Title
Ethernet for avionics
Technical field
The present invention relates to a system and a method for communication
networks in avionics.
Background of the Invention
In embedded control systems of today, developments in digital technology
have enabled complex functionality. However as a direct result from the
development, the need of additional system capacity provided by software and
various components such as sensors, processors, display units, data buses and
memory units is increasing.
Apart from implementing more functionality and interconnectivity in control
systems, using less Space Weight and Power, (SWaP) and a reduced number
of cabling are further important drivers. Updates of embedded hardware and
software during a products life span make adaptability and modularity another
interesting design parameter. Other incentives include achieving cost efficient
development, production and maintenance, where one possible route is to
implement Commercial Off-The-Shelf (COTS) technology instead of
expensive specialized technology.
Real-time systems for critical control applications, wherein typically data from
sensor/s are acquired, communicated and processed to provide a control signal
to at least an actuator pose strict demands regarding bandwidth, data delivery
time, redundancy, and integrity. Failure to meet one or several of these
demands can in applications including "brake-by-wire" or "steer-by-wire"
prove potentially dangerous.
One such area wherein reliable high-speed real-time communication is of
outmost importance is within avionics systems. Advances in technology
during late 1960 and early 1970 made it necessary to share information
between different avionics subsystems in order to reduce the number of
functional modules such as Line Replaceable Units (LRU:s). A single sensor
such as a position sensor provided information to weapon systems, cockpit
display system, autopilot and navigation system. The subsystems
communicated with one or several processing units using dedicated
standardized avionics communication protocols. Early communication
protocol includes the military MIL-STD-1 553 and the commercial
Aeronautical Radio Inc. (ARINC) 429 standard. ARINC 429 is a simplex
point-to-point protocol with limitation in maximum data transmission speed of
100 Kbps and maximum number of connections limited to 20 receivers. In a
system with many components requiring access to each other, this easily leads
to excessive wiring and thereby increased weight, cost and complexity. MILSTD-
1 553 is a protocol wherein the communication is half duplex and
asynchronous with a data transmission speed of 1 Mbps. In an avionics system
of today with high bandwidth requirement due to for example video
processing resulting from increased use of image generating sensors,
bandwidth of earlier protocol can prove insufficient. Avionics Standard
Communication Bus (ASCB) by General Aviation Manufacturers Association
(GAMA) provides a synchronized network protocol standard providing each
component attached to the network a guaranteed amount of bandwidth. The
ASCB protocol typically provides a data transmission speed of 10 Mbps.
Recently incentive has been raised within the field of avionics to implement
more and more COTS hardware and software to reduce the need of specialized
solutions with resulting higher cost.
COTS communication protocol standards that offer high bandwidth include
Institute of Electrical and Electronics Engineers (IEEE) 802.3 Ethernet,
Universal Serial Bus (USB), IEEE 1394 Firewire, Fibre Channel (FC)
developed by the InTernational Committee for Information Technology
Standards (INCITS) and Fibre Distributed Data Interface (FDDI). Ethernet
have been around for more than 30 years and employed for data transmission
applications such as internet access, multi-media, local office networks and
other general purpose network solutions. However standard Ethernet was not
originally developed with safety critical applications in mind. Transmitted data
have no guarantee to arrive at an intended receiver and may thus have to be
retransmitted.
Ethernet have recently started to be employed for communication purposes
within avionics systems. Standards including ARINC 664 and the closely
linked Avionics Full Duplex switched Ethernet (AFDX) presents protocols for
communication in avionics using Ethernet. AFDX provides a protocol with a
statistically bounded latency where the bandwidth is divided and each sender
is provided a guaranteed bandwidth. Despite the latency being bounded by the
protocol standard, it is still variable. Dedicated AFDX switches are also
required when implementing an AFDX network.
EP 1 4 138 A2 discloses a method and system for controlling synchronous
and asynchronous Ethernet communication in an avionic system. The
synchronous messages are allocated to corresponding predetermined timeslots
and the asynchronous messages are allocated to the one or more
unoccupied portions of the predetermined time-slots.
However, moving from dedicated communication protocols in attempts to
reduce costs and increase bandwidth tends to decrease determinism.
Accordingly, there is a need of an improved communication system and
method in order to be able to reliably transfer data requiring high bandwidth
between various avionics components in real-time.
Objective of the Invention
It is therefore an objective of the present invention to provide a system and a
method, which offer high bandwidth and increased determinism in regard of
the data traffic within an avionics communication network.
Summary of the Invention
This object is achieved according to the present invention by an avionics
switched Ethernet network, for communication of data, the network
comprising: a plurality of synchronized interconnected system nodes each
comprising: an avionics module arranged to periodically provide a set of data
messages, each comprising a set of data variables related to avionics functions,
a network module coupling the avionics module to the network said network
module comprising a network interface unit arranged to provide means for
transmissions of data packets comprising a number of said set of data
messages from said avionics module. The network module of each system
node comprise a transmission control unit arranged to control the network
interface unit to perform scheduled transmissions of said data packets, wherein
said transmission control unit is provided with access to memory means
comprising scheduling information relating to a timing table comprising a
plurality of time-slots of predetermined duration within at least one
periodically repeating communication time frame, wherein each of said
plurality of time-slots is statically allocated a predetermined number of data
packets, each comprising a predetermined subset of data messages from said
set of data messages.
