FlexRay Bus Architecture Model

Click here to view and Simulate the FlexRay VisualSim model

1         FlexRay Overview

FlexRay is a new network communication system targeted specifically at the next generation of automotive applications or "by-wire" applications. The by-wire applications demand high-speed bus systems that are deterministic, fault-tolerant, and capable of supporting distributed control systems. The technology is designed to meet key automotive requirements like dependability, availability, flexibility, and a high data rate to complement the major in-vehicle networking standards - CAN, LIN, and MOST.

 

FlexRay, as a scalable communication system, allows synchronous and asynchronous data transmission. The synchronous data transmission enables time triggered communication to meet the requirement of dependable systems. The asynchronous transmission, based on the fundamentals of the byteflight™ protocol, allows each node to use the full bandwidth for event driven communications. FlexRay's synchronous data transmission is deterministic with a minimum message latency and message jitter guaranteed. FlexRay supports redundancy and fault-tolerant distributed clock synchronization for a global time base, thus keeping the schedule of all network nodes within a tight, predefined precision window.

2         Introduction

Mirabilis Design provides a modeling and simulation environments around the FlexRay and CAN bus network architectures.  The tutorial and white paper below helps understand the operation of FlexRay in general and to use the VisualSim model to analyze different conditions.  The model link provided above helps the user to fall through using the model Applet.  This link opens a separate Web Page where the interactive FlexRay model will appear. The user can click on the icons of the communication channel, nodes, star and the top-level parameters, modify them and execute the simulation.  This will enable the user to study impact on their architectures and to optimize the FlexRay parameters.  A list of analysis is provided below. 

 

The Automotive Library contains the following components of FlexRay-

  1. Communication Controller
  2. Node
  3. Star
  4. Dual Channel
  5. Variety of Network topology examples including mesh,
  6. Example systems including rollover prevention in an Anti-lock Braking System (ABS).
  7. Traffic generators to activate the model and construct scenarios

 

The Automotive Library in VisualSim can be used to construct automotive network topologies and interconnect sub-system across this backbone. Architects can utilize the FlexRay Bus architecture model in VisualSim for validating the operation of a sub-system, evaluate performance for different FlexRay bus topologies, sizing an Electronic Control Units, or optimizing the FlexRay protocols for future revisions.  Architects can also use the FlexRay Bus model to optimize the FlexRay bus static and dynamic slot assignments in terms of number of slots and the size of slots that may be available from different vendors.   The FlexRay Bus Model is based on the FlexRay – Protocol – Specification – V2.1, 12May2005 release date.

 

The following analysis can be constructed using these models:

  1. Task Latency requiring data from multiple sensors and processed at an ECU
  2. Performance impact for different network topology or parameter configurations
  3. Determine the number of devices connected
  4. Performance vs. reliability trade-off by using the Dual Channel for additional bandwidth or redundant links.
  5. Create multi-system configuration to evaluate real-time performance impact of shared network topology and traffic loading.
  6. Size the over 35 variables of the FlexRay protocol such as number of slots slot size and node buffer
  7. Design protocol conversion algorithms and bus gateway between different speed FlexRay, CAN and in-system networks.
  8. Evaluate proprietary or alternate extension to the FlexRay standard such as BMW Byteflight™
  9. Evaluate system reliability by injecting an error into the channel or create downtime at a port

 

A Two Port FlexRay Bus Model in VisualSim is shown in Figure 1 and is described in detail in this document. Bus Statistics are generated for each bus channel, and each assigned bus slot in the protocol.  The collected statistics include:

 

  • Channel 1 and Channel 2 frame utilization percent
  • Channel 1 and Channel 2 aggregate throughput (Kbps), and latency (sec)
  • Per Channel, per Slot aggregate throughput (Kbps), and latency (sec)

 

Figure 1‑0 Two Port FlexRay Bus Model

3         FlexRay Bus Model Capabilities

 

The FlexRay Bus Model is based on the FlexRay – Protocol – Specification – V2.1, 12May2005 release date.  The FlexRay frame format is shown below.

