minislot is “expanded” to a
realslot,whichislargeenoughtoaccommodateaframetransmission.Itisonlyatertheendofthe
frame transmission that counting of the minislots continues. The expansion of a minislot reduced
the number of minislots available in this dynamic segment. he operation of the dynamic segment
is illustrated in Figure .: Figure .a shows the situation before minislot occurs. Each of the
channels offers minislots for transmission. The owner of minislot on channel —in this case
controller D—has data to transmit. Hence the minislot is expanded as shown in Figure .b. The
number of available minislots on channel is reduced to .
If there are no data to transmit by the owner of a minislot, it remains silent. The minislot is not
expanded and slot counting continues with the next minislot. As no minislot expansion occurred,
no additional bandwidth beyond the minislot itself is used; hence other lower-priority minislots that
are sequenced later within the dynamic segment have more bandwidth available.
This dynamic media access control scheme produces a priority and demand-driven access pattern
that optimally uses the reserved bandwidth for dynamic communication. A controller that owns an
“earlier” minislot, i.e., a minislot, which has a lower number, has higher priority. he further in the
dynamic segment a minislot is situated, the higher is the probability that it will not be in existence in
a particular cycle due to the expansion of higher priority slots. A minislot is only expanded and its
bandwidth used if the owning controller has data to transmit. As a consequence, the local controller
configuration has to ensure that each minislot is configured only once in a network. he minimum
duration of a minislot is mainly determined by physical parameters of the network (delay) and by
the maximum deviation of the clock frequency in the controllers. he duration of a minislot and the
length of the dynamic segment are global configuration parameters that have to be consistent within
all controllers in the network.
The symbol window is a time slot of fixed duration, in which special symbols can be transmitted
onthenetwork.Symbolsareusedfornetworkmanagementpurposes.
The network idle time is a protocol-specific time window, in which no traffic is scheduled on
the communication channel. The CCs use this time window to execute the clock synchronization
algorithm. he offset correction (see below) that is done as a consequence of clock synchronization
requires that some controllers correct their local view of the time forward and others have to cor-
rect backward. The correction is done in the network idle time. Hence no consistent operations of