By this is achieved a communication network that is deterministic and is able
to bound latency.
The system is in one option further characterized in that the scheduling
information is based on determined end-to-end communication delay
constraints associated to communication of the data messages between
processes allocated to different system nodes (N1-N3)
The system is in one option further characterized in that the scheduling
information is determined off-line on basis of communication requirements,
determined from execution characteristics of at least one process associated to
each of the system nodes, said at least one process arranged to periodically
provide at least one data message of said set of data messages.
The system is in one option further characterized in that the avionics module
of each system node is arranged to periodically provide data messages of the
set of data messages at a plurality of different period times and wherein the
duration of each of the at least one communication time frames is preselected
based on the shortest period time of the plurality of different period times.
The system is in one option further characterized in that the scheduling
information is configured so as to prevent time-slot over-run wherein at least
one of the data packets is transported on the network for a plurality of
consecutive time-slots.
The system is in one option further characterized in that at least one system
node is arranged to transmit synchronization data packets to the other system
nodes.
The system is in one option further characterized in that the network
comprises a dual redundant switched Ethernet network topology and each of
the system nodes are arranged to transmit data packets along two independent
paths formed in the network based on a plurality of traffic control means.
The system is in one option further characterized in that the transmission
control unit is arranged on top of IEEE 802.3 Ethernet conformant physical
layer.
By this is achieved a communication system that is a cost effective by means
of the ability to operate using standard Ethernet physical layer components
such as for example standard network interfaces, switches and cables.
This objective is also achieved according to the present invention by a method
for controlling transmissions in an avionics Ethernet network the method
comprising the steps of: providing periodically a set of data messages, each
comprising a set of data variables related to avionics functions, transmitting at
least one synchronization message onto the transmission medium, detecting
the beginning of at least one periodically repeating communication time frame
comprising a plurality of time-slots of predetermined duration, based on the at
least one synchronization message, perform scheduled transmissions of said
data packets, based on scheduling information relating to a timing table
statically allocating to each of said plurality of time-slots a predetermined
number of data packets, each comprising a predetermined subset of data
messages from said set of data messages.
The dependent claims define optional characterizing features corresponding to
those described in relation to the system.
This objective is also achieved by a computer programme comprising a
programme code for performing the above described method steps, when said
computer programme is run on a computer.
This objective is also achieved by a computer programme product comprising
a program code stored on a computer readable media for performing the above
described method steps, when said computer programme is run on the
computer.
This objective is also achieved by a computer programme product directly
storable in an internal memory of a computer, comprising a computer
programme for performing the above described method steps, when said
computer programme is run on the computer.
Brief Description of the Drawings
Fig. 1. shows schematically a block diagram of the communication system for
avionics.
Fig.2. shows schematically a time-slot scheme according to an example of the
present invention.
Fig.3. illustrates schematically a time and traffic schedule of the
communication system for avionics according to an example of the present
invention.
Fig.4. shows schematically a block diagram of the communication system for
avionics according to an example of the present invention.
Fig. . shows schematically a method of communicating according to an
example of the present invention.
Detailed Description
The following examples relates to the case where a communication system is
described with reference to aerial vehicles. However, various different
applications are possible, e.g. for use in land, sea or space vehicles.
Referring to the drawings, and initially to Fig. 1, there is illustrated a block
diagram of a communication network 1 in accordance with an example of the
present invention. The communication network 1 also referred to in the
description as the communication system comprises bidirectional transmission
links 5A-5C, communicatively connecting a plurality of system nodes N1-N3,
via a switch 6. The communication network 1 is configured to provide means
for communication of data packets comprising units of data arranged in data
messages between the system nodes N1-N3. The communication network 1 is
based on standard Ethernet physical layer conformant with IEEE 802.3, such
as for example 100 BASE-TX and/or 1000 BASE-T or the like. Each system
node N1-N3 comprises an avionics processing component A1-A3 coupled to a
network component E1-E3. Each of the avionics processing components Al-
A3 may be a line replaceable unit (LRU). A LRU describes units such as for
example a flight control computer (FCC), an integrated modular avionics
(IMA) unit, a display computer, an autopilot or the like. Each of the network
components E1-E3 is arranged to perform one or more operations related to
timing, data conversion, message encapsulation, message de-capsulation, data
transmission and data reception. The network components E1-E3 of each
system node N1-N3 may for example be coupled to via an internal bus
structure associated to the avionics processing components A1-A3 such as for
example a PCI, PCIe, IDE or the like in order to perform read and or write
operations regarding units of data stored in one or more memory components
associated to the avionics processing component A1-A3. The data messages
may each comprise one or more units of data also referred to as variables,
related to for example, a sensor reading, a periodically calculated output value
of a control loop or the like. A data packet may be forwarded in the network
from one or a plurality of sending nodes to one or a plurality of designated
receiving nodes. Each of the nodes N1-N3 may interchangeably send and
receive data packets. Access to the network 1 is restricted for each node Nl-
3 on basis of a time triggered media access protocol. This means that each
node N1-N3 is permitted to perform transmissions of one or more data packets
during a particular allocated period of time i.e. time-slot.
