4.3 AFDX / ARINC 664 Part 7 Avionics Full-Duplex Switched Ethernet

4.3 AFDX / ARINC 664 Part 7 Avionics Full-Duplex Switched Ethernet

The pre-12 June 2024 detailed Appendix I description for 5.4 referred to ARINC, other specifications, and aircraft network / Ethernet; the current consolidated Appendix I retains only the Data buses heading and levels. A representative civil architecture for that historical study scope is AFDX — Avionics Full-Duplex Switched Ethernet — published as ARINC 664 Part 7. It is taught here as an aircraft-network example at B1/B2 level 2: not laboratory Ethernet as used in an office, but a deterministic profile of switched Ethernet used as the on-board data network. OpenExamPrep publishes this material as independent exam preparation; it is not an EASA document and does not claim official approval or partnership.

Typical civil installations you should be able to name are the Airbus A380, the Airbus A350, and the Boeing 787 Common Data Network (CDN). Examination fees remain NAA/Part-147 matters; EASA does not publish a single Module 5 euro price.

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Why a switch instead of a shared bus

ARINC 429, ARINC 629 and MIL-STD-1553B are shared-medium buses. All listeners electrically see the same pair (or current loop). Bandwidth is global: one talker occupies the medium. Collisions are prevented by simplex rules (429), unique terminal gaps (629), or a bus controller (1553).

AFDX replaces the party line with a switched fabric:

• Each End System (ES) — the avionics computer or remote data concentrator with an AFDX interface — has a dedicated full-duplex link to a switch (or to a pair of switches, one per redundant network).
• Full-duplex means the ES can transmit and receive at the same time on that link. There is no CSMA/CD collision domain, because no two End Systems share a half-duplex wire.
• The switch is store-and-forward: it receives a frame, checks it, and forwards it only on the ports that the virtual-link table allows.

The physical signalling is Ethernet (typically 100 Mbit/s class 100BASE-TX teaching; some links may be 10 Mbit/s or, on later types, gigabit-class). Treat 100 Mbit/s as the usual Module 5 figure for AFDX links, not 2 Mbit/s (629) or 1 Mbit/s (1553). Cables and connectors are aviation-qualified; you do not randomly fit office CAT-5 into a pressure bulkhead and call it AFDX.

Because the fabric is switched, a technician cannot clip a 429-style high-impedance tap onto the party line and see every frame. Observation is at an End System test connector, a switch monitoring port if the design provides one, or a maintenance network access point described in the AMM. Wrong tool, wrong family.

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Virtual links (VL)

A virtual link is a unidirectional logical path from exactly one source End System to one or more destination End Systems. It behaves like a constrained multicast: only the configured source may emit that VL, and only the configured destinations should consume it. Bidirectional exchange uses two VLs, one in each direction — the Ethernet analogue of needing two 429 pairs for two-way traffic, except both VLs can share the same physical switch ports because the switch multiplexes many VLs.

The destination MAC address of an AFDX frame carries the virtual-link identifier. End Systems and switches classify traffic by VL, not by an ARINC 429 octal label and not by a 1553 RT address. IP and UDP ride inside the Ethernet frame as the usual AFDX communications profile; Module 5 does not require you to subnet an office LAN, but it does require you to know that AFDX is a profile of Ethernet, not a new analogue signalling standard.

A VL is not a bidirectional Ethernet collision domain and not a 1553 subaddress. If a question uses those distractors, return to the definition: one source, one or more sinks, unidirectional, identified in the frame, paced by BAG.

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Bandwidth Allocation Gap (BAG) and policing

The Bandwidth Allocation Gap is the minimum interval between the start of consecutive frames on one VL. Typical teaching BAG values are powers of two from 1 ms to 128 ms (1, 2, 4, 8, 16, 32, 64, 128 ms). A VL also has a maximum frame length (Lmax). Together they bound the bandwidth that VL may consume:

Reserved bit rate ≈ (Lmax in bits) / BAG

Worked example. A VL with BAG = 8 ms (0.008 s) and Lmax = 200 bytes = 1600 bits:

1600 bits / 0.008 s = 200 000 bit/s = 200 kbit/s reserved for that VL.

If BAG is halved to 4 ms with the same Lmax, the reservation doubles to 400 kbit/s. If Lmax doubles at the same BAG, the reservation also doubles. The source End System traffic-shapes: it must not emit the next frame on that VL until BAG has expired. The switch polices: if a babbling source violates BAG or Lmax, excess frames are dropped rather than flooding every other computer. That policing is the safety property that ordinary unmanaged office Ethernet does not provide.

Jitter around BAG is bounded by the specification so that receiving partitions in an IMA cabinet can be scheduled. Module 5 will not ask you for a microsecond jitter constant unpublished by EASA; it will ask what BAG is for.

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Dual independent networks

AFDX installations use two independent networks, conventionally Network A and Network B: separate cables, separate switches, separate power where the design requires it. An End System typically transmits the same frame on both networks, with a sequence number in the AFDX header. The receiving End System performs integrity checking (discard a corrupted frame) and redundancy management (accept the first valid copy of a sequence number, discard the duplicate that arrives on the partner network).

A single switch failure, loom cut or connector fault can therefore take Network A while Network B continues. That is the Ethernet form of 1553 Bus A/B and 629 dual buses, implemented with two switched fabrics instead of two party lines.

Never improve an installation by bridging A and B together with a commercial Ethernet switch. That destroys independence and can create forwarding loops the aircraft profile never allowed.

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A380, A350 and B787 context

The Airbus A380 was the first large civil airliner to use AFDX as the primary avionics network, interconnecting IMA modules, flight-control, cockpit displays and numerous subsystems that would previously have required forests of 429 pairs and 629-style backbones. The A350 continues that Airbus switched-Ethernet architecture. The Boeing 787 Common Data Network is the Boeing teaching example of ARINC 664 / AFDX-style aircraft Ethernet on a twin-aisle type.

429 has not vanished on these aeroplanes. Many sensors and actuators still speak 429 (or analogue) into remote data concentrators; the concentrators then ride the AFDX network. Module 5 therefore treats AFDX as the aircraft network / Ethernet clause, not as a claim that every wire in the aeroplane is Ethernet.

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Shared bus versus switched Ethernet — teaching snapshot

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Maintenance implications

Configuration of VL tables, BAG values and port maps is loadable software/data. An unapproved change can mis-route or starve a virtual link — which is why Module 5 later treats software management control as a catastrophic-risk subject. On the flight line, confirm that both networks are present, that the End System is powered, that connectors are seated, and that the central maintenance system / BITE names a VL, a switch, or an ES rather than a 429 label. Do not apply 429 twenty-receiver arithmetic to a switch port, and do not apply 1553 response-time windows to Ethernet frames.

Level 2 is this picture: AFDX is ARINC 664 Part 7 aircraft Ethernet; virtual links plus BAG make it deterministic; switches police rather than share a wire; dual networks provide integrity; A380, A350 and 787 are the typical types.