Avionics Full-Duplex Switched Ethernet

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Avionics Full-Duplex Switched Ethernet (AFDX) is a deterministic data network for safety critical applications that utilizes dedicated bandwidth while providing Quality of Service (QoS). AFDX is based on IEEE 802.3 Ethernet technology and utilizes commercial off-the-shelf (COTS) components. It is Part 7 of the ARINC 664 Specification 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.

1 History
2 Overview of AFDX
3 Virtual Links
4 External links

[edit] History

Prior to AFDX, Aircraft Data Networks utilized primarily the ARINC 429 standard. This standard, developed over 30 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 both Boeing and Airbus, including the Boeing 737, 747, 757, 767 and Airbus A330 and A340. ARINC 429 utilizes a unidirectional bus with a single transmitter and up to 20 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 off of this standard, as is formally defined in Part 7 of the ARINC 664 specification. AFDX was developed by Airbus Industries for the A380. 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 star topology network of up to 24 end systems that are tied to a switch, 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).

[edit] Overview of AFDX

AFDX adopted concepts from the telecom standard, Asynchonous Transfer Mode (ATM) to fix the shortcomings of IEEE 802.3 Ethernet. By adding key elements from Asynchonous Transfer Mode to those already found in Ethernet, 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 are eliminated. However it should be noted that although bandwidth and maximum end-to-end latency 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. It specifies interoperable functional elements at the following OSI Reference Model layers:

Data Link (MAC and Virtual Link addressing concept);
Network (IP and ICMP);
Transport (UDP and optionally TCP)
Application (Network) (Sampling, Queuing, SAP, TFTP and SNMP).

[edit] Virtual Links

The central point of an AFDX network are its Virtual Links (VL). 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 routes packets based on the Ethernet desitination 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 with the total amount of bandwidth defined by the system integrator. However total bandwidth can not exceed the maximum available bandwidth on 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 among the Virtual Links with data to transmit. Also it should be noted that sub-virtual links do not provide guaranteed bandwidth or latency.

Each switch can support up to 4096 Virtual Links. Therefore in a network with multiple switches cascaded (bridged together) the total number of Virtual Links are nearly limitless. There is no specified limit to the number of Virtual Links that can be handled by each End System, however the number sub-VLs that may be created in a single Virtual Link is limited to four.