Each of the data message intended to be transmitted from each of the system
nodes N1-N3 is encapsulated before transmission into one or more Ethernet
data packet. The data packet may comprise a header field, an application data
field and a fault detection field. The header field may be provided with
information related to identification of sending node and receiving node. The
application data field may be provided with application data from the sending
node such as for example message data comprising the result of a calculated
control loop. The fault detection field may be provided with error correcting
code (ECC) and/or checksum.
In one example to indicate what particular data message is being transmitted
and received, User Datagram Protocol (UDP) port numbers may be used to
identify each data message and where to place the units of data provided in
each of said data message. Hence, the network component E1-E3 of each
system node N1-N3 may use the UDP port numbers to identify the correct
place i.e. memory address to perform read and/or write operations regarding
data messages and associated one or more units of data stored in one or more
memory components associated to the avionics processing component A1-A3
n one example the data packets may be transmitted from each of the system
node N1-N3 using point-to-multipoint communication i.e. multicast, referring
to sending data packets to a group of system nodes. As an example system
node l may be arranged to multicast data packets to a system group
comprising the system nodes N2 and N3.
The switch 6 is in one example a standard Ethernet switch comprising a
plurality of data ports. The switch 6 may comprise a processor, a plurality of
input and output ports each associated with one of the system nodes N1-N3.
Upon reception of the data messages the switch 6 may be arranged to store the
received data messages in one or more input buffer memory devices. The
processor of the switch 6 may be arranged to route the received data messages
from the input buffer memory devices to output buffer memory device
associated with one or more designated receiving system node N1-N3. The
designated one or more receiving system nodes may be read by the switch by
means of destination addresses encapsulated by the sending node in each of
the data packets. The addresses may be Media Access Control (MAC)
addresses, defining one address for each transmission link 5A-5C coupling the
system nodes N1-N3 to the switch 6. The switch 6 may then be able to
correctly identify the designated one or more receivers of each data packet by
means of indentifying the address in a static lookup table provided in an
internal memory of the switch 6. The lookup table may then provide the
correct one or more output ports to route the received data to. Configuration of
the switch 6 may be performed by uploading a new version of the lookup table
to the internal memory of the switch 6.
In the illustrated example with reference to fig. 1 the communication system
comprises three system nodes N1-N3 and one switch 6, however other
examples may comprise a number of additional system nodes and/or switches
connected with standard Ethernet links.
in one example the time triggered media access protocol may be implemented
in the network component E1-E3 of each system node N1-N3. Each system
node N1-N3 may transmit data packets simultaneously oiito the network in
accordance with the time triggered media access protocol. Each member of the
communication network i.e. system nodes N1-N3 may be pre-assigned to
predetermined points in time i.e. when it should transmit data packets and
further points in time when it can expect to receive data packets. The
predetermined points in time may be detected and maintained by software
arranged in each of the system nodes 1-N3 such as for example by a driver
associated to a programmable interrupt timer. To be able to execute the time
triggered schedule each of the system nodes may be arranged to maintain time
by means of local hardware and/or software time keeping devices or a
combination thereof.
With reference to fig. 2 the time triggered media access protocol may be
defined by a cyclic i.e. repeating schedule comprising a major communication
cycle 145, comprising a set of minor communication cycles 125, 126 each
comprising a number of time-slots S1-S5, S10-S14 of predetermined duration.
At detection of the start of a time-slot, each system node may be arranged to
transmit its allocated one or more data packets one after another.
As an example each time period such as for example each 1 second time
period may be configured to be divided into 20 equally sized 50 s time-slots.
The major cycle may comprise a number of minor cycles i.e. sets of said
configured 50 ms time-slots. In case the major cycle comprise three minor
cycles then corresponding major cycle is 3 seconds long. After the major cycle
is completed it may be repeated indefinitely or until shutdown of system.
In one example each system node N1-N3 may be provided with a fixed traffic
schedule. The fixed traffic schedule may comprise one or more tables of
entries associating each of the time-slots with one or more corresponding data
messages to transmit in at least one data packet.
The fixed i.e. preconfigured traffic schedule may in one example be stored on
a memory portion of a memory associated to the network module of each of
the respective system nodes. Each of the system nodes N1-N3 connected to
the network may by means of the fixed traffic schedule statically be allocated
zero or more packets to transmit in each time-slot of a series of subsequent
time-slots in accordance with the time triggered media access protocol. Hence,
at each instant of time the traffic of data packets is known to a l system nodes.
