Available technologiesWhen selecting the Ethernet protocols, it isimportant that they have deterministic behaviour, bounded latency and precisesynchronization with Quality of service.
Avionics Full DuplexSwitched Ethernet (AFDX)AFDX is a deterministic networkstandard, developed by Airbus, for safety-critical applications thatutilizes dedicated bandwidth while providing deterministic quality of service(QoS). The AFDX is based on ethernetdata network (UDP, SNMP) and protocol specifications (IEEE 802.3 and ARINC 664,Part 7) for the exchange of data between Avionics Subsystems . The primary featuresof AFDX include full duplex, redundancy, determinism, highspeed performance, switched and profiled network. It supports for network basedon 10, 100 Mbps or 1 Gbps. The predecessor to AFDX, ARINC 429 is a bus systemthat supports 1-1 and 1-n connections.
Compared to the predecessor it hashigher data transfer rate (approximately one thousand times faster) and significantreduce in wiring which reduces the weight .OverviewAFDX uses the concept of Tokenbucket (Asynchronous Transfer Mode – virtual connection / circuit is being established before the actual dataexchange happens between the endpoints). The possibility for collision of datais eliminated using full duplex switched network – one for receiving and theother for transmitting. The network is designed for the critical data beingprioritized using the QoS policies there by achieving latency, jitter anddelivery within the set parameters.
A highly intelligent switch is used whichis capable to buffer packets for both reception and transmission. The messagesare encapsulated with in the UDP/IP and then the Ethernet Headers are placed.The main components in AFDX system are· AFDXEnd system: It is the interface between the sub systems (global positioningsystem) and the network· AFDXInterconnect: A full duplex switched Ethernet interconnect consists of switchesthat forward frames to the appropriate destinations· AFDXVirtual Links: It is a unidirectional virtual connection from 1 to 1 or 1 to nEnd systemsVirtual Links and MessageFlow Inthe traditional Ethernet, the frames are routed based on the Ethernetdestination address.
In AFDX the frames are routed using the 16-bit valuecalled as Virtual Links. The Virtual links shall partition the network into communicationchannels with predefined scheduling time and link bandwidth. These areunidirectional and the switched route the packets based on the virtual link ID.Each virtual link should at least have one or even more predefined receiving endsystems that the packets are transmitted to. When an application sends a messageto a communication port the source and destination end systems and the AFDXnetwork is configured to deliver the message successfully. For example, amessage M is transmitted to port S.
Then the port connected to Endpoint S shallencapsulate the message as per the AFDX protocol format and adds it on top UDPand sends it to the AFDX switched network on VLID 50. The forwarding tables inthe network switch are configured to deliver the frame to the End system D1 andEnd system D2. In the End systems, the message M is extracted from the frameand is transmitted to the port D1 and D2. Isolation and SchedulingThe virtual link is assigned with twoparameters· BandAllocation Gap (BAG) – it’s a value that range in the power of 2 from theinterval 1 to 128 ms. It represents the minimum interval that are transmittedin milliseconds. Depending on the value configured for a virtual link the framecan never be transmitted before that· Lmax– The largest frame in bytes that can be transmitted on the virtual linkFor example, if a VLID 50 has theLmax of 100 bytes and BAG is of 4ms, then the maximum bandwidth for VLID 50 isof (100*8*1000/4) 200 kbps. The choice of the BAG and Lmax depends on therequirement of the application and the bandwidth capability.
AFDX Switch TheAFDX switch forwards packets according to a static MAC table. And Each MACaddress in the table correspond to a virtual link Identifier. The Rx and Txbuffers store packets in a FIFO and the I/O processing unit in the switch willmove the packets according to the virtual link identifier. AFDX switch containfunctions for filtering, policing and monitoring. Filtering is done based onthe frame integrity, frame length and valid destination.