Figure 2‑0 FlexRay Frame Format

 

VisualSim provides the FlexRay_Frame_DS to set the above data as integers, if desired.

 

Figure 3‑0 VisualSim FlexRay_Frame_DS

 

 

 

A user can set the FlexRay_Frame_DS fields for Frame_ID, Payload_Length, Payload_Bytes, Header_CRC, Cycle_Count, Payload_Bytes, or Trailer_CRC external to the bus model.  The most important user field is the Payload_Bytes field, this tells the FlexRay Bus port how many

format frames may be needed.  The FlexRay Bus model operation is based on the detailed frame encoding for the static and dynamic segments.  Figure 4, below shows the frame encoding for the static segment of the FlexRay bus.

Figure 4‑0 FlexRay Static Segment Frame Encoding

 

The FlexRay Bus Model provides similar encoding with the parameters shown below.

 

Figure 5‑0 VisualSim Static/Dynamic Segment Frame Parameters

 

The VisualSim modeling parameters provide for the Number_of_Static_Slots parameter, 1023 maximum, according to the FlexRay specification.  The model can actually run with more slots than 1023, however, practical considerations of Kbps per slot may preclude more and more slots as a viable design solution.  More slots executing in a system may increase power consumption and add some bookkeeping complexity.  Each Dual Port in the Model can be configured according to an OEM bus slot configuration, and integrated into the model to see the overall effect on performance.  The next parameter Bytes_per_Static_Slot parameter, 254 maximum, can be set according to the optimum performance of time critical operations and the number of these ports in the system.  A bit time is calculated according to the following expression:

 

 

            Equation 1.    Bit Time (sec) = 1.0 / (FlexRay_Speed_Mbps * 1.0E06)

 

 

In addition, the following parameters can be set according to the FlexRay specification in terms of bit times:

 

  • Transmission_Start_Sequence
  • Frame_Start_Sequence
  • Byte_Start_Sequence
  • Frame_Ending_Sequence
  • Dynamic_Trailing_Sequence

 

Next the encoding for the dynamic segment is shown.

 

Figure 6‑0 FlexRay Dynamic Segment Frame Encoding

 

 

The VisualSim FlexRay Bus Model provides the Dynamic_Trailing_Sequence (bit times) parameter to the existing encoding.  Another parameter, Idle_Time (bit times) between Dynamic and Static segments, can be used to control the overall bus timing.

 

Slot assignments are made at the Dual Port level, much like a vendor may have specific slots for their specific sensor, or controller.  Figure 7-0 illustrates a typical FlexRay topology using Active Star linking nodes.  The VisualSim Star_Node is similar to the topology in Figure 7-0, containing four bi-directional ports that can be linked together individually, or in combination.  The FlexRay specification allows two Star_Nodes per FlexRay bus channel, while in the VisualSim model, more Star_Nodes could added to the model.  The one restriction being they link the same channel, according to the FlexRay specification.

 

 

 

 

 

  

Figure 7‑0 Typical FlexRay Bus Topology with Active Star Nodes

 

The VisualSim FlexRay bus model is identical to the above topology, with dual port bus nodes, and four port active star nodes.  In addition, the VisualSim FlexRay bus model has a common Communication Controller to define the Static and Dynamic slot configuration for all Dual Ports in the model, monitors channel/slot activity, and provides bus statistics of same.  Think of the VisualSim Communication Controller as containing the Communication Cycle setup in one convenient place in the model, as to reduce the information entered into each Dual Port bus module.  The Communication Controller parameters are shown in Figure 8-0.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 8‑0 FlexRay Communication Controller Parameters

 

One can turn off the plot for Channel 1 or 2 with the Channel_1_ON and Channel_2_ON parameters.  In addition, one can set the offset line they plot using the Channel_1_Offset or Channel_2_Offset parameters.  The Static and Dynamic slot offsets are calculated according to the following equations:

 

            Equations 2, 3.   