In one example the fixed traffic schedule may be determined on basis of the
communication requirement of each system node N1-N3. Each system node
may be arranged to periodically perform one or more functions associated
with various avionics systems such as for example calculating one or more
control loops. Each function of each of the system nodes N1-N3 may be
divided into a set of periodic tasks also referred to as processes. Each task may
be defined by a set of characteristics with respect to timing. Each task may
require a specific number of processor cycles constituting an execution time
(ET). Each task may further be defined by a worst case execution time
(WCET) wherein said WCET is generally longer than the ET based on the
actual physical processing including operations such as for example task
release, start, preemption, resume, completion and termination. One or more
of the periodic task may further be dependent on one or more units of data
provided by one or more other tasks which is arranged to execute on different
avionics processing components A1-A3. These one or more periodic tasks
need to be scheduled regarding communication of data on the communication
network 1.
In one example one or more associated data messages provided from each
periodic task may be determined to be scheduled for communication in one or
more data packets to be transmitted on the network 1 in a specific time-slot
based on the associated execution period of each of the tasks. As an example
each task may arranged to communicate at a periodic basis with respect to the
period associated to each task.
In one example each of the periodic tasks one or more associated data
messages provided from each periodic task may be determined to be
scheduled for communication in one or more data packets to be transmitted on
the network 1 in a specific time-slot based on the associated execution period
of each of the tasks and communication delay constraints associated to each of
the tasks. The communication delay constraints may be imposed on each task
by means of one or more tasks being dependent on units of data provided as
communication between several tasks. As an example a first task executing on
a first system node N may be arranged to transmit one or more associated
data messages comprising one or more units of data in one or more time-slots,
such that the one or more units of data provided by the first task is available
for processing by a second task executing periodically on a second system
node N2 before a scheduled release time of said second task.
In one example each task arranged to provide units of data associated to data
messages may be pre-assigned a task execution period time selected from a set
of system global task periods. Hence, communication by means of the network
1 between a plurality of system nodes and tasks executing on said system
nodes may be co-scheduled based on period time, CET and delay
constraints associated to each of the tasks. In order to provide means for
periodic communication of data messages provided from each task the minor
communication time frame may be adapted to correspond to the shortest task
period time of the set of system global task periods.
In one example the size of each data message is considered with respect to the
available bandwidth of the network 1 when determining the allocation of data
messages to be transmitted in time-slots of the fixed traffic schedule.
Worst case transmission time WCT need to be considered when determining
the fixed traffic schedule. The WCT may be referred to as the time from when
a particular unit of data is produced by completion of an instance from a first
task and is ready for transport until the time when the particular unit of data
has been transported and is ready for processing by an instance of a second
task. The actual transmission time may vary from time to time based on
variation in the arrival or departure times referred to as jitter caused by
switches, input/output processing and configuration of physical transmission
medium etc. Parameters that may affect the WCT may for example include
type and length of cabling along the respective intended data packet routing
and the policy and performance of the switch regarding memory access, the
amount of data traffic and processing power. The WCT may be determined
according to the following equation.
WCT = S P + P + S + ll2 + p
Wherein S,, denotes time it takes for the respective system node N1-N3 to
process operations related to transmission of data messages, wherein PR
and denotes propagation delay inflicted by transmitting electrical signals
corresponding to data packets along cables to and from the switch. 5 denotes
delay inflicted by queuing and routing the data packets in the switch . R is
the time it takes for the respective system node N1-N3 to process operations
related to receiving data packets and retrieving data messages from said data
packets.
In one example the fixed traffic schedule may be determined based on
preventing time-slot over-run wherein a data message is transported on the
network 1 for a plurality of consecutive time-slots. By the use of the condition
the design of the communication system is able to bound worst case
transmission delay.
By considering the communication requirements of the each of the respective
system nodes and the physical properties of the communication network 1 the
time triggered traffic schedule may be determined off-line.
The specific time triggered traffic schedule may be determined by hand or by
means of scheduling software executed on a computer. After the time
triggered traffic schedule is determined it may subsequently be transformed
into instructions readable by a computer such as for example compiled into a
binary format.
For purpose of illustration with reference to fig. 3 a first system node Nl may
be arranged to periodically execute instances of a first task 7A-7D, instances
of a second periodic task 8A-8B and instances of a third periodic task 9A
according to a cyclic execution schedule 200 defined by a major execution
cycle 110 comprising a number of minor execution cycles 115-118. A second
node N2 may be arranged to execute a first set of tasks being dependent on
units of data provided by the instances of the first 7A-7D and second task 8A-
8B. The instances of the first task 7A-7D may be arranged to produce an
output i.e. units of data corresponding to "positional data" on a 40 Hz basis
from using information provided by an inertial measurement unit IMU. The
instances of the second periodic task 8A-8B may be arranged to produce an
output corresponding to a "position report message" on a 20 Hz basis. The
instances of the third periodic task 9A may be arranged to produce an output
corresponding to a "distance to closest obstacle" on a 10 Hz.
Each instance of the first, second and third tasks may for example be defined
by the following characteristics regarding worst case execution time WCET,
worst case transmission time WCT and task frequency as illustrated by Table
1.