Traffic policing isbased on the token bucket algorithm which keeps token for all the virtuallinks. When a frame is received, it checks the account and if enough creditsare available the packet is sent and credits debited. The tokens are creditedas time progress (depending on the BAG and Lmax). Monitoring is used to log theswitch operation and health of the network. The traffic policing makes surethat no virtual links routed through the switch that exceeds the bandwidth.AFDX Message Structure Ethernet for ControlAutomation Technology (EtherCAT)EtherCAT is a high performanceindustrial network based on Ethernet system invented by Beckhoff Automation.The protocol is suitable for hard and soft real time requirements even for theapplications which require short update rates of less than 100 microsec withprecise synchronization of less than 1 microsec and is standardised in IEC61158.
It is used to achieve faster and more efficient communication network.The main features of EtherCAT include highly flexible, short update rate, lowcommunication jitter and minimal hardware costs. It supports network up to 100Mbps full duplex. The Ethernet frames shall have the capability of passing and receivingthe data at the same time, thereby the utilization of the data rate increases alot.Functionality EtherCATfunctions as a Master-slave network. The master shall control the network andpassage of the frames and slave shall provide I/O.
The master instead oftransmitting data specifically for each node, it shall transmit the framesthrough every node. The EtherCAT nodes shall read the data in a specific framewhich are addressed to them, when a frame passes through them. The frames inEtherCAT contains telegrams. If the slave node needs to transmit a data then itshall place it in the frame as a telegram. The frame is passed through to theother node and that node shall absorb data and / or feed to the telegram.
Inthis process, frames are only delayed by a few nanoseconds. Instead of waitingtill the complete frame is received by the node, the node shall startprocessing it immediately. After all the slave nodes received the frame, thenit shall be sent back to the master. Periodically the frames are send by themaster and the slaves shall process the data required and shall send back tothe master. The EtherCAT frames can be compared as a train and the telegramsare like the compartments of the train. TheEtherCAT protocol has its own the EtherType in the Ethernet frame, to specify aframe as EtherCAT frame, the EtherType is set to be 0x88a4. The EtherCAT frameshas an option to be placed on top of the UDP.
With the UDP, the EtherCATmessages can be transmitted to another subnet with a router. If an EtherCATmessage is received by another subnet with the UDP packaging, the UDP unpackingis done only at the first station. EtherCAT frames are still following thestandard frame sizes, the frames can be monitored using the standard tools. The communication between the masterand slave can be either Process Data communication – Where the cyclic datatransfer between the master and slave is achieved by mapping the logicalprocess data space in the frame to each slave node by the master, or Mailboxcommunication function – where the master sends a command to slaves and theslaves shall respond to the master. Synchronization Whensome distributed processes need to do a simultaneous task, precisesynchronization is particularly important. For the synchronization, theaccurate alignment of the distributed clocks through the network must be done.This is taken care by the master. As the EtherCAT communication shall utilize alogical ring structure (master sends data to slave and slave shall send thedata back to master), the master clock can determine the propagation delayoffset to the individual slave clocks accurately.
The distributed clocks areadjusted based by the value and the complete network shall achieve a precisenetwork wide timebase with a jitter less than 1 microsec. However, a highresolution distributed clocks are required and the accurate informationregarding the local timing during the data acquisition.EtherCAT FrameStructure TTEthernet TTEthernetwas developed by TTTech Computertechnik AG to enable the time triggeredcommunication over Ethernet and achieving the deterministic real time communicationfor safety related and highly real time applications. It’s been standardized bySAE International as SAE AS6802. TTEthernet integrates the AFDX Virtual links,so provides AFDX communication as Rate Constrained communication and being a transparentsynchronization protocol it can coexist with the other traffic on the samephysical communication network. It could be the solution for all theapplications ranging from control systems, Entertainment to safety related applications.Overview TTTEthernetoperates at the ISO/ OSI Level2, which is above the physical layer and belowthe network layer.