 

Static Offset      = (Transmission_Start_Sequence + Frame_Start_Sequence +

                              Byte_Start_Sequence) * Bit Time

 

Dynamic Offset = (Frame_End_Sequence + Transmission_Start_Sequence +

                              Frame_Start_Sequence +  Byte_Start_Sequence) * Bit Time

 

 

 

Figure 9‑0 FlexRay Dual Port Parameters

 

One will note that the user can set the color of the ports in Channel_1_Color and Channel_2_Color parameters.  These colors will be applied to the slots identified for the parameters Channel_1_Write and Channel_2_Write of this Dual Port block.  It should be noted that the colors selected plot best if Channel 1 and Channel 2 colors do not overlap.  Other Dual Port blocks can use the same colors for the same channel, data will be aggregated on the plot.  The  format for describing the slot assignments parameters Channel_1_Write, Channel_2_Write,

Channel_1_Read, Channel_2_Read is an array of slot number consistent with the Communication Controller.  If there are 7 static slots and 2 dynamic slots, the 1 -7 identify static assignments, then 8, 9 assign dynamic assignments.  If a Dual Port does not wish to write to a particular channel, then a single element array {0} with zero will prevent transmission in either Channel_1_Write or Channel_2_Write parameter entries.  This is part of the MAC layer functionality.

 

Figure 10‑0 FlexRay Star Node Parameters

 

 

Figure 11‑0 FlexRay Channel, Slot Setup Summary

 

  

Figure 12‑0 FlexRay Bus Modules

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 12‑0 FlexRay Modeling Modules

 

MAC Layer Processing

 

The MAC layer processing is predicated on assigning the Static and Dynamic slots, based on the user parameter settings.  If a Dual Port assigns the same slot ID as another Dual Port block, then the model will throw an exception identifying the two ports needing correction. 

 

The model Dual Port and Star Nodes operate identically to chip level components in terms of throughput/latency performance and port buffering (Max_Port_Queue_Length parameter), based on the static and dynamic slot assignments.  FlexRay slot assignments include the size (Bytes_per_Static_Slot, Bytes_per_Dynamic_Slot parameters) and the number of slots in the Communication Cycle(Number_Static_Slots, Number_Dynamic_Slots parameters).  Currently, the VisualSim FlexRay model expects the Bytes_per_Dynamic_Slot to simply be a multiple, or ratio, of the Bytes_per_Static_Slot.  For example, the Bytes_per_Dynamic_Slot could be 1X, 2X, 3X, or 4X of the Bytes_per_Static_Slot  parameter.  This minor constraint was added to improve overall model performance, so accesses to the input buffer can be in equal amounts from the buffer.

 

The MAC layer portion of the Dual Port block also supports fragmenting of the incoming transaction byte size into FlexRay frames that match the Bytes_per_Static_Slot parameter, while adding the header and trailer overhead bytes to each transaction transmitted over the bus.  The added convenience of fragmenting incoming transactions does impose a constraint on the Bytes_per_Static_Slot parameter to be greater than the FlexRay frame format header + trailer bytes (8), as the fragmentation is the incoming transaction byte size – Bytes_per_Static_Slot + 8.  In addition, the MAC layer accepts input transactions to the Dual Port block and calculates the media access time, based on the Communication Cycle information entered by the user, prior to running a simulation.

 

The MAC layer also processes intermediate FlexRay frames once a transmission completes, or if a Dynamic Mini-Slot encountered an ongoing transmission, the MAC layer will reschedule the frame transmission when the current Dynamic Slot completes.

4         FlexRay Bus Model Analysis

 

The FlexRay Bus Model provides the ability to analyze individual channel and slot performance in terms of throughput, latency, more importantly, the ability to see the data being written by Dual Port blocks to each channel and time slot.  Below is a plot of two FlexRay channels writing frames

to Dual Port blocks with the following port configurations:

 

Channel_1 Name Array (Static Slots 1 through 7 [red], Dynamic Slots 8, 9 [blue]):

 

{"Port_1", "Port_1", "Port_2", "Port_2", "null", "null", "null", "Port_1", "Port_2"}

 

Channel_2 Name Array (Static Slots 1 through 7 [red], Dynamic Slots 8, 9 [blue]):

 