TABLE 1
Task Frequency Period WCET WCT
First 40 Hz 25 ms 5 ms Nl-N2 6 ms
Second 20 Hz 5 ms 6 ms Nl-N2 6 ms
Third 10 Hz 100 ms 25 ms Nl-N2 6 ms
Tasks associated to each of the respective first and second system nodes N -
N2 in the example may be arranged to synchronously execute in priority order
based on task frequency i.e. task period in accordance with execution
schedules such as for example execution schedule 200 comprising a major
cycle 110 and a number of minor cycles 115-118. The instance of the first task
7A-7D may be arranged to execute first in order each 25 ms time period. The
instances of the second task 8A-8B may be arranged to execute second in
order in each 50 ms period and the instances of the third task 9A may be
arrange to execute third in order in each 100 ms period. The instances of the
third task 9A may be preempted during a second minor cycle 116 and resumed
in a fourth minor cycle 118 due to its associated WCET risk exceeding its
allocated execution time as defined by schedule 200. To construct a
corresponding preconfigured traffic schedule 300 each of the minor execution
cycles 125-128 of the execution schedule 200 may be divided into a number of
time-slots S1-S8 each corresponding to a predetermined time period. In the
shown example the time period each of the minor execution cycles are divided
into eight time-slots S1-S8 each corresponding to a time period of 3. 5 ms.
Each of the units of data i.e. output results from processing instances of the
first, second and third task may be scheduled for transmission TR1-TR7 in
one or more data packets D1-D7 in one or more corresponding time-slot S1-S8
associated to one or more of the minor communication cycles 125-128. The
transmissions TR1-TR7 may be scheduled based on time of completion of the
task performed by a system node N1-N2 and based on points in time relating
to when a particular data message comprising one or more units of data i.e.
particular output results of said task is needed by one or several other tasks
and/or other systems coupled to the communication network 1. Additional
factors that may be considered is the amount of time S needed for process
operations related to transmission of data messages in a transmitting node.
The WCT from the first node Nl to the second node N2 may be determined to
be 6 s. Delay constraints for transmission of units of data from instances of
the first and second task may be determined to be that the units of data from
all transmitted data messages need to be ready for processing in the one or
more receiving system nodes no later than at the end of the period wherein
which the unit of data is provided. A first transmission TR1 may represent a
first transmission of output data from a first instance of the first task 7A via
the network 1 to one or more of the second. The first transmission TR1 may
be scheduled in one or more of the time-slots starting from the first upcoming
time-slot after its time of completion. However to fulfill the condition of tasks
associated to the second node being dependant on one or more output values
from a previous execution period associated to the task providing the units of
data the worst case transmission time to the second node needs to be
considered when scheduling the first transmission TR1. Transmissions from
instances of the first task 7A-7D may then accordingly be scheduled into any
time-slot ranging from a third time-slot S3 to a sixth time slot S6 of each
minor communication cycle 125-128, such as for example in the third timeslot
S3 of each the minor communication cycles 125-128, corresponding to
four periodic transmissions illustrated in fig. 3 as first, third, fourth and sixth
transmission TR1, TR3, TR4 and TR6. A second transmission TR2 may
represent a transmission of output data from instances of the second task 8A-
8B to the second node. The constraint for the second transmission TR2 may
be determined from the WCET, period time associated to instances of said
second task and the WCT to the second node. Accordingly, the second
transmission TR2 may be scheduled into any time-slot ranging from a seventh
time-slot S7 of the first minor cycle 125 until a sixth time-slot S6 of a second
minor cycle 126, such as for example in the seventh time-slot of the first
minor communication cycle 125. Subsequently a fifth transmission TR5 may
be scheduled in the seventh time-slot S7 in the third minor cycle 127. Note
that there is no constraint presented for the illustrated seventh transmission
TR7 representative of a transmission of a data message comprising an output
value form an instance of the third task 9A.
The corresponding preconfigured traffic i.e. communication schedule 300 may
be repeated indefinitely in synchronicity with the task execution schedule 200.
In one example the first time-slot SI of each minor cycle 115-118 in the
communication schedule 300 may be assigned to a synchronization data
packet. The last time-slot S8 in each of the minor cycles 15-118 may be
configured as a "dead band" also referred to as a refusal window in order to
provide separation between subsequent minor cycles 15-118.
It should be understood that the communication system with reference to the
above example including three tasks, each described with a specific timing
requirements may be configured differently. Differences may include that the
number of tasks differ, each of the tasks may transmit more than one data
message and the tasks may be dependent on more than one different task or
other system connected to the communication network 1.
In one example a common time reference also referred to as local avionics
system time (AST) is provided. To be able to effectively implement the time
triggered media access protocol, restricting the access to the transmission
medium it is important that all nodes N1-N3 in the communication system 1
have one and the same view of time. To maintain substantially the same view
of system time among the system nodes, a synchronization protocol may be
implemented in the system.
In one example synchronization data packets indicating the start of the minor
cycle are sent on a cyclic basis from a master node, such as from a flight
management computer (FCC) configured for flight safety critical operations.
The synchronization data packets are received by the system nodes whereby
each receiving node can use the synchronization packet to detect if is out of
sync and respond by re-synchronizing its respective local clock.
