Trends in Automotive Communication Systems - Networked Embedded Systems - page 313

regular message transmissions can be used as “heartbeats” for detecting the station failures. he three
TDMA-based networks that could serve as gateways or for supporting safety-critical applications are
TTP/C (see Ref. []), FlexRay (see Section ...) and TTCAN (see Section ...). FlexRay,
whichisbackedbytheworld'sautomotiveindustry,isbecomingthestandardintheindustryand
isalreadyusedintheBMWXmodelsince(seeRef.[]).Inthefollowing,wechoosenot
to discuss further TTP/C which, to the best of our knowledge, is no more considered for vehicles
but is now used in aircraft electronic systems. However, the important experience gained over the
years with TTP/C, in particular regarding fault-tolerance features (see Ref. []) and their formal
validation (see Ref. []), will certainly be beneficial to FlexRay.

13.2.2.1 FlexRay Protocol
A consortium of major companies from the automotive field is currently developing the FlexRay
protocol. The core members are BMW, Bosch, Daimler, General Motors, NXP Semiconductors,
Freescale Semiconductor, and Volkswagen. The first publicly available specification of the FlexRay
Protocolhasbeenreleasedin,thecurrentversionofthespeciication[]isavailableat
http://www.flexray.com.
The FlexRay network is very flexible with regard to topology and transmission support redun-
dancy.Itcanbeconiguredasabus,astar,ormulti-star.Itisnotmandatorythateachstationpossesses
replicated channels nor a bus guardian, even though this should be the case for critical functions such
as the Steer-by-Wire. At the MAC level, FlexRay defines a communication cycle as the concatenation
of a TT (or static) window and an event-triggered (or dynamic) window. In each communication
window, size of which is set statically at design time, two distinct protocols are applied. The com-
munication cycles are executed periodically. he TT window uses a TDMA MAC protocol; the main
difference with TTP/C is that a station in FlexRay might possess several slots in the TT window, but
thesizeofalltheslotsisidentical(seeFigure.).Intheevent-triggeredpartofthecommunica-
tion cycle, the protocol is Flexible Time Division Multiple Access (FTDMA): the time is divided into
so-called mini-slots, each station possesses ∗ a given number of mini-slots (not necessarily consecu-
tive) and it can start the transmission of a frame inside each of its own mini-slots. A mini-slot remains
idle if the station has nothing to transmit which actually induces a loss of bandwidth (see Ref. [] for
a discussion on that topic). An example of a dynamic window is shown in Figure .: on channel B,
frames have been transmitted in mini-slots n and n +  while mini-slot n + hasnotbeenused.Itis

noteworthy that frame n  is not received simultaneously on channels A and B as, in the dynamic

window, transmissions are independent in both channels.

+

TDMA
Static window

FTDMA
Dynamic window

Node A
static
slot

Node B
static
slot

Node A
static
slot

Node C
static
slot

Node A
static
slot

Node D
static
slot

Node A
static
slot

Node B
static
slot

...

Mini-slots

FIGURE . ExampleofaFlexRaycommunicationcyclewithnodesA,B,C,andD.(FromNavet,N.,Song,Y.Q.,
Simonot-Lion, F., and Wilwert, C., Proc. IEEE ., (), , . With permission.)

∗ Different nodes can send frames in the same slot but in different cycles, this is called “slot multiplexing” and it is only
possible only in the dynamic segment.