{"Port_1", "Port_1", "Port_2", "Port_2", "null", "null", "null", "Port_1", "Port_2"}

 

Dual Port 1, Channel 1 Write Color: “red”

Dual Port 1, Channel 2 Write Color: “blue”

 

Dual Port 2, Channel 1 Write Color: “black”

Dual Port 2, Channel 2 Write Color: “orange”

  

 

 

Figure 13‑0 FlexRay Channel, Slot Activity Plot

This plot shows Dual Port 1 in red and blue in the plot, sending twice as much data as Dual Port 2 in black and orange, which is consistent with the top level system diagram.  The top level system modeling diagram has two traffic sources for Dual Port 1, while Dual Port 2 has only one traffic source.  The grey markers at the bottom of the plot indicate the static and dynamic portion of the Communication Cycle.

 

Two Port FlexRay Bus Model -- Statistics generated by the Model

 

Channel Statistics

 

A_Channel_1_Utilization             = 38.4000000384,

A_Channel_2_Utilization             = 38.4000000384,

 

B_Channel_1_Max_Kbps                        = 879.1208791208792,

B_Channel_1_Mean_Kbps                      = 549.4505494505496,

B_Channel_1_Min_Kbps             = 219.7802197802198,

B_Channel_1_StDev_Kbps                     = 219.7802197802194,

C_Channel_1_Max_Delay                       = 3.84E-4,

C_Channel_1_Mean_Delay                     = 2.368E-4,

C_Channel_1_Min_Delay                        = 1.024E-4,

C_Channel_1_StDev_Delay                     = 9.656721320752E-5,

 

D_Channel_2_Max_Kbps                        = 879.1208791208792,

D_Channel_2_Mean_Kbps                      = 549.4505494505496,

D_Channel_2_Min_Kbps             = 219.7802197802198,

D_Channel_2_StDev_Kbps                     = 219.7802197802194,

E_Channel_2_Max_Delay                       = 3.84E-4,

E_Channel_2_Mean_Delay                     = 2.368E-4,

E_Channel_2_Min_Delay                        = 1.024E-4,

E_Channel_2_StDev_Delay                     = 9.656721320752E-5,

 

 

Channel 1 Slot Statistics

 