The master node may in one example be provided with a high quality time
keeping device such as a high quality crystal oscillator, or be synchronized to
external high quality time keeping devices such as by means of receiving
Pulse per Second (PPS) signals from atomic clocks featured in Global
Positioning Systems (GPS). The latter may be beneficial in case the
communication system involves system nodes that perform calculations
involving positioning or other calculations that are performed on basis of an
external global time value, such as Greenwich Mean Time (GMT).
In one example the system nodes may be synchronized in time using the IEEE
1588, standard for a precision clock synchronization protocol for networked
measurement and control systems.
In one example the system nodes may be arranged to operate on basis of a fail
silent protocol. The fail silent protocol is based on that only system nodes that
are substantially synchronized in time i.e. within a tolerance level in respect of
the AST may be arranged to transmit data packets. As soon as each of the
system nodes is able to resynchronize, the respective system nodes may be
allowed to transmit its data packets according to the schedule. This means that
effects of local clock jitter, drift or effects resulting from a system startup
procedure of each system node may not affect transmitting packets being out
of sync with respect to the predefined schedule.
In one example the handling of input/output (I/O) processes in correspondence
to the preconfigured traffic schedule may within each system node N1-N3 be
performed by partitioned software executed on an IMA computer.
In one example the communication system with reference to Fig. 4 comprise
three system nodes N1-N3 arranged in communicative operation by means of
a plurality of Ethernet links L1-L6 in a dual redundant configuration
comprising two switches 25B and 25C. Each of the two switches 25B and 25C
comprises four ports 1-4. The avionics component A1-A3 of each of the
system nodes comprises a processor lOA-lOC arranged to process one or more
associated tasks in order to produce one or more data messages. Each of the
processors of the avionics processing components is coupled to a program
memory 14A-14C comprising processing instructions for executing said
associated tasks. The processor of the second N2 and third N3 avionics
component A1-A3 is further coupled to an input memory 11B-11C and an
output memory 12B-12C to provide means for communication of said data
messages. The system nodes N1-N3 comprise a network interface 15A-15C
comprising an Ethernet socket and an Ethernet controller arranged to provide
means for transmissions of data messages in one or more Ethernet data
packets. The network interface 15A-15C of each system node may further
comprise additional sockets and controllers to allow to be coupled to at least a
secondary bus Bl by means of bus links BL1-BL3. The secondary bus Bl
may be a MIL-STD-1553 bus providing connections to other parts of the
network such as additional system nodes, sensors and/or actuators. The second
and third of the system nodes N2, N3 are provided with an I/O data processor
(IOP) 20B, 20C. The IOP 20B, 20C of the at least one system node may be
arranged to be in operative connection with a portion of a memory comprising
information relating to the preconfigured traffic data schedule. The IOP 20B,
20C of the at least one system node is arranged to be in operative connection
with one or more input 11B-11C and/or output 12B-12C memory associated
to data messages provided from tasks. The IOP 20B, 20C may be arranged to
control all incoming and outgoing data transmissions from a system node Nl-
N3 based on the preconfigured traffic schedule. The IOP 20B, 20C may for
example be arranged to implement timer services, such as for example
periodic interrupts to trigger timed operations of the network interface 15A-
15C in accordance with the preconfigured traffic schedule in providing units
of data associated to one or more data messages from one or more portions of
the associated output memory 12B-12C to the network. The IOP 20B, 20C of
each system node N1-N3 may further for example be arranged to implement
timer services to trigger timed operations in accordance with the preconfigured
traffic schedule in providing data received from the network to one or more
portions of the associated input memory 12B-12C. The timer services
associated to several system nodes may be synchronized based on the
synchronization data packets. The first system node Nl is arranged to perform
control of its associated network interface ISA such as for example by means
of an input/output software module stored in the program memory 14A and
scheduled to execute periodically on its associated avionics processing
component Al. The input/output software module is further arranged to
operate in accordance with the preconfigured traffic schedule.
It should be understood that the example with reference to fig. 4 may be
configured differently regarding the associated switches, memory
configuration and processors. For example each avionics processing
component A1-A3 may comprise an associated IOP or alternatively several
avionics processing components A1-A3 may share access to a single IOP.
In one example one or more of the system nodes N1-N3 may be configured to
execute a partitioning operative system (OS), such as for example an OS
compliant with the commercial Aeronautical Radio Inc. (ARINC) 653
specifications. This enables different applications to execute on separate
partitions of a single processor, where the execution of each application is
partitioned regarding execution time and memory space. Each process may be
assigned a process execution frequency, whereupon the process is periodically
executed. Each task or set of task may be associated to communication ports
referred to as sampling port or queuing ports for communication of data
messages comprising units of data.
n one example with further reference to f g . 3 the communication system is
based on two parallel identical networks including the two switches 25B, 25C
and the Ethernet links, providing two independent communication channels
between each of the system nodes. Each data packet transmitted from a
sending node may be transmitted substantially in parallel on both channels. As
an example the first system node Nl may be arranged to transmit its data
packets to both channels. The network interface 15C of the third node then
receives the data packets both via a first communication channel 15A-L1-25BL4-
25C and via a second communication channel 15A-L2-25C-L6. This
means that each receiving node receives two copies of each transmitted data
packet. The receiving node may be provided with a filtering device arranged
to filter out copies of the received data packets.
