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MX Foundation 4
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MX Foundation 4
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A virtual link is a fixed path through the AFDX network that originates at only one end system and delivers packets to a fixed set of end systems. This is similar to an ARINC 429 multi-drop bus. Frames are routed by the AFDX switch using a Virtual Link Id which is encoded inside the Ethernet destination MAC address. The Ethernet link on an end system can support multiple virtual links. Each link shares the total bandwidth. In order to prevent packets on one virtual link from interfering with packets on another virtual link using the same physical link, each virtual link is assigned two parameters. The Bandwidth Allocation Gap (BAG) represents the minimum interval between frames on the virtual link. The Lmax represents the largest Ethernet frame that can be transmitted on the virtual link. These two values provide a bandwidth limit for each virtual link.
Avionic subsystems use communication ports to send messages to each other. These ports provide a programming mechanism for sending and receiving messages. AFDX end systems must provide both sampling and queuing port services as described in ARINC 653. The only real difference between sampling and queuing ports is at reception. A sampling port buffer has storage for a single message only. Arriving messages will constantly overwrite this buffer. Reading the message does not remove the port from the buffer, so it can be read continuously and by many applications. A sampling port must provide an indication of message freshness. Without this, it could not be determined if the transmitting system had stopped sending or is sending the same message repeatedly. In contrast, a queuing port has enough storage for a fixed number of messages. These messages are appended to a FIFO queue. As the messages are read by the end system, they are removed from the queue.
Two physical communication links are used by a typical virtual link to transmit data for redundancy reasons. They are based on Ethernet ports and named Net-A, Net-B.
The standards governing AFDX can be found in IEEE 802.3 and ARINC 664, Part 7.