F_Channel_1_Slot_0_Max_Delay            = 3.0410000008E-4,

F_Channel_1_Slot_0_Max_Kbps = 109.8901098901099,

F_Channel_1_Slot_0_Mean_Delay          = 1.6234000008E-4,

F_Channel_1_Slot_0_Mean_Kbps           = 91.5750915750916,

F_Channel_1_Slot_0_Min_Delay = 1.290000008E-5,

F_Channel_1_Slot_0_Min_Kbps  = 0.0,

F_Channel_1_Slot_0_StDev_Delay          = 1.0306210942922E-4,

F_Channel_1_Slot_0_StDev_Kbps           = 40.9536259615346,

F_Channel_1_Slot_1_Max_Delay            = 3.1710000004E-4,

F_Channel_1_Slot_1_Max_Kbps = 109.8901098901099,

F_Channel_1_Slot_1_Mean_Delay          = 1.7534000004E-4,

F_Channel_1_Slot_1_Mean_Kbps           = 91.5750915750916,

F_Channel_1_Slot_1_Min_Delay = 2.590000004E-5,

F_Channel_1_Slot_1_Min_Kbps  = 0.0,

F_Channel_1_Slot_1_StDev_Delay          = 1.0306210942922E-4,

F_Channel_1_Slot_1_StDev_Kbps           = 40.9536259615346,

F_Channel_1_Slot_2_Max_Delay            = 1.8450000004E-4,

F_Channel_1_Slot_2_Max_Kbps = 109.8901098901099,

F_Channel_1_Slot_2_Mean_Delay          = 1.1490000005333E-4,

F_Channel_1_Slot_2_Mean_Kbps           = 54.945054945055,

F_Channel_1_Slot_2_Min_Delay = 3.890000008E-5,

F_Channel_1_Slot_2_Min_Kbps  = 0.0,

F_Channel_1_Slot_2_StDev_Delay          = 5.9612973962385E-5,

F_Channel_1_Slot_2_StDev_Kbps           = 54.945054945055,

F_Channel_1_Slot_3_Max_Delay            = 1.3430000008E-4,

F_Channel_1_Slot_3_Max_Kbps = 109.8901098901099,

F_Channel_1_Slot_3_Mean_Delay          = 9.310000006E-5,

F_Channel_1_Slot_3_Mean_Kbps           = 36.6300366300366,

F_Channel_1_Slot_3_Min_Delay = 5.190000004E-5,

F_Channel_1_Slot_3_Min_Kbps  = 0.0,

F_Channel_1_Slot_3_StDev_Delay          = 4.120000002E-5,

F_Channel_1_Slot_3_StDev_Kbps           = 51.8026945924211,

F_Channel_1_Slot_7_Max_Delay            = 3.4600000008E-4,

F_Channel_1_Slot_7_Max_Kbps = 219.7802197802198,

F_Channel_1_Slot_7_Mean_Delay          = 1.9656000008E-4,

F_Channel_1_Slot_7_Mean_Kbps           = 183.1501831501832,

F_Channel_1_Slot_7_Min_Delay = 5.480000008E-5,

F_Channel_1_Slot_7_Min_Kbps  = 0.0,

F_Channel_1_Slot_7_StDev_Delay          = 1.0306210942922E-4,

F_Channel_1_Slot_7_StDev_Kbps           = 81.9072519230692,

F_Channel_1_Slot_8_Max_Delay            = 2.1370000008E-4,

F_Channel_1_Slot_8_Max_Kbps = 219.7802197802198,

F_Channel_1_Slot_8_Mean_Delay          = 1.4623333338667E-4,

F_Channel_1_Slot_8_Mean_Kbps           = 91.5750915750916,

F_Channel_1_Slot_8_Min_Delay = 8.090000004E-5,

F_Channel_1_Slot_8_Min_Kbps  = 0.0,

F_Channel_1_Slot_8_StDev_Delay          = 5.4236355187469E-5,

F_Channel_1_Slot_8_StDev_Kbps           = 98.6293920720605,

 

 

Channel 2 Slot Statistics

 

