wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

24-10 Industrial Communication Systems
e1
e2
e3
e4
v6, v8
v7, v9
vx, v1
v2, v3
S1
S3
v6, v8, v9
v6, v7
vx, v2
e5
v5
v1, v3
e6
v4
S2
S4
S5
v6, v8, v9
vx, v6, v7
v2, v5
v1, v3, v4
e7
e8
e9
e10
FIGURE 24.8
Example of AFDX VLs configuration.
The AFDX network architecture is composed of several interconnected switches. The inputs and outputs
of the network are the end systems (the little circles in Figure 24.7). Each end system is connected to
exactly one switch port and each switch port is connected to either an end system or another switch. The
links are all full duplex. In Figure 24.7, the values on the end systems indicate the number of VL that are
dispatched between the end systems and a given switch. Thus, the VL concept of virtual communication
channels has the advantage of statically defining the flows, which enter the network, and associating
some performance properties to each flow. Each VL can be statically mapped on the network of interconnected
AFDX switches. Transmitting an Ethernet frame from one end system to another is based
on a VL identifier, which is used for the deterministic routing of each VL (the switch forwarding tables
are statically defined after allocation of all VL on the AFDX network architecture). Each VL defines a
logical unidirectional connection from one source end system to one or more destination end systems.
For example, Figure 24.8 illustrates different kinds of VL: vx is a unicast VL with path {e3−S3−S4−e8},
while v6 is a multicast VL with paths {e1−S1−S2−e7} and {e1−S1−S4−e8}.
A VL definition includes the bandwidth allocation gap (BAG) value, the minimum frame size (S min ),
and the maximum frame size (S max ). The BAG is the minimum delay between two consecutive frames
of the associated VL (which actually defines a VL as a sporadic flow). BAG and S max values guarantee an
allocated bandwidth for each VL. Moreover, a jitter value is associated to each VL to establish an upper
bound on the maximum admissible jitter after multiplexing different regulated VL flows.
24.6.4 Virtual Link Properties
From the avionics systems designer’s point of view, classic ARINC 429 buses have many interesting
features. For example, the single-emitter assumption implies a dedicated link offered to the emitter, and
thus guaranteed access to the bus, guaranteed bandwidth, and high determinism. Moreover, the communication
paradigm used by many avionic applications is derived from the ARINC 429’s properties
behavior, which has been generalized on IMA through APEX port paradigm [ARI97]. This explains why
the VL concept is of importance in the definition of AFDX; it allows direct replacement of ARINC 429
buses, on a deterministic ARINC 664 network.
24.6.4.1 VL Bandwidth Guarantee
Each avionic function defines its VL bandwidth requirements in the form of two parameters: the BAG
and the maximum frame size (S max ). When the VL has no jitter, the BAG represents the minimum
interval between the first bits of two consecutive frames. Thus, the bandwidth offered to a VL is the one
obtained when emitting a maximum-sized frame every BAG, the latter being specified by an integer (k), to
give (create) a 2 k interval (in milliseconds). The minimal BAG is thus 1.ms for k = 0, and when combined
with standard maximum Ethernet frames, the maximum bandwidth offered attributed to a single VL
can be up to 12.Mbps.
The network integrator collates all the VL bandwidth requirements and verifies that the sum of VL
bandwidths on any physical link of the full-duplex switched Ethernet network does not exceed the
© 2011 by Taylor and Francis Group, LLC