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 standard, patented byinternational aircraft manufacturer Airbus,for safety-critical applications that utilizes dedicated bandwidth whileproviding deterministic quality of service(QoS). The AFDX is based onethernet data network 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 a deterministicdata network based on 10, 100 Mbps or 1 Gbps. The predecessor to AFDX, ARINC429 is a point to multipoint bus system that supports one-to-one or one-to-manyconnections. AFDX is an improvement compared to ARINC 429 due to the higherdata 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 data iseliminated using full duplex switched network – one for receiving and the otherfor transmitting. The network is designed for the critical data being prioritizedusing the QoS policies there by achieving latency, jitter and delivery withinthe set parameters. A highly intelligent switch is used which is capable tobuffer packets for both reception and transmission.
The messages areencapsulated 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 positioning system)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 In the traditional Ethernet, theframes are routed based on the Ethernet destination address. In AFDX the framesare routed using the 16-bit value called as Virtual Links. The Virtual linksshall partition the network into communication channels with predefinedscheduling time and link bandwidth. These are unidirectional and the switchedroute the packets based on the virtual link ID. Each virtual link should atleast have one or even more predefined receiving end systems that the packetsare transmitted to. When an application sends a message to acommunication port the source and destination end systems and the AFDX networkis configured to deliver the message successfully. For example, a message M istransmitted to port S.
Then the port connected to Endpoint S shall encapsulatethe message as per the AFDX protocol format and adds it on top UDP and sends itto the AFDX switched network on VLID 50. The forwarding tables in the networkswitch are configured to deliver the frame to the End system D1 and End systemD2. In the End systems, the message M is extracted from the frame and istransmitted to the port D1 and D2. Isolation and SchedulingThe virtuallink is assigned with two parameters· 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 the Lmax of 100bytes and BAG is of 4ms, then the maximum bandwidth for VLID 50 is of (100*8*1000/4)200 kbps. The choice of the BAG and Lmax depends on the requirement of theapplication and the bandwidth capability. AFDX Switch The AFDX switch forwards packetsaccording to a static MAC table.
And Each MAC address in the table correspondto a virtual link Identifier. The Rx and Tx buffers store packets in a FIFO andthe I/O processing unit in the switch will move the packets according to thevirtual link identifier. AFDX switch contain functions for filtering, policingand monitoring. Filtering is done based on the frame integrity, frame lengthand valid destination. Traffic policing is based on the token bucket algorithmwhich keeps token for all the virtual links.
When a frame is received, itchecks the account and if enough credits are available the packet is sent andcredits debited. The tokens are credited as time progress (depending on the BAGand Lmax). Monitoring is used to log the switch operation and health of the network.The traffic policing makes sure that no virtual links routed through the switchthat exceeds the bandwidth.AFDX Message Structure