In one example the sending node may be arranged to provide a sequence
marking on the data packet before transmitting the data packet on the network.
By providing each subsequent transmitted data packet with an incrementing
sequence number the filtering device may be arranged to filter out copies by
detecting non-incrementing sequence number of the received data packets. To
reduce the number of bits in a packet necessary for the sequence number the
incrementing sequence number can be subjected to a roll over at a
predetermined sequence number.
In one example the filtering device may be arranged to filter out copies of the
data packets on basis of detecting time of arrival. The filter may then be
arranged to filter out the copy by discarding the data packet with the longer
arrival time.
All data packets may be provided with a checksum at the sending node such as
cyclic redundancy check (CRC) and/or other error correcting code ECC. The
checksum can be verified at a receiving node to control integrity of the
received data.
In one example the system nodes N1-N3 may each comprise a non-volatile
memory, a data processing device such as a microprocessor and a read/write
memory. The non-volatile memory has a first memory portion wherein a
computer program, such as an operating system, is stored for controlling the
function of the communication network 1. Further, the system nodes N1-N3
comprises a bus controller, a serial communication port, I/O-means, an A/Dconverter,
a time date entry and transmission unit, an event counter and an
interrupt controller. The non-volatile memory also has a second memory
portion.
A computer program comprising routines controlling transmissions in an
avionics Ethernet communication network 1 is provided. The program may be
stored in an executable manner or in a compressed state in a separate memory
and/or in the read/write memory.
When it is stated that the data processing device performs a certain function it
should be understood that the data processing device performs a certain part of
the program which is stored in separate memory, or a certain part of the
program which is stored in read/write memory.
The data processing device may communicate with a data port by means of a
first data bus. The non-volatile memory is adapted for communication with the
data processing device via a second data bus. The separate memory is adapted
to communicate with data processing device via a third data bus. The
read/write memory is adapted to communicate with the data processing device
via a fourth data bus.
When data is received on the data port it is temporarily stored in the second
memory portion. When the received input data has been temporarily stored,
the data processing device is set up to perform execution of code in a manner
described above. According to one example, data received on the data port
comprises information regarding the detected time-slot and corresponding one
or more specific data packets to transmit as determined from the preconfigured
time triggered traffic schedule. This information can be used by the system
nodes so as to control transmissions in an avionics Ethernet communication
network, as described above.
One example of the invention relates to a computer programme comprising a
programme code for performing the method steps depicted with reference to
fig. 5 when the computer programme is run on a computer.
One example of the invention relates to a computer programme product
comprising a program code stored on computer-readable media for performing
the method steps depicted with reference to fig. 5, when the computer
programme is run on the computer.
One example of the invention relates to a computer programme product
directly storable in an internal memory of a computer, comprising a computer
programme for performing the method steps depicted with reference to fig. 5,
when the computer programme is run on the computer.
Fig. 5 schematically illustrates a flow diagram of a method according to an
example of the present invention. This example relates to controlling
transmissions of data messages from system nodes in the avionics Ethernet
network 1.
In a first method step S90 associated to system initialization writing data
tables is performed i.e. writing the preconfigured traffic schedule to the
memory associated to the processor arranged to handle the input and output
processing in each of the system nodes N1-N3. The first method step is
generally only performed once in response to system modifications. After the
method step S9 a subsequent method step S100 is performed.
n the method step S100 a set of data messages are provided by each of the
system nodes. This means that each system nodes processes one or more
periodic tasks resulting in that one or more data messages are provided
periodically, each comprising one or more units of data. After the method step
S 00 a subsequent method step SI10 is performed
In the method step SI 10 synchronization of the system nodes is performed.
This means that a synchronization message is transmitted onto the network
arranged to provide synchronization information to the system nodes. After
the method step SI 10 a subsequent method step S120 is performed.
In the method step S120 a plurality of time-slots within at least one
communication time frame are detected, i.e. each system node detects the
beginning of an upcoming time-slot by determining if the elapsed time period
since the prior time-slot was detected, corresponds to a predetermined value in
the preconfigured traffic schedule or by determining if the elapsed time since a
synchronization data message was received at the initialisation process
corresponds to a predetermined value in the preconfigured traffic schedule.
After the method step S120 a subsequent method step S130 is performed.
In the method step S130 a subset of data messages of the set of data messages
provided by each system node to transmit in the detected time-slot is
determined. Each detected time-slot corresponds to a point in time indicating a
permission to transmit a predetermined subset of data messages as indicated
by one or more entries in the data tables i.e. preconfigured traffic schedule.
In the method step S 40 the subset of data messages determined to be
transmitted in the time-slot is encapsulated into one or more Ethernet data
packets. After the method step S140 a subsequent method step S150 is
performed in the illustrated example.
In the method step S150 each of the system nodes N1-N3 transmit its one or
more allocated Ethernet data packets. After the method step S150 a subsequent
method step SI 60 is performed in the illustrated example.
In the method step sl60 data from received Ethernet data packets is retrieved.
