Editing Avionics Full-Duplex Switched Ethernet (section)

==Virtual links==
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 paths 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, which is carried in the same position in an AFDX frame as the MAC destination address in an Ethernet frame. However, in the case of AFDX, this virtual link ID identifies the data carried rather than the physical destination. The virtual link ID is a 16-bit [[Unsigned integers|unsigned integer]] value that follows a 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 [[Maximum Transmission Unit|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 complementary 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 are read in a [[Round-robin scheduling|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.