Avionics Full-Duplex Switched Ethernet (AFDX) is a data network for safety-critical applications that utilizes dedicated bandwidth while providing deterministic Quality of Service (QoS). AFDX is based on IEEE 802.3 Ethernet technology and utilizes commercial off-the-shelf (COTS) components. It is described specifically by Part 7 of the ARINC 664 Specification, as a special case of a profiled version of an IEEE 802.3 network per parts 1 & 2, which defines how Commercial Off-the-Shelf networking components will be used for future generation Aircraft Data Networks (ADN). The six primary aspects of AFDX include full duplex, redundancy, deterministic, high speed performance, switched and profiled network.
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Prior to AFDX, Aircraft Data Networks utilized primarily the ARINC 429 standard. This standard, developed over thirty years ago and still widely used today, has proven to be highly reliable in safety critical applications. This ADN can be found on a variety of aircraft from Boeing, Airbus and Bombardier CSeries, including the B737, B747, B757, B767, Airbus A330, A340, A380 and the upcoming A350, Bombardier CSeries CS100 and CS300. ARINC 429 utilizes a unidirectional bus with a single transmitter and up to twenty receivers. A data word consists of 32 bits communicated over a twisted pair cable using the Bipolar Return-to-Zero Modulation. There are two speeds of transmission: high speed operates at 100 kbit/s and low speed operates at 12.5 kbit/s. ARINC 429 operates in such a way that its single transmitter communicates in a point-to-point connection, thus requiring a significant amount of wiring which amounts to added weight.
Another standard, ARINC 629, introduced by Boeing for the 777 provides increased data speeds of up to 2 Mbit/s and allowing a maximum of 120 data terminals. This ADN operates without the use of a bus controller thereby increasing the reliability of the network architecture. The draw back of this system is that it requires custom hardware which can add significant cost to the aircraft. Because of this, other manufactures did not openly accept the ARINC 629 standard.
ARINC 664 is defined as the next-generation aircraft data network (ADN). It is based upon IEEE 802.3 Ethernet and utilizes commercial off-the-shelf hardware thereby reducing costs and development time. AFDX builds on this standard, as is formally defined in Part 7 of the ARINC 664 specification. AFDX was developed by Airbus Industries for the A380,[1] initially to address real-time issues for flight-by-wire system development.[2] It has since been accepted by Boeing and is used on the Boeing 787 Dreamliner. AFDX bridges the gap on reliability of guaranteed bandwidth from the original ARINC 664 standard. It utilizes a cascaded star topology network, where each switch can be bridged together to other switches on the network. By utilizing this form of network structure, AFDX is able to significantly reduce wire runs thus reducing overall aircraft weight. Additionally, AFDX provides dual link redundancy and Quality of Service (QoS).
AFDX adopted concepts (token bucket) from the telecom standard, Asynchronous Transfer Mode (ATM), to fix the shortcomings of IEEE 802.3 Ethernet. By adding key elements from Asynchronous Transfer Mode (ATM) to those already found in Ethernet, and constraining the specification of various options, a highly reliable Full-Duplex deterministic network is created providing guaranteed bandwidth and Quality of Service. Through the use of Full-Duplex Ethernet, the possibility of transmission collisions is eliminated. However, though bandwidth and maximum end-to-end latency and jitter, links are guaranteed, there is no guarantee of packet delivery. A highly intelligent switch, common to the AFDX network, is able to buffer transmission and reception packets. Through the use of twisted pair or fiber optic cables, Full-Duplex Ethernet uses two separate pairs or strands for transmit and receiving data. AFDX extends standard Ethernet to provide high data integrity and deterministic timing. Further a redundant pair of networks is used to improve the system integrity (although a VL may be configured to use one or other network only) It specifies interoperable functional elements at the following OSI Reference Model layers:
The main elements of an AFDX network are:
The central feature of an AFDX network are its Virtual Links (VL). In one abstraction, it is possible to visualise the VLs as an ARINC 429 style network each with one source and one or more destinations. Virtual Links are unidirectional logic path from the source end-system to all of the destination end-systems. Unlike that of a traditional Ethernet switch which switches frames based on the Ethernet destination or MAC address, AFDX routes packets using a Virtual Link ID. The Virtual Link ID is a 16-bit Unsigned integer value that follows the constant 32-bit field. The switches are designed to route an incoming frame from one, and only one, End System to a predetermined set of End Systems. There can be one or more receiving End Systems connected within each Virtual Link. Each Virtual Link is allocated dedicated bandwidth [sum of all VL Bandwidth Allocation Gap (BAG)rates x MTU] with the total amount of bandwidth defined by the system integrator. However total bandwidth cannot exceed the maximum available bandwidth on the network. Bi directional communications must therefore require the specification of a complimentary VL. Each VL is frozen in specification to ensure that the network has a designed maximum traffic, hence determinism. Also the switch, having a VL configuration table loaded, can reject any erroneous data transmission that may otherwise swamp other branches of the network. Additionally, there can be sub-virtual links (sub-VLs) that are designed to carry less critical data. Sub-virtual links are assigned to a particular Virtual Link. Data is read in a round robin sequence among the Virtual Links with data to transmit. Also sub-virtual links do not provide guaranteed bandwidth or latency due to the buffering, but AFDX specifies that latency is measured from the traffic regulator function anyway.
BAG stands for Bandwidth Allocation Gap, this is one of the main features of the AFDX protocol. This is the maximum rate data can be sent, and it is guaranteed to be sent at that interval. When setting the BAG rate for each VL, care must be taken so there will be enough bandwidth for other VL's and the total speed cannot exceed 100Mbit/s.
Each switch has filtering, policing, and forwarding functions that should be able to process at least 4096 VLs (this seems like a system specific derived requirement in part 7). Therefore, in a network with multiple switches (cascaded star topology), the total number of Virtual Links is nearly limitless. There is no specified limit to the number of Virtual Links that can be handled by each End System, although this will be determined by the BAG rates and max frame size specified for each VL versus the Ethernet data rate. However, the number sub-VLs that may be created in a single Virtual Link is limited to four. The switch must also be non-blocking at the data rates that are specified by the system integrator, and in practise this may mean that the switch shall have a switching capacity that is the sum of all of its physical ports.
Since AFDX utilizes the Ethernet protocol at the MAC layer, it is possible to use high performance COTS switches with Layer 2 routing as AFDX switches for testing purposes as a cost cutting measure. However some features of a real AFDX switch may be missing such as traffic policing and redundancy functions.
The AFDX bus is used in Airbus A380, Boeing 787, Airbus A400M, Airbus A350, Sukhoi Superjet 100, AgustaWestland AW101, Irkut MS-21, Bombardier CSeries.
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