G_Channel_2_Slot_0_Max_Delay            = 3.0410000004E-4,

G_Channel_2_Slot_0_Max_Kbps            = 109.8901098901099,

G_Channel_2_Slot_0_Mean_Delay          = 1.6234000004E-4,

G_Channel_2_Slot_0_Mean_Kbps           = 91.5750915750916,

G_Channel_2_Slot_0_Min_Delay = 1.290000004E-5,

G_Channel_2_Slot_0_Min_Kbps = 0.0,

G_Channel_2_Slot_0_StDev_Delay         = 1.0306210942922E-4,

G_Channel_2_Slot_0_StDev_Kbps          = 40.9536259615346,

G_Channel_2_Slot_1_Max_Delay            = 3.1710000008E-4,

G_Channel_2_Slot_1_Max_Kbps            = 109.8901098901099,

G_Channel_2_Slot_1_Mean_Delay          = 1.7534000008E-4,

G_Channel_2_Slot_1_Mean_Kbps           = 91.5750915750916,

G_Channel_2_Slot_1_Min_Delay = 2.590000008E-5,

G_Channel_2_Slot_1_Min_Kbps = 0.0,

G_Channel_2_Slot_1_StDev_Delay         = 1.0306210942922E-4,

G_Channel_2_Slot_1_StDev_Kbps          = 40.9536259615346,

G_Channel_2_Slot_2_Max_Delay            = 1.8450000008E-4,

G_Channel_2_Slot_2_Max_Kbps            = 109.8901098901099,

G_Channel_2_Slot_2_Mean_Delay          = 1.1490000006667E-4,

G_Channel_2_Slot_2_Mean_Kbps           = 54.945054945055,

G_Channel_2_Slot_2_Min_Delay = 3.890000004E-5,

G_Channel_2_Slot_2_Min_Kbps = 0.0,

G_Channel_2_Slot_2_StDev_Delay         = 5.9612973996382E-5,

G_Channel_2_Slot_2_StDev_Kbps          = 54.945054945055,

G_Channel_2_Slot_3_Max_Delay            = 1.3430000004E-4,

G_Channel_2_Slot_3_Max_Kbps            = 109.8901098901099,

G_Channel_2_Slot_3_Mean_Delay          = 9.310000006E-5,

G_Channel_2_Slot_3_Mean_Kbps           = 36.6300366300366,

G_Channel_2_Slot_3_Min_Delay = 5.190000008E-5,

G_Channel_2_Slot_3_Min_Kbps = 0.0,

G_Channel_2_Slot_3_StDev_Delay         = 4.119999998E-5,

G_Channel_2_Slot_3_StDev_Kbps          = 51.8026945924211,

G_Channel_2_Slot_7_Max_Delay            = 3.4600000004E-4,

G_Channel_2_Slot_7_Max_Kbps            = 219.7802197802198,

G_Channel_2_Slot_7_Mean_Delay          = 1.9656000004E-4,

G_Channel_2_Slot_7_Mean_Kbps           = 183.1501831501832,

G_Channel_2_Slot_7_Min_Delay = 5.480000004E-5,

G_Channel_2_Slot_7_Min_Kbps = 0.0,

G_Channel_2_Slot_7_StDev_Delay         = 1.0306210942922E-4,

G_Channel_2_Slot_7_StDev_Kbps          = 81.9072519230692,

G_Channel_2_Slot_8_Max_Delay            = 2.1370000004E-4,

G_Channel_2_Slot_8_Max_Kbps            = 219.7802197802198,

G_Channel_2_Slot_8_Mean_Delay          = 1.462333334E-4,

G_Channel_2_Slot_8_Mean_Kbps           = 91.5750915750916,

G_Channel_2_Slot_8_Min_Delay = 8.090000008E-5,

G_Channel_2_Slot_8_Min_Kbps = 0.0,

G_Channel_2_Slot_8_StDev_Delay         = 5.4236355154298E-5,

G_Channel_2_Slot_8_StDev_Kbps          = 98.6293920720605,

 

 

The slot statistics are 0-based numbers, meaning 0 corresponds to slot 1, etc.  These statistics provide the user with detailed information about the throughput and latency of each slot on each channel of the FlexRay bus.  In addition, the channel level statistics provides utilization values in percent for each channel, in terms of time transmitting frames.  The maximum utilization of each channel will be less than 100%, based on the settings of the inter-frame spacing.

 

Dynamic Slot Analysis

 

One will notice that the two dynamic slots in this model have similar maximum Kbps values, meaning they were filled without contention.  Again, the two Dynamic Slots are 7, 8 in the statistics listing, above.  The mean Kbps values for the two slots are in proportion to the traffic being double on Dual Port 1, versus Dual Port 2.  Similarly, the maximum delay on Dual Port 1 is greater than Dual Port 2 as the MAC layer buffers transactions for Dual Port 1 from two sources, resulting in the greater slot latency statistics.

 

Presently, the Two Port FlexRay Bus Model allows the user to assign a fixed number of dynamic slots, based on the Number_Dynamic_Slots parameter, and allows the user to define dynamic slot widths in proportion to the number of Bytes_per_Static_Slot.  One variation of this dynamic slot assignment might be to allow for over-subscription of dynamic slots, by limiting the “available” dynamic slots in the Communication Cycle.  This can be accomplished by a modification to the MAC layer processing of each dynamic frame.

 

Number and Size of Static/Dynamic Slots

 

The VisualSim FlexRay bus model can be used to determine the optimum number of slots and their size, by running different slot configurations, realizing that smaller slots reduce overall frame latency for incoming transactions, while larger slots improve bandwidth utilization with fewer overhead bytes as a percentage of each frame.  In addition, smaller static or dynamic slots may increase power consumption, as a result of more headers being transmitted as well.  The model can help establish the best combination of both throughput, and latency for time-critical information, extra-bandwidth critical information, or both.