The network needs to be a 100 Mbps or 1 Gbps full duplex andby the full duplex, it can avoid the unpredictable conflicts while accessing ashared physical medium. All the devices in the TTEthernet network are synchronizedusing the clock synchronization protocol and the traffic in the network ismanaged for achieving the Quality of Service (QoS). All the nodes aresynchronized to the common time base to sub microsecond accuracy. Multiple redundantmasters are used such that if any one fails, the network shall recoverimmediately with no loss of accuracy.
During the initial startup phase, theclocks in the network are synchronized and the periodic operation isestablished. And the clocks are periodically synchronized to the common clockto counter any possible deviation and this is called as integration cycle. Data flow and trafficClasses TTEthernetintegrates different time criticality traffic in one network and broadly thetraffic in TTEthernet is classified into three classes. They are in descending order of the priorityas Time Triggered (TT), Rate Constrained (RC) and Best Effort (BE) traffic. Time Triggered Traffic Thishas the highest priority and nearly less than micro sec jitter can be achieved.TT messages are transmitted periodically at the same time interval. Thesynchronized local clocks play vital role for the TT messages to be transmittedsuccessfully.
These class is suited for the deterministic distributed system.The schedule shall be calculated offline and then shall be loaded individuallyto all the devices. By this the temporal isolation is achieved and is faulttolerant. As the frame is scheduled to be transmitted, so does the receiver isaware that it should receive the frame at that time.
If the message is receivedafter the acceptance window then the message is discarded. When a faulty devicesend error frames, these are contained and prevents the network congestion. Rate Constrained Traffic Thisclass is used for traffic which are less stringent determinism and real time applications.The bandwidth is predefined to each application with the temporal deviationsand delays. Typically, these are used for the multimedia systems.
The framesare event driven and is implemented in the virtual links fashion. The virtuallinks determine the routing of the messages from one end system to thedestination end system. The parameters BAG, Lmax determine the bandwidth usageof the bandwidth.
It used traffic policing for the data to be placed at theintervals and the bandwidth allocation. The RC messages are not with respect tothe system synchronized time as they are event based rather time based, so, theRC messages are queued at the at the network switches as in AFDX.Best Effort Traffic Theseare the lowest priority messages and doesn’t guarantee any maximum delay or evenwhen it will be delivered.
Even the standard ethernet devices can communicate inthis traffic without knowing whether the other nodes are TTEthernet. This givesthe flexibility for the TTEthernet and can coexist with the other traffic inthe network. These use the remaining bandwidth of the network. Clock Synchronization TTMessages are always transmitted in the predefined intervals with a precision ofmicro second. If the message is received after the acceptance window themessage is discarded.
Synchronization between the nodes in vital for the TTmessages to be delivered deterministically. To maintain the stable clocks atall the end systems, TTEthernet shall transmit clock synchronization messages periodically.And it relies on the hierarchical master – slave method where the master shallprovide time to the system.
There are redundant masters in case a master fails,the redundant master shall come into action. This shall guarantee fail safeoperation and high quality of synchronization. Thesynchronization happens two steps · Inthe first step the masters sends the Protocol Control Frames (PCF) to thecompression masters· Inthe second step the compression masters calculates the average of the values fromthe relative arrival time from these PCFs and sends the new PCF is sent back.
Each node uses the PCF from the compression master and adjusts their clocksThemasters, clients and compression masters are predefined during the systemarchitecture. TT Ethernet Switch Theswitches play a crucial role for successful communication of TTEthernetmessages. They should can differentiate different time of traffic and accordinglywith the priority these messages should transmitted or buffered. Switches whenthere is a contention, they follow Preemption – higher priority messages shall pre-emptthe lower priority messages, Timely Block – if time taken to transfer a lowerpriority message shall interrupt on global time when a triggered message isscheduled, then the lower priority is blocked and Shuffling – if a higherpriority message arrives when lowerpriority message is already been transmitted, then the higher priority messageis suspended till the lower priority message is transmitted. The TT messagetime schedule is uploaded to all the switches and the switch shall have theinformation of about when a TT message shall arrive.Frame Format