Each system node may determine which data messages of which received
Ethernet data packets to forward to the a particular memory address of the
system node on basis of one or more entries in the preconfigured traffic
schedule and data message identification information embedded in the
Ethernet data packet. After the method step S 60 the method ends and may be
repeated starting with the method step SlOO to perform a new controlled data
transmission.
Many modifications and variations will be apparent to practitioners skilled in
the art without departing from the scope of the invention as defined in the
appended claims. The examples were chosen and described in order to best
explain the principles of the invention and its practical applications, thereby
enabling others skilled in the art to understand the invention for various
examples and with various modifications as suited to the particular use
contemplated.
Claims
1.An avionics switched Ethernet network (1), for communication of data, the
network comprising:
- a plurality of synchronized interconnected system nodes (N1-N3) each
comprising:
- an avionics module (A1-A3) arranged to periodically provide a
set of data messages, each comprising a set of data variables related to
avionics functions,
- a network module (E1-E3) coupling the avionics module (Al -
A3) to the network (1), said network module (E1-E3) comprising a network
interface unit (15A-15C) arranged to provide means for transmissions of data
packets comprising a number of said set of data messages from said avionics
module (A1-A3),
characterized in that the network module (E1-E3) of each system node (Nl-
N3) comprise a transmission control unit arranged to control the network
interface unit ( 15A- 5C) to perform scheduled transmissions of said data
packets, wherein said transmission control unit is provided with access to
memory means comprising scheduling information relating to a timing table
comprising a plurality of time-slots (S1-S8) of predetermined duration within
at least one periodically repeating communication time frame (125-128),
wherein each of said plurality of time-slots (S1-S8) is statically allocated a
predetermined number of data packets, each comprising a predetermined
subset of data messages from said set of data messages.
2. The network (1) according to claim 1, wherein the scheduling information is
based on determined end-to-end communication delay constraints associated
to communication of the data messages between processes allocated to
different system nodes (N1-N3).
3. The network (1) according to claim 1 or 2, wherein the scheduling
information is determined off-line on basis of communication requirements,
determined from execution characteristics of at least one process (7A-7D, 8A-
8B, 9A) associated to each of the system nodes (N1-N3), said at least one
process (7A-7D, 8A-8B, 9A) arranged to periodically provide at least one data
message of said set of data messages.
4. The network (1) according to claim 3, wherein the avionics module (Al¬
A3) of each system node (N1-N3) is arranged to periodically provide data
messages of the set of data messages at a plurality of different period times
and wherein the duration of each of the at least one communication time
frames is preselected based on the shortest period time of the plurality of
different period times.
5. The network according to claim 1-4, wherein the scheduling information is
configured so as to prevent time-slot over-run wherein at least one of the data
packets is transported on the network for a plurality of consecutive time-slots.
6. The network (1) according to claim any of claims 1-5, wherein at least one
system node (N1-N3) is arranged to transmit synchronization data packets to
the other system nodes (N1-N3).
7. The network (1) according to any of claims 1-6, wherein the network (1)
comprises a dual redundant switched Ethernet network topology and each of
the system nodes (N1-N3) are arranged to transmit data packets along two
independent paths formed in the network (1) based on a plurality of traffic
control means (SW2, SW3).
8. The network (1) according to any of claims 1-7, wherein the transmission
control unit (20B, 20C) is arranged on top of IEEE 802.3 Ethernet conformant
physical layer.
9. A method for controlling transmissions in an avionics Ethernet network (1),
the method comprising the steps of:
- providing periodically a set of data messages, each comprising a set of data
variables related to avionics functions,
- transmitting at least one synchronization message onto the transmission
medium,
- detecting the beginning of at least one periodically repeating communication
time frame comprising a plurality of time-slots of predetermined duration,
based on the at least one synchronization message,
characterized by the further steps of in each system node (N1-N3) to:
- perform scheduled transmissions of said data packets, based on scheduling
information relating to a timing table statically allocating to each of said
plurality of time-slots (S1-S8) a predetermined number of data packets, each
comprising a predetermined subset of data messages from said set of data
messages.
10. The method according to claim 9, characterized by the step of in each
system node (N1-N3) to:
- perform scheduled retrieval of the data packets based on scheduling
information relating to a timing table statically allocating to each of said
plurality of time-slots (S1-S8) a predetermined number of data packets, each
comprising a predetermined subset of data messages from said set of data
messages.
11. The method according to claim 10, characterized by the further step of in
each system node (N1-N3) to:
- transmit the data packets along two independent paths formed in the network
(I), based on a plurality of traffic control means (SW2, SW3).
12. The method according to claim 1, characterized by the step of in each
system node (N1-N3) to:
- filter the data packets received via the two independent paths in order to
retrieve one correct copy of each of the data packets.
13. A computer programme comprising a programme code for performing the
method steps of any of claims 9-12, when said computer programme is run on
a computer.
14. A computer program product stored on a computer readable media for
performing the method steps of any of claims 9-12, when the computer
program is run on the computer.
15 . A computer program product directly storable in an internal memory of a
computer, comprising a computer program for performing the method steps of
any of claims 9-12, when the computer program is run in the computer.